Topics covered
- Climate and weather
- Evolution of atmosphere
- Composition of atmosphere
- Air
- Dust particle
- Green House Gases
- Structure of Atmosphere
- Troposphere
- Stratosphere
- Mesosphere
- Thermospher
- Exosphere
- Insolation and terrestrial radiation
- Variability of insolation at the surface of the Earth
- Albedo
- Heat Budget
- Temperature Distribution on the Earth
- Controlling factors
- Isotherm
- Air moisture
- Water in the atmosphere
- Humidity
- Temperature lapse rate
- Rising and Falling Air Parcel
- Adiabatic Lapse Rate
- Types of condensation
- Temperature Inversion
- Ground surface inversion
- Upper air inversion
- Frontal or cyclonic inversion
- Surface inversion of temperature
- Valley inversion
- Temperature inversion implications
- Types of precipitation – Rain, Frost, Sleet, Hail
- Rainfall Types
- Atmospheric circulation
- Vertical and horizontal variation
- Coriolis force
- Frictional force
- Pressure gradient force
- Geostrophic wind
- Pressure and wind
- World distribution of isobars
- Pressure Systems of the World (Vertical and horizontal Pressure Belts)
- El-Nino Southern Oscillation (ENSO)
- Wind Systems of the World
- Permanent- Planetary Winds
- Variable- Seasonal and Local winds
- Jet Streams
- Air Mass
- Fronts
- Cyclones- Tropical & Temperate Cyclone, their Formation, their Distribution, their Differences
- Thunderstorm
- Tornados or Twisters
- Rossby Waves
- Polar Vortex
- Ozone Depletion & Hole
- Polar Stratospheric Clouds
- Aurora
Syllabus for GS Paper
- Prelims Paper I
- Indian and World Geography-Physical, Social, Economic Geography of India and the World.
- Paper 2 (GS-I)
- Salient features of the world’s physical geography.
| Climate | Weather |
| Climate is the weather of a place averaged over a period of time, often 30 years. | It is the mix of events that happen each day in our atmosphere (One atmosphere on the Earth but different weathers at different locations). |
| Describes what the weather is like over a long period of time in a specific area. | Refers to short-term changes in the atmosphere. |
| Its study known as Climatology | Its study known as Meteorology |
The Evolution of Earth’s Atmosphere: From Primordial Gases to Oxygen
- There are three stages in the evolution of the present atmosphere.
- Loss of primordial atmosphere.
- The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a result of the solar winds.
- Evolution of the atmosphere by the hot interior of the earth.
- During early life of the earth, Nitrogen, Sulphur, Carbon Dioxide, Water Vapour, and Argon came out due to the extensive volcanism and degassing.
- Modification of atmospheric composition by the living world through the process of photosynthesis.
- Water vapor condensed, which led to the formation of clouds and hence the rainfall washed out the bulk of Carbon Dioxide into the Oceans.
- Oxygen was produced from anaerobic respiration of bacteria like Cyanobacteria (and not from degassing).
- Evolution of the atmosphere by the hot interior of the earth.
- The present composition of earth’s atmosphere is chiefly contributed by nitrogen and oxygen.
Properties of Atmospheric Gases
- Nitrogen, Oxygen, Hydrogen and Argon are permanent gases.
- Water vapour, Carbon Dioxide, Ozone are variable gases.
- Neon, Argon – inert gases
- Atmospheric gases
- No chemical interaction among them
- They don’t lose their properties
- They act as a single unified gas
The Composition and Role of Earth’s Atmosphere
- Atmosphere is a mixture of different odorless, tasteless and colorless gases, dust and vapour.
- It is a relatively thin layer enveloping the earth all round and held by the Earth’s gravity. It extends several thousands of kilometers above the earth surface.
- It is a protective boundary between outer space and the biosphere.
Air: The Vital Mixture of Gases Surrounding Earth
- Air is mostly gas.
- Air in motion is called wind.
- Atmosphere is an envelope of air composed of numerous gases. These gases support life over the earth’s surface.
- The air in Earth’s atmosphere is made up of approximately 78% nitrogen and 21% oxygen.
- Air also has small amounts of lots of other gases, too, such as carbon dioxide, neon, and hydrogen.
Dust Particles: Origins, Distribution, and Transport in Earth’s Atmosphere
- Small solid particles originate from different sources.
- It includes sea salts, fine soil, smoke-soot, ash, pollen, dust and disintegrated particles of meteors.
- They are generally concentrated in the lower layers of the atmosphere.
- They can be transported to great heights by convection air currents.
- The higher concentration of dust particles is found in subtropical and temperate regions due to dry winds in comparison to equatorial and Polar Regions.
Hygroscopic nuclei: They are the dust particles around which the water vapor condense to form clouds.
Greenhouse Effect: Earth’s Atmospheric Heating Mechanism
- The greenhouse effect is a process that occurs when gases in Earth’s atmosphere trap the Sun’s heat.
- This process makes Earth much warmer than it would be without an atmosphere.
- The greenhouse effect is one of the things that make Earth a comfortable place to live.
Greenhouse Gases: Earth’s Atmospheric Heat Regulators
- Greenhouse gases are gases in Earth’s atmosphere that trap heat.
- They let sunlight pass through the atmosphere, but they prevent the heat that the sunlight brings from leaving the atmosphere.
- GHGs absorb long longwave terrestrial radiation.
- Some of the main greenhouse gases are:
- Water vapor
- Carbon dioxide
- Methane
- Ozone
Factors Affecting Greenhouse Gases’ Impact on Climate Change
- How much is in the atmosphere?
- Concentration, or abundance, i.e. the amount of a particular gas in the air.
- Larger emissions of greenhouse gases lead to higher concentrations in the atmosphere.
- How long do they stay in the atmosphere?
- Each of these gases can remain in the atmosphere for different amounts of time, ranging from a few years to thousands of years.
- How strongly do they impact the atmosphere?
- Some gases are more effective than others at making the planet warmer and “thickening the Earth’s blanket.”
For each greenhouse gas, a Global Warming Potential (GWP) has been calculated to reflect how long it remains in the atmosphere, on average, and how strongly it absorbs energy. Gases with a higher GWP absorb more energy than gases with a lower GWP, and thus contribute more to warming Earth.
Role of Water Vapor in Atmospheric Dynamics
- A variable gas in the atmosphere that decreases with altitude and on moving from the equator towards the poles.
- Its concentration is higher in warm and wet tropics in comparison to the dry and cold areas of desert and Polar Regions.
- It also absorbs parts of the incoming solar radiation and preserves the radiated heat from the earth (terrestrial radiation).
- Hence, it acts like a blanket allowing the earth neither to become too cold nor too hot.
- It also contributes to the stability and instability in the air.
Impact of Carbon Dioxide on Earth’s Radiation Balance
- Meteorologically a very important gas because of its transparency to the incoming solar radiation but opaqueness to the outgoing terrestrial radiation.
- It absorbs a part of terrestrial radiation and reflects back some part of it towards the surface of earth.
- It is largely responsible for the greenhouse effect.
The Significance of Methane as a Greenhouse Gas
- One of the most important greenhouse gases.
- It is produced from decomposition of animal wastes and biological matter.
Nitrous Oxide: Sources and Implications
- Nitrous oxide is emitted during agricultural and industrial activities, combustion of fossil fuels and solid waste, as well as during treatment of wastewater.
Fluorinated Gases: Industrial Sources and Environmental Impact
- These are synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes.
- Fluorinated gases are sometimes used as substitutes for stratospheric ozone-depleting substances.
- These gases are typically emitted in smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as High Global Warming Potentialgases (“High GWP gases”).
Ozone: Atmospheric Shield Against Harmful UV Radiation
- Important component of the atmosphere is found between 10 and 50 km above the earth’s surface.
- Prevents the surface of the earth by absorbing the harmful ultraviolet radiations coming from the sun.
Ground-Level Ozone vs. Stratospheric Ozone: A Comparison
| Ground-level Ozone | Stratospheric Ozone | |
| Description | · Bad Ozone
· Part of Photochemical Smog (We will read about it later) · Found in Troposphere |
· Good Ozone
· Act as natural filter which absorbs the Sun’s UV rays · Found in Stratosphere |
| Sources | · Forms when Nitrous Oxides (NOx) react with Volatile Organic Compounds (VOCs). | · Naturally forms when Oxygen is in the presence of UV radiation. |
| Effects | · Eye and respiratory irritation
· Lung disease · Corrosion in buildings |
Thinning of ozone shield leads to
· Crop damage · Aquatic life death · Eye irritation · Skin cancer |
STRUCTURE OF ATMOSPHERE
Thermal Zone Segregation
Troposphere: Earth’s Dynamic Atmospheric Layer
- Lowermost layer of the atmosphere.
- Also known as convective region(all convection occurs till the Tropopause).
- Tropopause: Zone separating the troposphere from stratosphere. The temperature here is nearly constant.
- Average height = 13 km
- Highest at equator (18 km) because heat is transported to great heights by strong convection currents
- Lowest at poles (8 km)
- The temperature is also lowest at equator (-80ºC) as compared to poles (-45ºC). This is because convection currents are strongest at the equator.
- 90% of total mass.
- Contains dust particles and water vapor.
- Most clouds appear here as approx 99% of water vapor is found here.
- All changes in climate and weather take place in this layer.
- Seasons and jet streams affect the troposphere.
- Most important layer for all biological activity.
- Temperature reduces at 6.5ºC/km or 1ºC▼/165m (normal lapse rate) as we move up.
Stratosphere: Home to Earth’s Protective Ozone Laye
- Extends from tropopause to 50 km.
- Important Feature = It contains the Ozone Layer (Shields life on the earth by absorbing intense, harmful ultra-violet radiation)
- Temperature inversion: Normal Lapse Rate ends This warming of the stratosphere with altitude is caused largely due to absorption of solar energy by ozone.
- The air movements are almost horizontal. This is because the effect of convection currents is almost negligible in comparison to the troposphere. This in turn prevents vertical mixing of pollutants from troposphere to stratosphere.
- Ideal region for flying jets as clouds are almost absent (sometimes layers possess cirrus clouds in lower level).
- Winds blow from west to east.
Mesosphere: Where Meteors Blaze and Noctilucent Clouds Glow
- Ranges 50-80 km (Stratopause and Mesopause)
- Temperature again starts falling with elevation (because no GHGs exist here, i.e. no heat absorbing layer nor ozone layer).
- Temperature decrease from 0 ºC to -90 ºC.
- Meteors burn in this layer.
- Mesospheric or Noctilucent clouds visible at high latitude during summer season due to the condensation of moisture around the meteoric dust.
- Mesopause= Upper limit of mesosphere
- Very thin layer causes difficulty in breathing.
