Solar Radiation: Earth’s Energy Exchange

The Earth receives almost all of its energy from the sun. The earth in turn radiates back to space the energy received from the sun. As a result, the earth neither warms up nor does it get cooled over a period of time. Thus, the amount of heat received by different parts of the earth is not the same. This variation causes pressure differences in the atmosphere. This leads to the transfer of heat from one region to the other by winds. 

Solar Radiation: Earth’s Shape, Slanting Rays, and Energy Exchange

• Insolation: The earth’s surface primarily receives energy in short wavelengths, which is termed as “incoming solar radiation” or “insolation.”
• Geoid Shape: The Earth resembles a geoid shape (sphere-like shape). 
• Angle of Solar Rays: The sun’s rays fall slantwise at the top of the atmosphere. 
• Interception of Solar Energy: Out of this, the Earth intercepts a small portion of the sun’s energy.
• Solar Constant: On average, the earth receives 1.94 calories per sq. cm per minute at the top of the atmosphere.

The Passage of Solar Radiation through the Atmosphere

Distribution of Solar Radiation: Earth’s Orbital Tango, Tilt, and Latitude Influence

• Changing Earth-Sun Distance:  The changing distance between the Earth and the Sun varies the solar output throughout the year.
  • The annual insolation on 3rd January is slightly more than on 4th July is due to perihelion.
• Axial Tilt: The Earth’s axis is angled at 66½° with its orbital plane. 
  • This angle significantly influences insolation received at different latitudes.
• Latitude: The amount of insolation is also determined by the angle of the sun’s rays which  depends on the latitude of a place. 
  • The higher the latitude the less is the angle they make with the surface of the earth resulting in slant sun rays which cover more area and energy get distributed and the net energy received per unit area decreases.
• Atmospheric Effects: Moreover, slant rays pass through a greater depth of the atmosphere, leading to more absorption, scattering, and diffusion.

Atmospheric Influence on Insolation: Solar Radiation Alchemy

• Atmospheric Transparency to Solar Radiation:  The atmosphere is mostly transparent to short-wave solar radiation. 
  • As solar radiation passes through the atmosphere, gases like water vapour and ozone absorb much of the near-infrared radiation.
• Scattering of Visible Light: Small particles in the troposphere scatter the visible spectrum of the light. 
  • This scattering results in the blue colour of the sky and the red colour of the rising and setting sun.

Spatial Distribution of Insolation on Earth’s Surface: Solar Radiation Tapestry

• Subtropical deserts: It receives the highest insolation due to minimal cloudiness.
• The equator:  It receives less insolation compared to the tropics.
• Continents vs. Oceans: At the same latitude, continents generally receive more insolation than oceans.
• Seasonal Variations: During winter, middle and higher latitudes receive less radiation compared to summer.

Heating and Cooling of Atmosphere: Solar Radiation Dynamics

There are different ways of heating and cooling of the atmosphere. These are the following:

Heat Transfer: Warming Air through Layered Conduction

• Transfer of Heat: The air in contact with the land gets heated slowly and the upper layers in contact with the lower layers also get heated due to the transfer of heat from the warmer to the cooler body. 
  • This process is called conduction.
• Layered Heat Transfer: This process continues until both bodies reach the same temperature or contact is broken. 
  • The conduction primarily heats the atmosphere’s lower layers.

Rising Heat: Convection in Earth’s Atmosphere

• Definition: Air in contact with the earth rises vertically when heated, forming currents that transmit atmospheric heat. 
• This vertical heating process is termed as convection.
• Location: The convection-based energy transfer is limited to the troposphere.

Advection’s Impact: Heat Shifts in Air Movement

• Definition: The transfer of heat through horizontal movement of air is called advection. 
• Importance: Horizontal air movement is more important  than vertical movement.
• Importance: In middle latitudes, daily weather variations (diurnal) are mainly due to advection. In tropical regions, especially in northern India during summer, local winds like ‘loo’ result from the advection process.

Earth’s Heat Exchange: Insolation to Radiation

• Earth’s Absorption: The earth absorbs insolation in short waveforms, warming its surface. 
• Radiation: Once heated, the earth becomes a radiator, emitting energy back to the atmosphere in long waveforms. 
  • This emitted energy is termed “terrestrial radiation.”

Atmosphere’s Role: Absorbing, Radiating Earth’s Heat

• Atmospheric Absorption: The atmosphere absorbs long-wave radiation, especially from gases like carbon dioxide and other greenhouse gases.
• Atmospheric Radiation: This means the atmosphere gets its warmth indirectly from the earth’s radiation, rather than directly from the sun.
  • After absorbing this energy, the atmosphere then radiates heat back into space.
• Energy Balance: This ensures a balance, where the heat the earth receives from the sun is eventually returned to space, maintaining a constant temperature both on the earth’s surface and in the atmosphere.

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