Atmospheric Pressure and Wind Dynamics

Air expands when heated and gets compressed when cooled generating atmospheric pressure. This change in atmospheric pressure sets the air in horizontal motion called wind. This wind redistributes the heat and moisture across the planet, thereby, maintaining a constant temperature for the planet as a whole. The vertical rising of moist air cools it down to form the clouds and bring precipitation.

Atmospheric Pressure: Key to Wind Dynamics

• Atmospheric pressure: is the force exerted by a column of air in a unit area from sea level to the top of the atmosphere.
  • It is commonly measured in millibars (mb), and the average atmospheric pressure at sea level is about 1,013.2 millibars.
• Gravitational Effect:
  • Near the Earth’s surface, the force of gravity causes the air to be more densely packed, resulting in higher atmospheric pressure.
  • As one ascends in altitude, the gravitational pull decreases, leading to a reduction in air pressure.
• Pressure Difference and Wind:
  • Variations in atmospheric pressure across different locations create pressure gradients, which are the primary drivers of air movement, or wind.
  • Air naturally flows from regions of higher pressure to those of lower pressure, which is a fundamental mechanism governing atmospheric circulation and weather patterns.

Atmospheric pressure is a crucial element in understanding weather systems, as it influences the movement of air masses and the formation of weather patterns.

Vertical Variation of Pressure: Atmospheric Pressure Changes

In the lower atmosphere, there is a swift decline in pressure as one goes higher. 

• This decline is roughly 1 millibar for every 10 meters of elevation gain, although it’s important to note that this rate of decrease is not uniform.

• Vertical Pressure Gradient Force vs. Gravity: The vertical pressure gradient force is significantly greater than the horizontal pressure gradient force. 
  • However, it is typically counteracted by an almost equal and opposite force of gravity. 
  • As a result, we do not encounter strong upward winds.

Horizontal Distribution of Pressure: Decoding Atmospheric Pressure

• Small pressure variations: They are crucial for wind patterns. 
  • Isobars, lines connecting equal pressure areas, are used to study horizontal pressure distribution.
• The adjoining image shows the patterns of isobars corresponding to pressure systems. 
• The Low Pressure system: It is enclosed by one or more isobars with the lowest pressure in the center. 
• High-pressure system: It is also enclosed by one or more isobars with the highest pressure in the center.

Fig. . Isobars, pressure and wind systems in Northern Hemisphere

World Distribution of Sea Level Pressure: Atmospheric Pressure Across Latitudes

• Based on the latitudinal pressure distribution regions/strips of varying pressure are found. These are called Pressure Belts. These are listed as under:
  • Equatorial Low: It is the region close to  the equator where sea level pressure is low. 
  • Subtropical Highs: This is the high-pressure area along 30° N and 30° S latitude. 
  • Sub Polar Lows: These are the low-pressure belts towards the pole along 60° N and 60° S latitude.
  • Polar High: It is the high pressure region close to the poles.

Distribution of pressure (in millibars) — January

• The pressure belts are not fixed.
  • They shift with the apparent sun movement. 
  • In the northern hemisphere, they shift southward in winter and northward in summer.

  Distribution of pressure (in millibars) — July

Forces Affecting the Velocity and Direction of Wind: Winds Unleashed: Atmospheric Pressure at Play

The horizontal winds near the earth’s surface are influenced by  the combined effect of three forces: the pressure gradient force, the frictional force, and the Coriolis force.

• Pressure Gradient Force:
  • The rate of change of pressure with respect to distance is called the pressure gradient, which is fueled by the force generated due to the difference in atmospheric pressure.
  • The pressure gradient is strong where the isobars are close to each other and is weak where the isobars are apart.

• Frictional Force:
  • The movement of wind over the earth’s surface causes friction which affects the speed of the wind.
  • Frictional force is greatest at the surface and minimal over the sea surface and generally extends up to an elevation of 1 – 3 km.

• Coriolis Force:
  • Result of Earth’s Rotation: The Coriolis force is a result of the Earth’s rotation about its axis and plays a crucial role in determining the direction of wind movement.
  • Named After French Physicist: It is named after Gustave-Gaspard Coriolis, a French physicist who identified this force in 1844.
  • Northern Hemisphere Deflection: In the Northern Hemisphere, the Coriolis force deflects the wind to the right, affecting its direction.
  • Southern Hemisphere Deflection: In the Southern Hemisphere, the Coriolis force deflects the wind to the left, influencing its direction.
  • Direction Dependence on Wind Velocity: The magnitude of this deflection depends on the wind’s velocity.
  • Proportional to Latitude Angle: The Coriolis force is directly proportional to the angle of latitude, meaning it is more significant at higher latitudes and absent at the equator.
  • Maximum at Poles: At the poles, the Coriolis force is at its maximum effect.
  • Perpendicular to Pressure Gradient Force: The Coriolis force acts perpendicular to the pressure gradient force, creating the characteristic deflection in wind direction.
  • Stronger Pressure Gradient Force: When the pressure gradient force is stronger, the wind’s velocity is higher, and the deflection caused by the Coriolis force is more pronounced.
  • Absence of Cyclones at Equator: Cyclones are not formed at the equator because the Coriolis force is virtually absent. 
  • At the equator, winds blow nearly perpendicular to the isobars, causing low-pressure systems to get filled rather than intensifying.