Thermosphere: Where Ions Dance and Auroras Illuminate
- It extends from 80-400 km and contains electrically charged particles known as ions (Region is known as Ionosphere).
- Temperature rises with height again due to proximity to the sun (Ions absorb heat).
- Even though the temperature is high but because of the rarified atmosphere the heat could not be felt.
- International Space Station Satellites orbit in this layer
- Aurora Also form in this layer.
The Exosphere: Where Earth’s Atmosphere Fades into Space
- Uppermost layer of the atmosphere.
- Above the thermosphere.
- Highest layer and extremely rarefied.
- It gradually merges with outer space.
Ozonosphere and Ionosphere: Protective and Communicative Layers of Earth’s Atmosphere
Ozonosphere
- It spans the stratosphere and lower mesosphere and lies at an altitude between 30 km and 60 km from the earth’s surface.
- This layer reflects the harmful ultraviolet radiation due to the presence of ozone molecules.
- The ozonosphere is also known as chemosphere because of immense chemical activity goes on here.
- The temperature increases at a rate of 5°C/km.
Ionosphere
- Where electron density is very high (100-300 km).
- Ions useful for Radio communication(reflects radio waves).
Electromagnetic Spectrum, Sky Coloring, Temperature, and Heat Transfer Mechanisms
Planck–Einstein relation ( Planck’s energy–frequency relation)
- Frequency = Number of occurrences of a repeating event per unit of time.
- Wavelength = Distance between identical points in the adjacent cycles of a waveform signal.
- Higher the Frequency = High Energy or Short Wavelength
Much of the near infrared radiation absorbed by the water vapor, ozone and other gases present in the atmosphere (mainly in the troposphere) while solar radiation passes through it.
Coloring of the sky
- Sunlight reaches Earth’s atmosphere and is scattered in all directions by all the gases and particles in the troposphere.
- A clear cloudless day-time, sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light.
- When we look towards the sun at sunset, we see red and orange colors because the blue light has been scattered out and away from the line of sight.
Temperature
An objective measurement of how hot or cold an object is. It can be measured with a thermometer or a calorimeter.
Heat
It is a form of energy that is transferred between two substances at different temperatures.The effects of this energy transfer usually, but not always, is an increase in the temperature of the colder body and a decrease in the temperature of the hotter body.
- A substance may absorb heat without an increase in temperature by changing from one physical state (known as phase) to another. This absorbed heat is known as Latent Heat.
Heat versus Temperature
While heat represents the molecular movement of particles comprising a substance, the temperature is the measurement in degrees of how hot (or cold) a thing (or a place) is. The interaction of incoming solar radiation with the atmosphere and the earth’s surface creates heat which is measured in terms of temperature.
- Plank’s law states that hotter a body, the more energy it will radiate and shorter the wavelength of that radiation.
- Specific heat is the energy needed to raise the temperature of one gram of substance by one Celsius.
Heat Transfer mechanisms
- Conduction– Heat transfer by direct contact of particles of matter.
- Convection– Transfer of heat by the movement of a heated fluid (liquids and gases) molecules. Heat transfer by convection is caused by differences in temperature and density within a fluid.
- Advection– Heat transfer through horizontal movement of the air.
- Radiation– The transfer of energy through empty space. There is no direct contact between the heat source and an object.
Insolation and Terrestrial Radiation: Earth’s Energy Balance
- Earth’s surface receives most of its energy in the form of short wavelengths known as incoming solar radiation (Insolation).
- The earth absorbs shortwave radiation (Short wavelength = High Energy) during daytime and reflects back the heat received into space as long-wave radiation (mostly infrared radiation) during night. It makes the Earth a radiating body.
- The long wave energy radiated by the Earth known as Terrestrial Radiation.
- Terrestrial Radiation heats the atmosphere from below as it is absorbed by the atmospheric gases particularly the greenhouse gases.
- The atmosphere in turn radiates and transmits heat to space.
- Finally, the amount of heat received from the sun is returned to space thereby maintaining constant temperature at the earth’s surface and in the atmosphere.
- This is why earth neither warms up nor does it get cooled over a period of time.
- The amount of heat received by different parts of the earth is not equal which causes pressure differences in the atmosphere.
- This leads to transfer of heat from one region to the other by winds.
Factors Influencing Variability of Insolation on Earth’s Surface
The factors that cause these variations in insolation are:
- Rotation of earth on its tilted axis (at 66.5 degree with the plane of its orbit around the sun).
- Angle of inclination of the sun’s rays
- Area under the insolation increases with increasing latitude as a result of slant sun rays.
- Length of the day
- Duration of the day affects the amount of insolation received.
- Shorter the duration results in less received insolation.
- Transparency of the atmosphere
- Affects reflection, absorption or transmission of insolation.
- Depends upon the cloud cover and its thickness, dust particles, water vapor, etc
- Thick cloud hinders the solar radiation from reaching the earth’s surface.
- Water vapor absorbs solar radiation resulting in less amount of insolation reaching the surface.
- Slant rays are required to pass through greater depth of the atmosphere resulting in more absorption, scattering and diffusion.
- Configuration of land in terms of its topography
- Sun facing slopes receive more vertical rays of sun.
Sub-solar point and Sun’s declination
- The point on earth where the sun is directly overhead at a given point of time is called sub-solar point.
- The latitude of the sub-solar point is called the Sun’s declination.
Albedo
- This is the amount of insolation reflected by the body.
- It is defined as the ratio of the reflected radiation to the total intercepted radiation.
- It is described in terms of percentage of reflected radiation.
- When the sun is overhead, the albedo is less.
- Albedo commonly refers to the “whiteness” of a surface, with 0 meaning black and 1 meaning white.
- A value of 0 means the surface is a “perfect absorber” that absorbs all incoming energy and the object having this surface is known as Blackbody.
Heat Budget
- The earth as a whole does not accumulate or lose heat. It maintains its temperature.
- This can happen only if the amount of heat received in the form of insolation equals the amount lost by the earth through terrestrial radiation (insolation=terrestrial radiation).
- 35% of insolation is radiated (27% from clouds, 2% from ice) and 14% of insolation is absorbed by the atmosphere.
- Rest 51% of insolation reaching earth’s surface gets absorbed by it and later radiated back.
- 34% is absorbed by the atmosphere again (19% via latent heat of condensation).
- 17% is radiated directly to space.
- Atmosphere together radiates back 48% to space.
Latitudinal Heat Balance
- Latitudinal Heat Balance is the state of balance which exists between the latitudinal belts by maintaining net incoming solar radiation and the outgoing terrestrial radiation.
- The amount of insolation received by the earth surface varies from latitude to latitude.
- At latitudes below 40º = Insolation ≥ Outgoing Radiation (surplus of net radiation)
- At latitudes above 40º = Insolation ≤ Outgoing Radiation(deficit of net radiation)
- Heat transfer takes from the heat surplus zone to the heat deficit zone by ocean currents (20%) and atmosphere (80%).
- The temperature of the earth as a whole remains constant due to this equilibrium.
- If there is no latitudinal heat balance, the deficit heat belt will become extremely cold and the surplus heat belt will become extremely hot to live in.
Temperature Distribution Factors
Factors controlling the distribution of Temperature (LACTO PAE) (BHUMIKA BANDHO)
- Latitudes: Intensity of insolation decreases with the increase in latitude. Maximum temperature is not at the equator but at 20ºN.
- Major portion is reflected by the clouds and a sizable amount is lost in evaporation.
- At 45º latitude, insolation is about 75% of that at the equator.
- At 66.5º latitude, it is about 50% of that at the equator.
- At poles, it is about 40% of that at the equator.
- Altitude: Temperature decreases with increasing height at an average rate of 6.5ºC/km.
- The layers of air are denser at the earth’s surface and become lighter with increasing altitude.
- The lower layers contain water vapor and dust particles.
- Distance from the Coast: Temperature is moderated by marine environments because of sea breeze and land breeze.
- Terrestrial radiation: Major source of atmospheric heat is the earth’s surface from where heat is transferred to the atmosphere.
- Ocean Currents: Warm currents raise temperature whereas cold current reduces. (We will read Ocean Currents in detail in Oceanography Chapter)
- Prevailing Winds: Winds transfer heat from one latitude to another as well as between land and water bodies.
- The oceanic winds bring a moderating effect from the sea to coastal areas (cool summers and mild winters).
- This effect is pronounced only on the windward side. The leeward side or the interior experiences extreme temperature as it does not get a moderating effect of the sea. (Grammer)
- Air mass: Places having warm air mass experiences higher temperature than places that come under influence of cold air mass. (We will read Air Mass in detail ahead)
- Effect of continentality: Daily Range of temperature is less in marine climate, while extremely high in continental climate.
Temperature Distribution: Isotherms and Anomalies
- The global distribution of temperature can well be understood by studying the temperature distribution in January and July.
- The temperature distribution is generally shown on the map with the help of isotherms.
- Isotherms: Lines joining places having equal temperature at a given time or on average over a given period.
- Effects of altitude are not considered while drawing an isotherm.
- Temperature anomaly: The difference between mean temperature of a place and the mean temperature of its latitude is called temperature anomaly.
- Positive Anomaly = Local Temperature > Latitude Temperature
- Negative Anomaly = Local Temperature < Latitude Temperature
Above 40º N, continents have Negative Anomaly and oceans have Positive Anomaly for the year as a whole and vice versa for ocean.
Isotherms and their general characteristics
- Generally, follow the latitude parallels (because all the points located on the same latitude receive the same amount of insolation).
- Sudden bends at ocean-continent boundaries even on the same latitude (because of the differential heating of land and water).
- High thermal gradient (rapid change in temperature) indicated by narrow spacing between isotherms.
- Low thermal gradient (small or slow change in temperatures) indicated by wide spacing between isotherms.
Global Temperature Distribution Patterns
- Highest temperatures = Tropics and sub-tropics (high insolation)
- Lowest temperatures = Polar and SubPolar Regions
- The interiors of continents have the highest diurnal and annual temperature range because of the continentality effect (No moderating effect of oceans).
- Temperature gradients are usually low over the eastern margins(because of warm ocean currents) and high over the western margins (because of cold ocean currents) of continents.
- The isotherms show a poleward shift while passing through an area with warm ocean currents.
- An enhanced land-sea contrast makes isotherms irregular over the northern hemisphere.
- Northern hemisphere is warmer than the southern
- hemisphere due to the predominance of landmass.
- Temperature contrast between continents and oceans are greater during winters than in summers.
- Maximum insolation is received over the subtropical Deserts due to less cloudiness.
The cold currents and warm currents would be discussed in detail in Oceanography.
Global Temperature Distribution Patterns in January
- Summer in the southern hemisphere and winter in the northern hemisphere.
