Atmospheric Pressure & Air Motion

Atmospheric Pressure

Pressure is defined as the force exerted on a surface, taking into account the influence of gravity. Atmospheric pressure is a specific type of pressure that results from the weight of a column of air pressing down on a surface. Atmospheric pressure can be measured using a barometer. A barometer indicates the changes in atmospheric pressure, by the rise or fall of a column of mercury in a glass tube.

What is barometric pressure?

We use a barometer to measure atmospheric pressure. Because we use a barometer, atmospheric pressure is also known as barometric pressure.

Atmospheric pressure at sea level

At sea level, the standard atmospheric pressure is 1013.25 millibars. This value can fluctuate due to changes in temperature, altitude, and other meteorological factors. The barometer serves as a tool for measuring these changes and providing information about atmospheric conditions.

Why does pressure fall with an increase in height?

The atmospheric pressure is just the weight of the column of air on the surface. It decreases as the altitude increases because the size of the column decreases as we rise up.

Factors affecting pressure

Horizontal Distribution of Pressure

planetary pressure belts

Planetary pressure belts refer to regions on a planet where the atmospheric pressure is relatively constant. These regions are typically characterized by relatively stable weather patterns and are formed by the dynamics of the planet’s atmosphere.

On Earth, there are three main pressure belts. The equatorial low-pressure belt, the subtropical high-pressure belts, and the polar low-pressure belts.

The equatorial low-pressure belt is also known as the Intertropical Convergence Zone (ITCZ). It is a region around the Earth’s equator where the trade winds from the northern and southern hemispheres converge. This convergence leads to rising air and the formation of low-pressure areas, resulting in high precipitation and thunderstorms.

The subtropical high-pressure belts are also known as the Horse Latitudes. They are located around 30 degrees north and south of the equator. These regions are characterized by descending air and the formation of high-pressure areas. This results in clear skies and relatively dry conditions.

The polar low-pressure belts are located at around 60 degrees north and south of the equator. They are characterized by low pressure and cold temperatures.

Similarly, pressure belts can be found on other planets as well. On Jupiter, there are equatorial low-pressure belts, subtropical high-pressure belts, and polar low-pressure belts. However, the dynamics of these pressure belts differ from that of Earth. This is due to the different atmospheric compositions and conditions of these planets.

In summary, planetary pressure belts are regions on a planet where the atmospheric pressure is relatively constant. These regions are characterized by relatively stable weather patterns, and are formed by the dynamics of the planet’s atmosphere.

Laws of Horizontal Motion of Air

Three factors influence the movement of air on Earth’s surface.

  1. Pressure Gradient Force
  2. Coriolis Force
  3. Frictional Force

Pressure Gradient Force

pressure gradient force

The wind blows across the isobars from high-pressure isobar to low-pressure isobar. When there is a difference in temperature in fluids (air and water), they flow from high-pressure to low-pressure. This means pressure creates a slope, resulting in the movement of fluids from high to low. This phenomenon results not only in the movement of planetary winds but also creates ocean currents.

Coriolis Force

Also known as Earth’s Rotational Deflective, Coriolis force is a force that deflects any moving object (air or water) on a rotating surface. Earth’s rotation and the conservation of angular momentum cause this effect. This effect is most substantial at the poles and weakest at the equator.

As a result of this force, any moving object is deflected towards the right in the northern hemisphere and towards the left in the southern hemisphere. Read more…

Frictional Force

When wind moves over the surface of the Earth, it encounters friction. Friction resists the movement of air. This results in changing the direction and speed of the wind. The wind slows down and begins to deviate from its original path. The strength of friction depends on various factors, including the roughness of the surface, the presence of obstacles such as trees and buildings, and the interaction of the wind with the surface, such as crops or forests. The effect of friction on wind is an important factor to consider. It impacts the distribution and intensity of wind patterns.

The resultant direction of the wind on the surface of the Earth

wind direction

The resultant direction of the wind on the surface of the Earth is the result of the combined effects of pressure gradient force (PGF), Coriolis force (CF), and frictional force (FF).

The pressure gradient force drives the wind from high-pressure areas to low-pressure areas. The Coriolis force, caused by the Earth’s rotation, causes the wind to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Frictional force, which slows down the wind as it moves over the Earth’s surface, further modifies the direction of the wind.

The combination of these forces results in the wind blowing at an angle between the direction of the pressure gradient and the direction of the Coriolis force. The magnitude of this angle is dependent on the relative strengths of each force and the speed of the wind.

The direction of the wind at any given location is determined by the balance between these forces and the specific meteorological conditions present in that area.

Read about related concepts of Ekman Spiral, Ocean Gyres and Upwelling of water

Geostrophic Winds

geostrophic wind parallel to isobars

As we move upwards from the earth’s surface, the frictional force decreases and becomes negligible at around 8 to 15 km below the tropopause. Under normal circumstances, air is influenced by the pressure gradient, Coriolis, and frictional forces, causing it to follow a certain path. However, when one of these forces is missing, the wind direction changes. In the absence of frictional force, the wind has only two opposing forces, the pressure gradient force and the Coriolis force. As a result, the wind does not flow from high-pressure isobars to low-pressure isobars but instead blows parallel to the isobars, forming ‘geostrophic winds’. Geostrophic winds are a type of wind that form as a result of the balance between the pressure gradient and Coriolis forces in the atmosphere.  Jet streams are also a type of geostrophic wind, which are fast-flowing, narrow air currents found in the upper levels of the atmosphere.

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