- The thermal equator lies to the south of the geographical equator (because ITCZ shifts southward with the apparent southward movement of the sun) and the high temperature belt runs somewhere along 30°S latitude.
- The western margins of continents are warmer than the eastern margins due to the Westerlies that carry high temperatures into the landmasses.
- The eastern margins of continents have a close temperature gradient.
- The effect of the ocean makes isotherms almost parallel to the latitudes in the southern hemisphere.
- Land Masses are cooler than the oceans in the northern hemisphere.
- The isotherms bend towards the poles while crossing oceans and to the equator while crossing landmasses.
- Oceans are cooler than the landmasses in the southern hemisphere.
- Isotherms bend towards the equator while crossing oceans and towards the poles while crossing landmasses.
Global Temperature Distribution Patterns in July
- Summer in the northern hemisphere and winter in the southern hemisphere.
- The thermal equator lies to the north of the geographical equator (due to the northward shift of ITCZ with the apparent northward movement of the sun).
- The southern hemisphere has a regular gradient but shows a slight bend towards the equator at the continent’s edge.
- The deviation of isotherms is not that much pronounced in July as in January, especially in the northern hemisphere.
- Oceans are cooler than the landmasses in the northern hemisphere.
- Isotherms bend towards the equator while crossing oceans and towards the poles while crossing landmasses.
- Land Masses are cooler than the oceans in the southern hemisphere.
- The isotherms bend towards the poles while crossing oceans and to the equator while crossing landmasses.
AIR MOISTURE
Water in the Atmosphere
Hydrological Cycle: The Continuous Exchange of Water
- The water between the atmosphere, the oceans and the continents continuously exchanges through the processes of
- Evaporation (Moisture driven from atmosphere by transforming liquid into gaseous state)
- Transpiration (Moisture driven from plants)
- Condensation (In the form of clouds)
- Precipitation ((In the form of rain)
- The hydrological cycle maintains the balance between these processes so that the total amount of moisture in the entire system remains constant.
- Water vapor in air varies from zero to four percent by volume of the atmosphere and plays an important role in weather phenomena.
Humidity: Absolute, Specific, and Relative Humidity
Water vapor present in the air is known as humidity.
Absolute humidity
- It is the weight of water vapor per unit volume of moist air.
- It is an actual amount of water vapor present in the atmosphere.
- It is expressed as grams of moisture per cubic meter of air (g/m3).
- Change in temperature or pressure may impact volume hence affects absolute humidity.
Specific Humidity
- It is the weight of water vapor per unit weight of dry air.
- Since it is the weight of the air now, it is not impacted by change in temperature or pressure.
Relative humidity (RH)
- Proportion of actual water vapor present in the air to its water vapor carrying capacity at a constant temperature.
- Saturated Air parcel = 100% RH (The air is at full moisture carrying capacity and no further moisture addition is possible)
- Relative humidity can be affected by two ways
- By adding moisture through evaporation (by increase in absolute humidity)
- RH = Over Ocean (greater availability of water for evaporation) > Over Continent
- By changing temperature of air (Change in saturation point)
- By adding moisture through evaporation (by increase in absolute humidity)
- Change in temperature can affect the moisture carrying capacity of the atmosphere.
- Dew Point = Temperature at which the sample of air becomes saturated and it cannot hold moisture any further.
Measurement
- Hygrometer is an instrument used to measure humidity.
- Psychrometer is a hygrometer with one dry bulb thermometer and one wet bulb thermometer. The difference between the two readings gives the humidity.
Temperature Lapse Rate
- It is the rate of change in temperature observed with rising altitude.
- Positive = Temperature decreases with altitude (Normal Lapse Rate)
- Zero = Temperature is constant with altitude
- Negative = Temperature increases with altitude (known as temperature inversion)
Factors Contributing to Temperature Decrease with Altitude
- The temperature falls with rising altitude is primarily due to two reasons.
- Atmospheric pressure falls (Pressure is directly proportional to Temperature and vice versa)
- Reduced greenhouse gases concentration (leading to low heat absorption capacity of atmosphere).
- This fall in temperature with altitude is called Temperature Lapse and the rate at which it falls is known as Temperature Lapse Rate.
Behavior of Air Parcels in Fluids
- The object sinks or rises in a fluid based on its relative density with fluid.
- Air parcel density is more than the surrounding environment = It will fall
- Air parcel density is less than the surrounding environment = It will rise
Behavior of Rising Air Parcels
- Air parcel heated more than surrounding (Heat exchange hence non-adiabatic) = Temperature increase à Volume increase (due to expansion, Charles’s law) à Density decrease
- At constant Pressure, Change in Volume is directly proportional to Temperature change (i.e. Increase in Temperature àIncrease in Volume)
- Since only heat interaction takes place with no addition of any mass in the air parcel, volume increases at constant mass.
- As density is given by mass divided by volume, volume increment at constant mass causes decrease in density.
- Rising air parcel à Less pressure above (Atmospheric pressure decreases with altitude) à Volume increases (Removing pressure from object increases volume, Boyle’s law) due to decreased pressure à Temperature falls (due to internal changes rather than heat exchange hence adiabatic)
- The increased volume causes further decrease in density. The air parcel keeps rising. As there is no interaction of heat, only expansion leads to temperature decrease. The rate at which this temperature fall occurs is called Positive Adiabatic Lapse Rate.
Behavior of Falling Air Parcels
- Air parcel at upper level à Heat exchange between the air parcel and the surrounding environment (hence non-adiabatic process) à Temperature falls à Volume decreases (Charles’s law) à Density increases
- This can also occur if an air parcel comes in contact with cooler surfaces like mountain slopes.
- Air parcels start falling when its density becomes greater than surrounding.
- With fall, internal temperature of a falling air parcel increases adiabatically due to the increased atmospheric pressure (Gay-Lussac’s law).
- The rate at which this temperature rise occurs is called Negative Adiabatic Lapse Rate.
Adiabatic Lapse rate
- Fall in temperature of a rising air parcel without losing any internal heat.
- Air expands and cools adiabatically when it rises.
- Rate of cooling depends on the water vapor content of the air. Hence, ALR is usually differentiated as dry or wet (moist) air.
- Higher the water vapor = Lower the rate of cooling due to release of latent heat of condensation.
- Dry adiabatic rate is about twice of the wet adiabatic rate.
Wet Adiabatic Lapse Rate (WALR)
- Saturated air parcel cool down slower than the unsaturated one due to the release of latent heat of condensation.
- The WALR varies considerably due to the high variability of water vapor amount in the air.
- High amount of vapor = Low ALR
- More release of internal heat in the form of latent heat of condensation, hence less temperature reduction (Phase change occurs at constant temperature).
- Average WALR for the Earth’s atmosphere = 4°C/km.
- WALR is mainly associated with unstable conditions (due to the high moisture content).
Dry Adiabatic Lapse Rate (DALR)
- Dry or unsaturated air parcel cool down early than the saturated air as there is less release of latent heat of condensation.
- Low amount of vapor = High ALR
- Less internal heat releases heat in the form of latent heat of condensation, hence more temperature reduction.
- Average DALR for the Earth’s atmosphere = 8°C/km
- DALR is mainly associated with stable conditions (because it has less moisture).
Significance in Meteorology
- The difference between the Normal Adiabatic Lapse Rate (NALR) in the atmosphere and the DALR & WALR determines the vertical stability of the atmosphere.
Weather conditions at different Adiabatic Lapse Rates (ALR)
- ALR = Adiabatic Lapse Rate of entire atmosphere = 6°C/km
- If ALR > 6°C/km = DALR
- Less moisture than normal = more stable than normal
- If ALR > 6°C/km = DALR
- If ALR < 6 °C/km = WALR
- More moisture than normal = less stable than normal or instability
Atmospheric Stability
- Conditional stability: WALR < NALR < DALR
- Normal moisture conditions = It may or may not rain
- Absolute stability: NALR < WALR < DALR
- Little moisture in the air parcel = It won’t rain
- Absolute instability: WALR < DALR < NALR
- Excess moisture in the air parcel = It will rain violently
Types of Condensation and Their Outcomes
| Place of Condensation | Outcome |
| At height | Cloud |
| At lower level | Fog |
| On the cold surface | Dew |
| On a surface having below freezing point temperature | Frost |
Cloud Formation and Characteristics
- A cloud is an aggregation of moisture droplets and ice crystals that are suspended in air.
- They are great enough in volume and density to be visible to the naked human eye.
- Each cloud particle’s diameter ranges from 20 to 50 mm. It is formed around a solid matter called condensation nucleus.
- They vary from sea level to 13,700 meters.
Classification of Clouds Based on Altitude and Shape
3 shape division
| //5466’/ |
|
| cumuliform |
|
| Cirriform |
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Altitudinal Divisions and Classification of Clouds
| LOW | |
| Stratocumulus |
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| Nimbostratus |
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| Cumulus |
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| Cumulonimbus |
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| Stratus |
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| MEDIUM- 2 TO 6 KM | |
| Altocumulus |
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| Alto-stratus |
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| HIGH ALTITUDE- ABOVE 6 KM | |
| Cirrus |
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| Cirrostratus |
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| Cirro cumulus |
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Dew
- It is the condensation of water vapor on a cold surface that causes formation of water droplets.
- Condition => Clear Sky, calm air, high relative humidity temperature is above freezing point, long and cold nights.
- Dew point should be above freezing point.
White frost
- When under dew forming conditions, the dew point of the air is below or at 0º C, water vapor condenses as minute ice. This is called white frost.
Fog
- Fog is ground level cloud reducing horizontal visibility to less than 1km.
- It consists of very small water droplets in suspension in the lower layer of the atmosphere.
- Depending on the temperature, the water may be frozen which would result in freezing fog.
- Fog is a real danger for general aviation pilots.
Types of Fog and Their Formation Processes
Radiation fog
- When the ground cools rapidly due to radiation and the adjacent air becomes too cool, its water vapor condenses.
- Such fog is not very thick.
Advection fog
- When moist warm air moves horizontally over a cold surface.
- Such fogs are thick and persistent.
Frontal fog
- Condensation and precipitation take place when warm air mass is forced to rise over the cold air mass and cools down.
- If the cold air below is near the dew point, its temperature falls further and excess moisture condenses as fog.
- It is formed at the convergence zone.
Upslope Fog
- This fog forms adiabatically.
- Moist winds up glides while blowing toward a mountain and this causes the air to rise and cool.
- The cooling of the air from rising causes it to meet up with the dew point temperature.
- Fog forms on top of the mountains.
Valley Fog
- Valley fog forms in the valley when the soil is moist from previous rainfall.
- As the skies clear, solar energy exits earth and allows the temperature to cool near to dew point.
- This forms a deep and dense fog.
Ocean current
- At the meeting point of cold current and warm current.
Mist
- Mist is a phenomenon consisting of a large amount of water droplets/ice crystals present in a layer of the atmosphere.
- In mist, each nuclei contains a thicker layer of moisture.
- Fogs are drier than mist and they are prevalent where warm currents of air come in contact with cold currents.
- Relative humidity is generally between 60% and 100%.
- It contains more moisture than fog.
- Mist does not represent a real danger for commercial aviation pilots (visibility is between 1 km and 5 km).
- Mists are frequent over mountains as the rising warm air up the slopes meets a cold surface.
Haze
- Contrary to fog and mist, haze is a horizontal visibility reduction due to non-aqueous particles.
- Particles can be dust, sand grains, pollen grains, chemical pollution, etc.
- These particles are invisible to the naked eye, but sufficient to give the air an opalescent appearance.
- There is no condensation in the haze. Smog is similar to haze but with condensation.
Extra Knowledge
Primary and Secondary Air Pollutants
Primary pollutants
- Primary pollutants are emitted directly from the source.
- They are found in the 9 atmospheres in the form they are emitted.
- Ash, smoke, dust, oxides of carbon, sulphur and nitrogen are primary pollutants.
Secondary pollutants
- Secondary pollutants are not emitted directly from the source but are formed due to chemical reactions.
- They are found as products of chemical reaction between the atmospheric constituents and primary pollutants.
- SO3.03 ketones and hydrogen cyanide are secondary pollutants.
Types and Effects of Smog: Sulfurous and Photochemical Smog
Smog is a kind of air pollution, originally named for the mixture of smoke and fog in the air.
Types of Smog
- Sulfurous smog or “London smog”
- Sulfurous smog is the result of a high concentration of sulfur oxides in the atmosphere.
- This is usually caused by the burning of fossil fuels like coal.
- It is intensified by dampness and a high concentration of suspended particulate matter in the air.
- Photochemical smog or “Los Angeles smog” or “Summer smog”
- Photochemical smog is created when sunlight reacts with nitrogen oxides (PP) and at least one volatile organic compound (VOC, a PP) in the atmosphere.
- Nitrogen oxides are emitted in the atmosphere from automobiles, power plants, factory emissions.
- Volatile organic compounds are released in the atmosphere due to paints, gasoline and cleaning solvents.
- Occurs most prominently in urban areas or the places having large numbers of automobiles (Nitrogen oxides are the primary emissions).
- This kind of smog requires neither smoke nor fog.
- This Ozone forms near the earth’s surface and causes several ill effects in comparison of stratospheric Ozone
Effect on Visibility = Mist > Haze > Fog > Smog
Temperature Inversion
Temperature Inversion: Reversal of Tropospheric Temperature Gradient
- It is a phenomenon in which the normal behavior of temperature in the troposphere gets reversed. There is a cooler air mass near the ground and warmer air at higher altitudes. (Temperature usually decreases with altitude under the normal conditions).
- This happens when earth’s surface is able to radiate solar energy directly into space.
- Negative Lapse Rate = Increase in temperature with increasing altitude.
Exploring the Varieties of Temperature Inversions in Atmospheric Dynamics
Non-advectional Inversion
- Ground or surface inversion or radiation inversion.
- Upper air inversion.
Advectional Inversion
- Frontal inversion or cyclonic inversion.
- Valley inversion due to vertical air movement.
- Surface inversion due to horizontal air movement.
Mechanical Inversion
- Subsidence inversion.
- Turbulence and convective inversion.
Non-advectional Inversion
Ground Surface Temperature Inversion: Radiation Phenomenon
- Radiation inversion occurs near the earth’s surface due to the radiation mechanism.
- It is non-advectional as there is no movement of air either vertical or horizontal.
- It requires some necessary conditions like
- Long cold winter nights.
- Cloudless and clear sky.
- Presence of dry air near the surface.
- Slow movement of air to avoid mixing.
- Snow covered ground surface.
- Air coming in contact with the cool ground surface also becomes cold while the air layer lying above is relatively warm.
The heat of the day is radiated off during the night by the earth. By early morning, the air near the surface becomes cool due to conduction and settles. The air above it remains warm as convection currents are not possible. Over polar areas, temperature inversion is normal throughout the year.
Dew Formation Due to Temperature Inversion: Mechanism and Effects
- Temperature inversion results in a cooler surface of earth than the above air.
- Moisture laden air comes into contact with the cold surface and releases heat.
- At a certain point, the release of heat becomes unable to further reduce the temperature of air due to which phase change occurs.
- The change of phase causes condensation that results in dew formation leading to low visibility.
Types and Characteristics of Upper Air Inversion: Thermal Upper Air Inversion
Upper air inversion is of two types
Thermal upper air inversion
- This warming of the stratosphere with altitude.
- It is caused due to the absorption of solar energy by ozone.
- The temperature of this layer becomes much higher than the air layers lying above and below the ozone layer.
- Occurs only when there is no vertical movement of air (either ascent or descent of air).
- This creates stability condition hence discouraging rainfall.
Mechanical upper air inversion
- At higher heights in the atmosphere due to subsidence of air.
- This inversion also relates to anti-cyclones.
- Inversion results when the upper layer of air moves down during an initial anti-cyclone.
Advectional Inversion: Characteristics and Causes
- Also called as dynamic inversion because it is always caused due to either horizontal or vertical movements of air.
- Strong wind movement and unstable conditions of the atmosphere are prerequisite conditions for advectional inversion of temperature.
Frontal Inversion: Formation and Characteristics
- It is caused in the temperate zones due to temperate cyclones.
- Temperate Cyclones = Formed due to the convergence of warm westerlies and cold polar winds in the northern hemisphere.
- The warm air is pushed up by the cold polar air and thus the warm air overlies the cold air because it is lighter than the cold air.
- The existence of warm air above and cold air below reverses the normal lapse rate and inversion of temperature occurs.
- It is important to note that air moisture increases upward in frontal inversion of temperature while it decreases upward in other types of temperature inversion.
Surface Temperature Inversion: Causes and Impacts
- It is caused by horizontal movement of air occurs in several situations.
- Such inversion is caused when warm air invades the area of cold air or cold air moves into the area of warm air.
- Warm air being lighter is pushed upward by relatively denser cold air.
- When the warm air moves, such inversion is caused over the continents during winter and over the oceans during summer.
- When the cold air becomes active and invades the areas of warm air, such inversion occurs over the continents during summer and over the oceans during winter.
- Such surface inversion occurs generally in the low latitudes.
- The convergence of cold and warm ocean currents also causes such inversion of temperature.
Valley Inversion and Its Impact on Local Climate
- Inversion takes place in hills and mountains due to katabatically air drainage.
- A katabatic wind (or fall/downslope/gravity wind) is the technical name for a drainage wind.
- Katabatic wind carries high-density air from a higher elevation down a slope under the force of gravity.
- The temperature of the upper parts of the mountains becomes cold because of rapid rate of loss of heat whereas the valley temperature does not reduce because of slow loss of heat.
- The warmer valley air ascends and the cooler air descends.
- This situation is responsible for severe frost in the valley floors leading great damage to fruit orchards and agricultural crops whereas the upper parts of the valleys are free from frost.
- This is why the valley floors are avoided for human settlements while the upper parts are inhabited in the mountainous valleys of middle latitudes.
Theories of Precipitation
Precipitation Theories: Collision-Coalescence and Ice Crystal Hypotheses
- Collision-Coalescence hypothesis of precipitation: This explains precipitation in tropical areas where the temperature in clouds is too high for the formation of ice. So water droplets condense, positive charge attracts negative charge, they come together, become big and fall.
- Ice crystal hypothesis / Bergeron-Findesein hypothesis: Saturation vapor pressure is lower over ice than over water surface. Initially a cloud may contain both ice and water. Since vapor pressure is lower over ice, it attracts more water vapor in the cloud. Thus the vapor present in the cloud begins to decrease and the water droplets evaporate to replenish the diminishing vapor. So ice crystals grow at the expense of water droplets. As they descend, they may melt and form as rain or snow.
Types of Precipitation: Rain, Drizzle, Virga, Sleet, and Hail
Rain
- This is the wet stuff that nourishes plants and for which umbrellas were invented.
- It occurs when both the cloud temperature and ground temperature are above freezing.
- It can take three forms.
- Simple rain = When the drops are about 0.5 mm (0.02 in) in diameter.
- Drizzle = When the drops are smaller than Simple Rain.
- Virga = When the drops are so small they don’t reach the ground (Evaporated).
Sleet
- They are refrozen ice.
Hail
- Precipitation in the form of hard round pellets.
- Strong ascending currents take water vapor to great heights where it condenses and precipitates as snow.
- As it comes down, it melts but strong currents push them up again increasing the size. Thus size keeps on increasing until it becomes very hard and big.
Classification of Rainfall: Convectional, Orographic (Relief), and Cyclonic (Frontal)
Rainfall has been classified into three main types based on their origin
- Convectional Rainfall
- Orographic or Relief Rainfall
- Cyclonic or Frontal Rainfall
Convectional Rainfall
- Occurs mostly in tropics where it is hot.
- Air naturally rises up in convection current when it heats up.
- It cools and condenses due to the expansion while moving higher altitude leading cumulus clouds formation.
- Heavy rainfall with lightning and thunder takes place which does not last long.
- If the air is hot enough, it rises very quickly and can cause thunderstorms.
- Such rain is usual in the summer or in the hotter session of the day.
- This can happen over land or water as long as moisture is present.
- When it happens over tropical oceans (where the air is saturated with water), the combination of wind and moisture can create a tropical cyclone or hurricane.
Orographic or Relief Rainfall
- Relief rainfall occurs very frequently near mountains beside the sea.
- The moisture-laden air is forced to rise on encountering a mountain range. As it rises upwards, it is cooled and cloud is formed.
- The cloud becomes saturated with water vapor and it begins to precipitate on the side of the mountain facing the sea (known as windward side)
- The cloud precipitates the most on the windward side of the mountain.
- The cloud becomes almost exhausted by the time they reach another side (known as leeward side) so it rains very little there.
- This makes leeward sides of a mountain very sheltered from rain and they hardly ever get much rain.
Cyclonic or Frontal Rainfall
- Frontal rainfall occurs when warm air is forced to rise over cold air.
- The moisture in the warm air condenses as it cools which causes clouds and rain.
Atmospheric Circulation
Atmospheric Pressure and Its Characteristics
- The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure.
- It is expressed in units of milibar (mb).
- Due to gravity the air at the surface is denser and hence has higher pressure.
- Wind = It is horizontal movement of air which flows from high pressure areas to low pressure areas.
- Air current = The vertical or nearly vertical movement of air is called air current.
Air Pressure at ground surface
- The atmospheric pressure = 1013.25 mb = 76 cm of Mercury (Hg) column.
- It normally falls at a rate of 34 mb per 300 meters of ascent.
Vertical Variation of Pressure in the Atmosphere
- The pressure decreases rapidly with height in the lower atmosphere.
- It does not always decrease at the same rate due to the variations in the factors controlling air density (temperature, amount of water vapor and gravity).
- The vertical pressure gradient force is much larger than that of the horizontal pressure gradient.
- We do not experience these strong upward winds as they are generally balanced by a nearly equal but opposite gravitational force.
- A rising pressure indicates stable weather whereas a falling pressure indicates cloudy and unstable weather.
Horizontal Distribution of Pressure and Isobars
- It is studied by drawing isobars at constant levels.
- Isobars are lines connecting places having equal pressure after being reduced to sea level.
- Low pressure system is enclosed by one or more isobars with the lowest pressure in the centre.
- High pressure system is enclosed by one or more isobars with the highest pressure in the centre.
Pressure and wind
Important Laws of atmospheric circulation
- Buys Ballot Law: If you stand with your back to the wind in the Northern Hemisphere, air pressure is lower on your left than on your right.
- Winds are strong where isobars are crowded and weak where they are spread.
- Pressure distribution affects wind speed in high and mid latitudes. Between 10 N and 10 S, it is difficult to relate winds to pressure distribution.
- Near Earth’s surface, wind direction is influenced by surface features.
- Maximum speed of wind at noon and minimum just before sunrise.
- Winds are named after the direction they come from.
- The wind circulation around a low pressure is called cyclonic circulation. Around a high pressure it is called anticyclonic circulation.
- Generally, over a low pressure area the air will converge and rise. Over high pressure areas the air will subside from above and diverge at the surface.
Forces Affecting Wind Velocity and Direction: Coriolis, Frictional, and Pressure Gradient Forces
- Horizontal winds near the earth surface respond to the combined effect of three forces in addition with downward gravitational force
- Frictional Force
- Coriolis Force
- Pressure Gradient Force (PGF)
Coriolis Force
It is given by the formula 2vw sinX (v = Wind velocity; w = Earth’s particular point angular speed, X = Angle of latitude)
- It is not a force, but an effect causes due to rotation of the earth.
- It turns the object to right or clockwise in the Northern Hemisphere and to the left or anti-clockwise in the Southern Hemisphere.
- It affects wind direction and not the speed.
- Higher the wind speed greater is the coriolis effect.
- Maximum at poles as poles rotate slow and becomes zero at the equator.
- It always acts at right angle to the direction of the wind.
Frictional Force
- Friction is the resistance to motion of one object moving relative to another.
- The friction force drags the wind as it moves across surfaces.
- As the surface friction decrease wind speed, it reduces the effect of Coriolis force.
Pressure Gradient Force (PGF)
- The rate of change of pressure with respect to distance is the pressure gradient.
- Pressure Gradient is denoted by the spacing of isobars that expresses the rate and direction of pressure changes
- Close spacing = Steep or strong pressure gradient
- Wide spacing = Weak gradient
- PGF is produced by the differences in atmospheric pressure.
- It operates from the high pressure area to a low pressure area.
- The Pressure Gradient Force acts perpendicular to the Coriolis force and to an isobar.
- The higher the pressure gradient force, the more is the velocity of the wind and the larger is the deflection in the direction of wind.
Geostrophic Wind and Pressure Belts
- Geostrophic winds come about because pressure gradient force and Coriolis force come into balance after the air begins to move.
- Under the influence of both the Pressure Gradient Force and Coriolis Force, air tends to move parallel to isobars in conditions where friction is low (1000 meters above the surface of the Earth) and isobars are straight.
- At the surface level wind blows at an angle, but above it becomes parallel to isobars.
Pressure Belts
- There are distinctly identifiable zones of homogeneous horizontal pressure regimes or ‘pressure belts’.
- On the earth’s surface, there are in all seven pressure belts.
- equatorial low
- 2 sub-tropical highs
- 2 sub-polar lows
- 2 polar highs
Two Main Factors Controlling Pressure Systems: Thermal and Dynamic Factors in Pressure Systems
Thermal Factors
- Heating and cooling of air causes expansion (density decreases hence pressure reduces) and contraction (density increases hence pressure increases) respectively.
Dynamic Factors
- Arising out of Pressure Gradient Forces and rotation of the earth (Coriolis Force).
Equatorial Low Pressure Belt: The Doldrums and ITCZ
- These winds are roughly in between 5° N and S.
- The belt is generally known as doldrums (zone of calm and weak winds).
- Doldrums = Characterized by convergence, rising air, and heavy rainfall
- This area is called the Inter Tropical Convergence Zone (ITCZ) or the thermal equator or the “equatorial belt of variable winds and calms”.
- The trade winds converge in the equatorial trough (or tropical low).
The Dynamics of Weather in the Equatorial Low Pressure Belt
- A very moist air heated by the sun tends to expand and rise creating the area of low pressure.
- Due to the convergence of trade winds, only vertical current creates and the moisture laden air rises upward. This forms cumulonimbus clouds leading to thunderstorms.
- This region coincides with the world’s latitudinal belt of heaviest precipitation and most persistent cloud cover.
- Old sailing ships often remained becalmed in the doldrums for days at a time.
Sub-Tropical High Pressure Belt: Exploring the Horse Latitudes
- Areas of sinking and settling air from higher altitudes.
- Winds blow poleward to become the westerlies and equator-ward as the trade winds.
- These areas located between latitudes 25° N and S.
- Often called the subtropical belts of variable winds, or the “horse latitudes.”
- Name comes from the occasional need by the Spanish sea captains to throw their horses overboard in order to conserve drinking water and lighten the weight when their ships were becalmed in these latitudes.
- The subtropical highs are areas like the doldrums in which there are no strong prevailing winds.
Clear Skies and Arid Conditions: Sub-Tropical High Pressure Belt Characteristics
- Weather conditions are typically clear, sunny, and rainless, especially over the eastern portions of the oceans where the high pressure cells are strongest.
- As the subsiding air is warm and dry, most of the deserts are present along this belt.
- Tropical and extra-tropical disturbances are frequent in this belt.
Formation of the Sub-Tropical High Pressure Belt: Coriolis Effect and Air Descent
- The warm air rises from the low pressure equator and starts cooling. It begins to move towards poles after reaching the upper layers. It further cools down, becomes dense and by 25-35º latitude it begins to subside.
- Due to Coriolis Effect, the movement of air becomes effectively west to east instead of going north in these latitudes. This produces a blocking effect and the dense air begins to subside
- Hence, sub-tropical high belt is dynamically produced Pressure Belt due to
- Coriolis Force (Produced by rotation of the earth on its axis).
- Descent of air (due to the convergence of Trade winds and Westerlies).
Exploring the Sub-Polar Low Pressure Belt: Dynamics and Jet Stream Formation
- Located between 45° N&S latitudes and the Arctic and the Antarctic circles (66.5° N and S latitudes)
- This is dynamically produced Pressure Beltdue to
- Coriolis Force (Produced by rotation of the earth on its axis).
- Ascent of air (due to the convergence of Westerlies and polar easterlies).
- Polar Jet Streams are formed due to the contrasting areas between cold and warm air masses.
Emperate Cyclones: Weather Phenomena in the Sub-Polar Low Pressure Belt
- Temperate cyclones are produced in this region due to a great contrast between the temperatures of the winds from sub-tropical and polar source regions.
Polar High Pressure Belt: Atmospheric Dynamics at the Poles
- These are small areas that extend around the poles (lie around poles between 80°-90° North and South latitudes).
- The saturated dry air from the sub-polar low pressure belts becomes cold while moving towards poles through the upper troposphere. This air subsides and diverge near the pole creating a high pressure belt at the surface of earth.
- The lowest temperatures are found over the poles.
Seasonal Shift of Pressure Belts: Hemisphere Variances and Latitudinal Effects
- The shift is less in the Southern hemisphere due to abundant water.
- The shift of the pressure belts is also higher in lower latitudes than in higher ones.
- The ITCZ can shift about 20º N and only 10º S of the equator.
Global Distribution of Sea Level Pressure: Influence of Land-Ocean Contrasts on Seasonal Variations
- The continents and oceans distribution influence the distribution of pressure.
- In winter, the continents are cooler than the oceans causing development of high pressure (reverse with the oceans).
- In summer, continents are relatively warmer causing development of low pressure (reverse with the oceans).
July: Seasonal Pressure Distribution and Equatorial Shifts
- The equatorial low pressure belt shifts towards the north (Apparent northward movement of the sun). This shift is maximum in Asia.
- The landmasses of the northern hemisphere become excessively hot and low pressure areas develop over them.
- The sub-tropical high pressure belt of the southern hemisphere extends continuously. In contrast, it is broken over the continents and remains confined to the North Atlantic and North Pacific Oceans in the northern hemisphere.
- Sub-polar low is deep and continuous in the southern hemisphere, while there is only a faint oceanic low in the northern hemisphere.
January: Seasonal Pressure Distribution and Equatorial Shifts
- The equatorial low pressure belt shifts a little south of its mean equatorial position (due to the apparent southward movement of the sun).
- The lowest pressure pockets occur on the land masses of Southern Hemisphere (because land masses become much hotter than the adjoining oceans).
- Sub-tropical high pressure belt of the southern hemisphere is broken over the continents and remains confined to the oceans only.
Models of Atmospheric Circulation: Headley’s and Ferrel’s Perspectives
Headley’s Model
- His model assumed only one cell in each hemisphere.
- Low pressure at equator and high pressure at pole with air from pole flowing towards equator.
- It assumed a non-rotating earth and uniform earth surface.
Ferrel’s Model
- The cell between equator low pressure belt and sub-tropical highs.
- The one between sub-tropical high and sub-polar low is called Ferrel cell and the one between sub-polar low and poles is called polar cell.
- It assumes a rotating earth, uniform surface (i.e. either land or water throughout) and sun being stationary overhead at equator.
Pressure Cells
The Hadley Cell: Dynamics of Equatorial Circulation
Occurs between ITCZ and 30° N&S
- Ground is intensely heated by the sun. This leads to the rise of air which creates a low-pressure zone on the Earth’s surface.
- The air separates and starts moving towards poles in both north and south hemispheres.
- The air cools and sinks towards the ground after reaching about 30° north and south forming the subtropical high-pressure zone.
- The sinking air becomes warmer and drier. This creates a region of little cloud and low rainfall (where deserts are found).
- The air completes the cycle by flowing back to the equator as the trade winds.
The Ferrel Cell: Mid-Latitude Atmospheric Dynamics
Occurs between 30° to 60° N and S
- Air on the surface is pulled towards the poles forming
- warm south-westerly winds in the northern hemisphere
- north-westerly winds in the southern hemisphere
- These winds gain moisture while travelling through the oceans.
- They meet cold air (drifting from the poles) at around 60° N& S.
- Due to the relative light weight of warm air mass from the tropics in comparison to cold air mass, it rises as the two air masses meet. This air upliftment causes low pressure at the surface.
- The unstable weather conditions are associated with this mid-latitude depression.
The Polar Cell: Polar Atmospheric Circulation
Occurs between 60° N and S to the pole.
- At the poles, air is cooled and sinks towards the ground forming high pressure known as the Polar high. It then flows towards the lower latitudes.
- At about 60° degrees N&S, the cold polar air mixes with warmer tropical air and rises upwards, creating a zone of low pressure called the sub-polar low.
- The boundary between the warm and cold air is called the polar front.
- It accounts for a great deal of the unstable weather experienced in these latitudes.
El Niño Southern Oscillation (ENSO) and Walker Circulation: Normal Year Conditions
Normal year condition and the Horizontal Pressure Belt (Walker Cell)
- The easterly trade winds move water and warmed air towards the west.
- The western side of the equatorial Pacific is characterized by low pressure weather with warm and wet air.
- The Walker circulation leads to movement of warm and wet air from western side of the equatorial Pacific to the Eastern side of the Equatorial Pacific.
- The oceanic cycle develops below the water surface.
- Warm water starts moving from Western side of the Equatorial Pacific to the Eastern side of the Equatorial Pacific.
- Cold water upwelling brings nutrients to the surface at Peru which helps in Plankton development and pisciculture.
- This is how water and air are returned to the east. Both are now much cooler, and the air is much drier.
El Niño Phenomenon: Oceanic and Atmospheric Anomalies
- El Nino means The Little Boy, or Christ Child in Spanish.
- El Nino is an Oceanic and Atmospheric phenomenon that leads to reversal of normal year conditions by unusual warming of water in the Peru coast.
- Prevailing conditions
- Warm water as well as low pressure condition develops in the Eastern Pacific (Peru)
- Cold conditions as well as high pressure in Western Pacific (Australia).
- Due to the inverse relationship (increase of one causes decrease in another) between Pressure and amount of rainfall, El Nino creates drought situations in Australia and South East Asia.
La Niña Phenomenon: Intensification of Normal Climate Conditions
- La Nina means The Little Girl in Spanish.
- It is a climate pattern that intensifies the normal year conditions.
- It creates a cooling effect on surface ocean waters along the tropical west coast of South America.
- Effect of La Nina year on winter temperatures
- Warmer than normal in the Southeast
- Cooler than normal in the Northwest
El Niño-Southern Oscillation (ENSO): Understanding the Dynamics of Climate Variability
- The El-Nino event is closely associated with the pressure variations in the Eastern and Western Pacific. This change in pressure condition over the Pacific is known as the southern oscillation.
- The combined phenomenon of southern oscillation and El Nino is known as ENSO.
- Only El-Nino = Warm water in Eastern Pacific + Cold water in Western Pacific
- Only SO = Low Pressure over Eastern Pacific + High Pressure over Western Pacific
- ENSO = Warm water and Low Pressure near Eastern Pacific + Cold water and High Pressure near Western Pacific
- El Nino and La Nina are opposite phases of what is known as the El Nino-Southern Oscillation (ENSO) cycle.
- La-Nina is sometimes referred to as the cold phase of ENSO and El-Nino as the warm phase of ENSO.
- The ENSO cycle is a scientific term that describes the fluctuations in temperature between the ocean and atmosphere in the east-central Equatorial Pacific.
- This deviation from normal surface temperatures causes large-scale impacts not only on ocean processes, but also on global weather and climate.
- While their frequency can be quite irregular, El-Nino and La-Nina events occur on average every two to seven years. Typically, El-Nino occurs more frequently than La-Nina.
Wind Types
Primary Winds or Prevailing Winds or Permanent Winds or Planetary Winds
- Prevailing as they prevail throughout the year.
- Planetary because they are almost global in nature.
Trade Winds: Steady Winds of the Subtropical Highs
- Occur is in the vicinity of the subtropical highs.
- Can be identified between latitudes 5° and 25° North and South latitudes.
- On Earth’s surface, it blows out of the subtropical highs toward the equatorial trough in both the Northern and Southern Hemispheres
- Because of the Coriolis effect, the
- Northern trades move in a clockwise direction out of the northeast.
- Southern trades move in a counterclockwise direction out of the southeast.
- Also known as the tropical easterlies (Because the trades tend to blow out of the east)
- It tends to be constant, steady winds, consistent in their direction. This is most true when they cross the eastern sides of the oceans (near the eastern portion of the subtropical high).
- The area of the trades varies during the solar year. It moves north and south a few degrees of latitude with the sun.
- The weather of the trades is clear and dry near their source in the subtropical highs, but the trades have a high potential for stormy weather after crossing large expanses of ocean.
- Early Spanish sea captains depended on the northeast trade winds to drive their galleons to destinations in Central and South America in search of gold, spices, and new lands.
- Going eastward toward home, navigators usually tried to plot a course using the westerlies to the north.
Westerlies: Prevailing Winds of the Mid-Latitudes
- Occur between about 35° and 65° North and South latitudes.
- Winds flow poleward out of the subtropical high pressure cells deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
- Northern Hemisphere = Blow from the southwest
- Southern Hemisphere = Blow out of the northwest
- Tend to be less consistent in direction than the trades.
- Usually stronger winds may be associated with stormy weather.
- Westerlies of the Southern hemisphere are stronger (known as Roaring forties, Furious fifties, and Screaming sixties) and more consistent in direction due to predominance of water.
- The westerlies attain their greatest consistency and strength in the Southern Hemisphere due to the less land than in the Northern Hemisphere.
Polar Easterlies: Cold Winds from the Poles
- Dry, cold prevailing winds that blow from the high-pressure areas of the polar highs at the North and South Poles towards low-pressure areas.
- Cold air subsides at the poles creating the high pressure, forcing an equator-ward outflow of air that deflects westward by the Coriolis Effect.
- There are extremely cold winds as they blow from the Tundra and Icecap regions.
- More regular in the southern hemisphere than in the northern hemisphere.
- Unlike the westerlies in the middle latitudes, the polar easterlies are often weak and irregular.
Local Winds
- Differences in the heating and cooling of earth surfaces creates local differences of temperature and pressure. This develops daily or annual cycles that can create several common, local or regional winds.
| Name | Nature of wind | Place |
| Chinook (Snow eaters) | Hot, dry wind | The Rockies mountains |
| Foehn | Hot, dry wind | The Alps |
| Khamsin | Hot, dry wind | Egypt |
| Siroco | Hot, moist wind | Sahara to the Mediterranean Sea |
| Solano | Hot, moist wind | Sahara to the Iberian Peninsula |
| Harmattan (Guinea Doctor) | Hot, dry wind | West Africa |
| Bora | Cold, dry wind | Blows from Hungary to North Italy |
| Mistral | Cold wind | The Alps and France |
| Punas | Cold dry wind | Western side of Andes Mountain |
| Blizzard | Cold wind | Tundra region |
| Purga | Cold wind | Russia |
| Levanter | Cold wind | Spain |
| Norwester | Hot wind | New Zealand |
| Santa Ana | Hot wind | South California |
| Karaburun (black storm) | Hot dusty wind | Central Asia |
| Calima | Dust-laden dry wind | Saharan Air Layer across the Canary Islands |
| Elephanta | Moist wind in monsoon | Malabar coast |
Periodic Winds: Sea and Land Breezes
Sea and Land Breeze
Due to differential heating of land surface and sea water.
Sea Breeze
- Day time = Land gets heated à Warm air rises up à Low pressure develops
- Sea being less warm à High pressure develops at sea à Winds blow from sea to land causing sea breeze
Land Breeze
- Night time = Land cools faster than sea à High pressure over land (Low pressure over ocean)
- Winds blow from land towards sea.
Diurnal Mountain Wind Systems: Katabatic and Anabatic Winds
Katabatic Wind (Mountain Breeze)
- During the night, the slopes get cooled and the dense air descends into the valley as the mountain wind.
- This cool air of the high plateaus and ice fields draining into the valley is called Katabatic wind.
Anabatic Wind (Valley Breeze)
- In mountainous regions, the slopes get heated up during the day and air moves upslope.
- The air from the valley blows up the valley to fill the resulting gap.
- This air flow travelling up on an orographic surface is known as anabatic wind.
Seasonal Wind Patterns: Monsoons in South and Southeast Asia
- The pattern of wind circulation is modified in different seasons due to the shifting of regions of maximum heating, pressure and wind belts.
- The most pronounced effect of such a shift is noticed in the monsoons, especially over Southeast Asia.
Monsoon
- A seasonal prevailing wind in the region of South and South-East Asia.
- It arises due to a difference in temperatures between a land mass and the adjacent ocean.
- It blows from the south-west between May and September and brings rain (the wet monsoon), or from the north-east between October and April (the dry monsoon).
- The rainy season in SE Asia accompanied the wet monsoon.
- The winds reverse again at the end of the monsoon season.
Air Masses: Homogeneous Atmospheric Entities
Air mass
- It is a large mass of air that has similar characteristics of temperature and humidity within it along with little horizontal variations.
- It forms an integral part of the global planetary wind system.
Source Regions: Origins of Air Mass Characteristics
- The area (land or water) above which air mass lies and acquires its characteristics.
- Air mass picks up the distinct temperature and humidity characteristics of the region over which it sits for several days.
- Ideal Source Regions = High pressure areas with little pressure difference (pressure gradient)
- subtropics (the source for tropical air masses)
- poles (the source for polar air masses)
- Mid-latitudes have no major source regions due to the dominance of cyclonic and other disturbances.
Geographical Classification of Air Masses: Characteristics and Origins
- They are classified based on the source region and air mass modification.
- Types of air masses are recognized:
- Maritime tropical (mT) = Warm tropical and subtropical oceans
- Warm, humid and unstable
- Weather
- Winter = Mild temperatures, overcast skies with fog
- Summer = High temperatures and humidity, cumulous clouds, and convectional rainfall.
- Maritime tropical (mT) = Warm tropical and subtropical oceans
- Continental tropical (cT) = The subtropical hot deserts
- Dry, hot and stable
- Do not extend beyond the source
- Dry throughout the year
- Maritime polar (mP) = The relatively cold high latitude oceans
- Cool, moist and unstable
- These are the regions which cannot lie stagnant for long.
- Weather
- Winters = High humidity, overcast skies, and occasional fog and precipitation.
- Summer = Clear and stable
- Continental polar (cP) = The very cold snow covered continents in high latitudes
- Dry, cold and stable conditions
- Weather
- Winter = Frigid, clear, and stable
- Summer = less stable
- Continental arctic (cA) = Permanently ice covered continents in the Arctic and Antarctica
- Tropical air masses are warm and polar air masses are cold.
Thermodynamic Modification of Air Masses: Influence of Surface Heating and Cooling
- When the air mass is heated or cooled from the surface below, it is a thermodynamic change.
- A warm air moves over a cold surface leads to temperature inversion. It inhibits further vertical cooling.
- A cold air mass moving over a warm surface creates convection currents. This leads to formation of vertical clouds (cumulus) and air turbulence.
- Addition or loss of latent heat also is an example of thermodynamic modification.
Dynamic Modification of Air Masses: Influences Beyond Surface Heating or Cooling
- These modifications are independent of surface heating or cooling.
- Examples are subsidence caused by anti-cyclones or cyclones.
- Surface friction adds to the turbulence of air flow aiding the upward transfer of the effect of thermodynamic modifications.
Fronts and Frontogenesis: Dynamics of Air Mass Boundaries
- Front is that slopping boundary which separates two opposing air masses having contrasting characteristics.
- The frontal activities are invariably associated with cloudiness and precipitation due to the ascent of warm air which cools down adiabatically, condenses and causes rainfall.
- Frontal zone is neither parallel nor vertical to the ground surface, rather it is inclined at a low angle.
- The intensity of precipitation depends on the slope of ascent and amount of water vapor present in ascending air.
- Frontogenesis: The process associated with creation of new fronts or the regeneration of decaying fronts already in existence.
- Requires certain necessary conditions:
- Temperature Difference
- Opposite directions of Air Masses
- Frontolysis: The process of destruction or dying of existing fronts.
- Requires certain necessary conditions:
Cold Fronts: Dynamics and Weather Characteristics
- When the cold air moves towards the warm air mass, its contact zone is called the cold front.
- As the cold front nears your region, the barometer falls.
- The cold air behind the front wedges under the warm air and lifts it sharply off the ground.
- Large cumulonimbus clouds appear (often bring thunderstorms and rain showers).
- As the cold front passes, the wind changes direction.
- The weather becomes clear and colder and the barometer rises again.
- Cold front moves up at about double the speed than warm fronts.
Warm Fronts: Characteristics and Weather Effects
- If the warm air mass moves towards the cold air mass, the contact zone is a warm front.
- The warm air behind the front rises up over the cold air.
- The barometer falls leading to a long, steady rain.
- The front passes gradually and the sky clears.
- As the warm air moves up the slope, it condenses and causes precipitation.
- Unlike a cold front, the changes in temperature and wind direction are gradual.
- Such fronts bring moderate to gentle precipitation over a large area for several hours.
- Cirrostratus clouds ahead of the warm front create a halo around the sun and moon.
Occluded Fronts: Weather Phenomena and Formation
- It results when a cold air front overtakes a warm front and lifts the warm air mass completely off the ground.
- Steady rain falls at an occluded front.
- The occluded front causes complex weather – a mix of cold and warm front type weather. These fronts are common in west Europe.
- A combination of clouds formed at cold front and warm front.
- The formation of Mid-latitude cyclones involve the formation of an occluded front.
Stationary Fronts: Characteristics and Behavior
- A stationary front forms when a cold front or warm front stops moving.
- The surface position of a front does not change.
- This happens when two masses of air are pushing against each other but neither is powerful enough to move the other.
- Winds blowing parallel to the front instead of perpendicular can help it stay in place.
Jet Stream: High-Speed Upper Atmospheric Wind Patterns
- The Jet Stream is a geostrophic wind which meanders with high velocity in the upper layers of the troposphere and encircles the globe.
- The meandering or the whirl movement of the Jet Stream is called ‘Rossby Wave’. (We will read about Rossby wave ahead)
- Reason that causes high velocity
- Low friction at upper troposphere due to less air density.
- Higher air temperature difference enhances speed (The Jet stream has high velocity in winter in comparison to summer).
- These slim strips of strong winds are like rivers of wind high above in the atmosphere.
- Generally, blow from west to east near the tropopause at very high speeds (120 kmph in winters and 50 kmph in summers). That is why it is also referred to as westerlies or upper level westerlies.
- Polar jet streams flows 6-9 km above the ground.
- It flows from the temperate region towards the polar region and gets deflected right in the northern hemisphere and left in the southern hemisphere due to the Coriolis Effect.
- Sub-tropical jet streams flows 10-16 km above the ground.
- It flows from sub-tropical region towards temperate regions and gets deflected right in the northern hemisphere and left in the southern hemisphere due to the Coriolis Effect.
Sub-Tropical Jet Stream: Dynamics and Characteristics
- They prevail over the lower latitudes of westerlies.
- It is produced by the rotation of earth and its spherical shape (dynamically induced).
- The air over the equator has the highest velocity.
- As it rises and moves towards north, it has a higher velocity than the air at lower altitude prevailing at the same latitude. So it begins to flow from west to east around 30º latitude.
- It flows all-round the year.
- They flow to conserve angular momentum in the upper atmosphere.
- The sub-tropical westerly jet does not seem to affect surface weather as much as the polar fronts jets do.
Mid-Latitude or Polar Front Jet Stream: Dynamics and Seasonal Variations
- It is more variable and is produced by a temperature difference (thermally induced).
- Its position shifts towards poles in summers and towards equator in winters.
Tropical Easterly Jet (TEJ): Seasonal East-to-West High-Altitude Winds
- They are seasonal jet streams flowing east to west.
- These are found only in the northern hemisphere and generate only in the summer season.
- These are also thermally induced.
- The reason for the establishment and maintenance of the TEJ is still not clear.
- It is believed that these jets may be developing due to uniquely high temperatures and heights over the Tibetan Plateau during summer.
- The TEJ is the upper-level venting system for the strong southwest monsoon.
Weather system of mid-latitudes and high latitudes
- The weather of mid and high latitude regions is more complex than the equatorial or tropical regions.
- The heat surplus areas of equatorial or tropical regions create thermally induced weather systems.
- The higher latitudes weather systems are dynamically induced. They consist of localised and upper troposphere circulations known as Jet Streams.
- Jet Streams have a huge influence on climate as it can push air masses around and affect weather patterns.
- These differences of thermally and dynamically induced weather system create convectional and frontal rainfall system
Extra-Tropical / Middle-Latitude / Temperate Cyclones
- The system develops in the mid and high latitude (beyond the tropics).
- The passage of the front causes abrupt changes in the weather conditions over the area in the middle and high latitudes.
Formation and Dissipation of Extra-Tropical Cyclones
- Front is stationary initially.
- Warm air blows through the northern hemisphere from the south.
- Cold air blows from the north of the front
- Pressure drops along the front lead movement of the warm air northwards and the cold air southward.
- Results in counter-clockwise cyclonic circulation.
- The cyclonic circulation leads to a well-developed extra tropical cyclone (consisting of a warm front and a cold front).
- There are pockets of warm air or warm sector wedged between the forward and the rear cold air or cold sector.
- The warm air glides over the cold air. A sequence of clouds appears over the sky ahead of the warm front and causes precipitation.
- The cold front approaches the warm air from behind and pushes the warm air up. As a result, cumulus clouds develop along the cold front.
- The cold front moves faster than the warm front, ultimately overtaking the warm front.
- The warm air is completely lifted up and the front is occluded. Ultimately the cyclone dissipates.
Path of Extra-Tropical Cyclones
Distribution of Extra-Tropical Cyclones
Tropical Cyclones: Formation, Characteristics, and Impact
- Violent storms that originate and intensify over warm tropical oceans.
- It moves towards the coastal areas and causes large-scale destruction due to violent winds, very heavy rainfall and storm surges.
- This is one of the most devastating natural calamities.
- Favorable conditions for the formation and intensification tropical cyclone are:
- Large sea surface with temperature higher than 27° C.
- Presence of the Coriolis force.
- Small variations in the vertical wind speed.
- A pre-existing weak-low-pressure area or low-level-cyclonic circulation.
- Upper divergence above the sea level system.
- The place where a tropical cyclone crosses the coast is called the landfall of the cyclone.
- Cyclones that cross 20° N latitude generally recurve and they are more destructive.
- A mature tropical cyclone is characterized by the strong spirally circulating wind around the centre. This centre is called the eye.
- The diameter of the circulating system can vary between 150 and 250 km.
Cyclone Formation: Dynamics of Low Pressure Systems
- Warm, moist air over the ocean rises upward leaving less air near the surface. This causes a low air pressure area below.
- Air from surrounding areas with higher air pressure pushes into the low pressure areaand undergoes deflection due to Coriolis force creating a cyclonic vortex (spiraling air column).
- The new “cool” air becomes warm and moist and rises too and the cycle continues.
- The condensation of the rising, warmed, moist air leads to the formation of clouds.
- Heat is emitted during this process and a reaction between the moisture from the evaporation of water takes place that produces Thunderstorms.
- The whole system of clouds as well as wind spins and grows, fed by the ocean’s heat and water evaporating from the ocean surface.
Eye and Eye-wall in Cyclones: Calm Center and Violent Surroundings
- The air in the vortex is forced to form a region of calmness called an eyeat the center of the cyclone due to the centripetal acceleration.
- Higher pressure air from above flows down into the eye.
- The inner surface of the vortex forms the eye wall. It is the most violent region of the cyclone.
Regional names of Tropical Cyclone
Path of Tropical Cyclones: Factors Influencing Trajectory and Movement
- Start with a westward movement because of
- Earth rotation from west to east
- Zone of cyclone formation is under the influence of easterlies
- Turn northwards around 20° latitude and further north-eastwards around 25° latitude (Deflection towards right due to the Coriolis force).
- Eastwards turn around 30° latitude because of westerly winds.
- Loss of energy and subsidence because of
- Ocean water at 30° latitude is not warm enough to sustain a cyclone.
- Increasing wind shear due to westerlies doesn’t facilitate the formation of cyclonic vortexes.
Differences between Tropical and Extra-Tropical Cyclone
| Dimension | Tropical | Extra-Tropical | ||
| Origin | Thermal | Dynamic (Air masses movement and Coriolis Force) | ||
| Latitude | Confined to 10°-30° N&S of Equator | Confined to 35°-65° N&S of Equator | ||
| Formation | Only on seas having temperature more than 26°-27° C (On reaching the land the moisture supply is cut off and the storm dissipates) | Land as well as seas | ||
| Season | Late summers (August-October) | Irregular (Few in summers and more in winters) | ||
| Shape | Elliptical | Inverted ‘V’ | ||
| Rainfall | Heavy but does not last beyond a few hours | Slow and continues for many days, sometimes even weeks | ||
| Size | Limited to small area | Cover a large area | ||
| Path | East à West | West à East | ||
| Wind velocity and destruction | Much greater | Comparatively low (Flood causes more destruction than winds) | ||
| Temperature distribution | The temperature at the centre is almost equally distributed | All the sectors of the cyclone have different temperatures | ||
| Calm region | The centre (Eye) that have no rainfall | No single place where winds and rain is inactive | ||
| Driving force | Latent heat of condensation | Densities of air masses | ||
| Influence on India | Both coasts are affected but eastern is more vulnerable | Bring rain to North-West India (Associated instability is called ‘Western Disturbance’) | ||
Thunderstorms: Dynamics and Characteristics
- It is a well-grown cumulonimbus cloud which produces thunder and lightning.
- It is caused by intense convection on moist hot days.
- Since thunder comes from lightning, all thunderstorms have lightning.
- When the clouds extend to heights where sub-zero temperature prevails, hails are formed and they come down as hailstorms.
- If there is insufficient moisture, a thunderstorm can generate dust storms.
Essential Ingredients for Thunderstorm Formation
- Moisture
- Rising unstable air (air that keeps rising when given a nudge)
- Lifting mechanism to provide the “nudge”
Formation of Thunderstorms: Atmospheric Processes and Dynamics
- The sun heats the surface of the earth, which warms the air above it.
- This warm surface air is forced to rise—
- Hills or mountains (Orographic thunderstorm)
- Areas where warm/cold or wet/dry air bump together can cause rising motion (Frontal thunderstorm)
- It will continue to rise as long as it weighs less and stays warmer than the air around it.
- The rising air transfers heat from the surface of the earth to the upper levels of the atmosphere (the process of convection).
- The water vapor it contains begins to cool, releases the heat, condenses and forms a cloud.
- The cloud eventually grows upward into areas where the temperature is below freezing.
- As a storm rises into freezing air, different types of ice particles can be created from freezing liquid drops.
- The ice particles can grow by condensing vapor (like frost) and by collecting smaller liquid drops that haven’t frozen yet (a state called “supercooled”).
- When two ice particles collide, they usually bounce off each other. During this the particle can rip off a little bit of ice from each other and grab some electric charge.
- Lots of these collisions build up big regions of electric charges to cause a bolt of lightning, which creates the sound waves we hear as thunder.
Thunderstorm Life Cycle Stages: Developing, Mature, Dissipating
Developing stage
- Marked by a cumulus cloud that is being pushed upward by a rising column of air (updraft).
- The cumulus cloud soon looks like a tower (called towering cumulus) as the updraft continues to develop.
- There is little to no rain during this stage but occasional lightning.
Mature stage
- The updraft continues to feed the storm, but precipitation begins to fall out of the storm, creating a downdraft (a column of air pushing downward).
- The downdraft and rain-cooled air spreads out along the ground and forms a gust front, or a line of gusty winds.
- The mature stage is the most likely time for hail, heavy rain, frequent lightning, strong winds, and tornadoes.
Dissipating stage
- Eventually, a large amount of precipitation is produced and the updraft is overcome by the downdraft beginning the dissipating stage.
- At the ground, the gust front moves out a long distance from the storm and cuts off the warm moist air that was feeding the thunderstorm.
- Rainfall decreases in intensity, but lightning remains a danger.
Types of Thunderstorms: Single Cell Storms, Multicell Cluster Storms, Multicell Line Storms, Supercells
Single Cell Storms
- Typically last 20-30 minutes.
- Pulse storms can produce severe weather elements such as downbursts, hail, some heavy rainfall and occasionally weak tornadoes.
Multicell Cluster Storms
- A group of cells moving as a single unit, with each cell in a different stage of the thunderstorm life cycle.
- Multicell storms can produce moderate size hail, flash floods and weak tornadoes.
Multicell Line Storms
- Multicell line storms consist of a line of storms with a continuous, well developed gust front at the leading edge of the line.
- Also known as squall lines.
- These storms can produce small to moderate size hail, occasional flash floods and weak tornadoes.
Supercells
- Defined as a thunderstorm with a rotating updraft.
- These storms can produce strong downbursts, large hail, occasional flash floods and weak to violent tornadoes.
Tornadoes: Characteristics and Occurrence
- Much about tornadoes remains a mystery. They are rare, unpredictable and deadly.
- Tornado is a narrow, violently rotating column of air that extends from a thunderstorm to the ground.
- Because wind is invisible, it is hard to see a tornado unless it forms a condensation funnel made up of water droplets, dust and debris.
- Tornadoes can be among the most violent phenomena of all atmospheric storms we experience.
- The U.S. typically has more tornadoes than anywhere else in the world, though they can occur almost anywhere.
- Most tornadoes come from rotating thunderstorms, called supercells.
- Tornadoes generally occur in middle latitudes.
- The tornado over the sea is called Water Sprouts.
Gyanbazi
Rossby Waves: Oceanic and Atmospheric Impacts
- Rossby waves are naturally occurring planetary waves in rotating fluids.
- They are of two types – Oceanic and Atmospheric Rossby waves
- These waves affect the planet’s weather and climate.
Atmospheric Rossby Waves: Role in Weather Patterns
- It forms primarily as a result of the Earth’s geography.
- Rossby waves help transfer heat from the tropics toward the poles and cold air toward the tropics in an attempt to return atmosphere to balance.
- They also help locate the jet stream and mark out the track of surface low pressure systems.
- The slow motion of these waves often results in fairly long, persistent weather patterns.
Polar Vortex: Definition and Characteristics
- The term “vortex” refers to the counter-clockwise flow of air that helps keep the colder air near the Poles.
- The polar vortex is a large area of low pressure and cold air surrounding both of the Earth’s poles.
- It develops in the upper troposphere or stratosphere.
- It always exists near the poles, but weakens in summer and strengthens in winter.
Disruption of Polar Vortex: Causes and Effects
- Rossby Waves can disrupt the circulation around the polar vortex.
- It originated during winters due to the sharp temperature differential created between poles and equator.
- Many times during winter in the northern hemisphere, the polar vortex will expand, sending cold air southward with the jet stream.
- This occurs fairly regularly during wintertime and is often associated with large outbreaks of Arctic air in the United States.
Ozone Hole: Causes and Characteristics
- It is an area of depleted layers of ozone above the Antarctic region.
- Manufactured chemicals deplete the ozone layer.
- Atmospheric ozone is destroyed by chemical processes in each spring over Antarctica.
- This creates the ozone hole, which occurs because of special meteorological and chemical conditions that exist in that region.
Causes of Ozone Depletion: CFCs and Other Factors
- Depletion of ozone is due to many factors. The most dominant of which is the release of chlorine from CFCs (Chlorofluorocarbons) which destroys the ozone.
- CFCs are released by products such as hairsprays, old refrigerators etc.
Vienna Convention: Multilateral Environmental Agreement for Protecting the Ozone Layer
- A Multilateral Environmental Agreement.
- It was agreed upon at the 1985 Vienna Conference and entered into force in 1988.
- It is one of the most successful treaties of all time.
- It has been ratified by 197 states.
- It acts as a framework for the international efforts to protect the ozone layer.
- These are laid out in the accompanying Montreal Protocol.
- It is not legally binding.
Montreal Protocol: International treaty under the Vienna Convention aimed at phasing out ozone-depleting substances
- It is a protocol to the Vienna Convention for the Protection of Ozone Layer.
- It was the first treaty in history to achieve universal ratification (i.e. ratified by every member state of the United Nations).
- It is an international treaty and aims to protect the ozone layer by phasing out ozone depleting gases.
Kigali Agreement
- The Kigali Agreement amends 1987 Montreal Protocol to phase out Hydrofluorocarbons (HFCs), a family of potent greenhouse gases by the late 2040s.
- Montreal Protocol conceived only to phasing out gases that were destroying the ozone
- This move will help to prevent a potential 0.5 degree Celsius rise in global temperature by the end of the century.
Polar Stratospheric Clouds (PSCs) Characteristics and Role in Ozone Depletion
- Polar stratospheric clouds are clouds that form in the polar regions during the winter.
- Type-I PSCs
- Form when the stratospheric temperature drops below -78°C.
- They are primarily composed of nitric acid, water, and sulfuric acid.
- Type-II PSCs
- Form when the stratospheric temperature drops below -83°C.
- They are composed of crystals of water ice.
- They are referred to as nacre clouds or mother-of-pearl clouds due to their iridescence.
- Only Type-II clouds are necessarily nacreous whereas Type-I clouds can be iridescent under certain conditions, just as any other cloud.
- PSCs play a critical role in facilitating ozone depletion during the polar spring and summer.
- Type I clouds are now known as sites of harmful destruction of stratospheric ozone over the Antarctic and Arctic.
- Their surfaces act as catalysts that convert human-made chlorine into active free radicals (for example ClO, chlorine monoxide).
- These radicals destroy many ozone molecules in a series of chain reactions during the return of spring sunlight.
- Cloud formation is doubly harmful because it also removes gaseous nitric acid from the stratosphere that can combine with ClO to form less reactive forms of chlorine.
Aurora: Natural Light Display in the Sky and Its Characteristics
- An Aurora is a display of light in the sky which is predominantly seen in the higher latitude of northern and southern regions (Arctic and Antarctic). Due to this, it is also known as a Polar Light.
- It is less frequent at mid-latitudes and seldom seen near the equator.
- 2 Types
- Aurora Borealis (Northern Lights)
- Aurora Australis (Southern Lights)
- It is usually milky greenish in color but can also be seen in red, blue, violet, pink, and white colors.
- Auroras affect communication lines, radio lines and power lines.
Cause of Aurora Occurrence: Interaction between Solar Wind and Earth’s Magnetic Field
- These light shows are the result of interaction between sun energy (in the form of solar wind) and electrically charged particles trapped in Earth’s magnetic field.
- It is an outcome of collisions between the oxygen and nitrogen in Earth’s upper atmosphere with the fast-moving electrons from space.
- The electrons coming from the Earth’s magnetosphere (a region of space controlled by Earth’s magnetic field) enhances the energy of oxygen and nitrogen atoms and makes them “excited”.
- During returning to their normal state, these gases emit photons and small bursts of energy in the form of light.
- The color of the aurora depends on
- Which gas – oxygen or nitrogen – is being excited by the electrons, and on how excited it becomes and by what extent.
- How fast the electrons are moving, or how much energy they possess at the time of their collisions.
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