Pressure and Pressure belts
Pressure - Force per unit area is known as pressure.
Atmospheric Pressure - It is the pressure exerted by air on unit area of Earth's surface. The force is exerted due to the weight of the air column above the surface.
It is highest at the sea level and with increase in altitude, atmospheric pressure decreases.
It is measured using Barometer.
Average atmospheric pressure at the Sea level is 1013 mB
Isobars - It is the line connecting all the points on the surface of the Earth having same atmospheric pressure.
Factors affecting Atmospheric pressure
1. Temperature
- As we know that in an open system; with increase in temperature, pressure decreases and in a closed system with increase in temperature, pressure also increases.
- Since, Atmosphere is an open system; with increase in temperature, pressure decreases.
2. Altitude
- With increase in altitude, pressure decreases (because of convicting/rising air).
- And with decrease in altitude, pressure increases (because of subsidence/sinking air).
3. Rotation
- If we neglect the effect of temperature and altitude, pressure at equator is higher than pressure at poles.
In the atmosphere, when air is heated from the surface, it expands and the outward pressure of molecules is spread over a large area. Therefore, pressure of air decreases. Whereas when air is cooled, it contracts causing increase in pressure.
The pressure of air is high at the ground level than that of the air at higher altitude. When air rises, its volume increases and pressure decreases. Whereas when air sinks its volume decreases and pressure increases.
Rotation causes air at the poles to be thrown away towards the equator. If the effect of rotation is alone taken into consideration, the air piling up along the equator produces a belt of high pressure and at the poles a belt of low pressure.
Formation of Pressure belt
- At 90° N - Temperature ↓ 🠊 Air sinks 🠊 Pressure ↑ (Polar High)
- At 60° N - Low Pressure (due to divergence of air near to the surface) 🠊 Air rises (Temperate low)
- At 30° N - Due to sinking of air, Pressure ↑ (Sub-tropical high)
- At 0° - Temperature ↑ 🠊 Pressure ↓ 🠊 Air rises (Equatorial low)
- At 30° S - Due to sinking of air, Pressure ↑ (Sub-tropical high)
- At 60° S - Low Pressure (due to divergence of air near to the surface) 🠊 Air rises (Temperate low)
- At 90° S - Temperature ↓ 🠊 Air sinks 🠊 Pressure ↑ (Polar high)
Near to the equator due to high temperature, the air rises and causes expansion resulting in the formation of a low pressure belt at Equator.
The rising air spreads out as it reaches the top of the troposphere and moves towards the poles. The air moving towards the poles from the equator starts to converge and causes an increase in pressure and begin to sink leading to the formation of high pressure belt at 30° N and 30° S (Sub-tropical high).
Some part of the air moves towards the equator and some towards the pole. The air moving towards the equator replaces the air rising there.
Low temperature at the poles results in contraction of air and development of high pressure (Polar high).
The air blowing away from the poles (due to rotation of the Earth) spreads out to greater space and pressure falls leading to low pressure belt at 60° N and 60° S (Temperate low).
Some of the air from Sub-tropical high moving towards pole reaches 60° N and 60° S and converges with the air from poles leading to convection along 60° N and 60° S.
Shifting of Pressure belts
The apparent movement of the Sun between the tropics causes different pressure belts to shift towards the north and south of the Equator.
Distribution of Pressure belts
Northern Hemisphere - Pressure belts are nearer to each other.
Southern Hemisphere - Pressure belts are spread to a larger extent.
Movement of Pressure belts
- From January to July - Pressure belts shifted Northwards
- From July to January - Pressure belts shifted Southwards
During January -
- Equatorial low - Shifted down
- Sub-tropical high (Southern Hemisphere) - not a continuous belt, it is divided into smaller cells because of heating of land which creates too low pressure at land.
- Sub-tropical high (Northern Hemisphere) - almost continuous as it is winter in Northern Hemisphere.
- Siberian high would form a very high pressure cell because of its huge landmass.
During July -
- Equatorial low - Shifted upward
- Sub-tropical high (Northern Hemisphere) - not continuous, present as different cells over oceans and land
- Sub-tropical high (Southern Hemisphere) - continuous belt because of large ocean expanse.
In January, the Equatorial low pressure zone extends well into the Southern hemisphere.
The Sub-tropical high pressure belt of Southern hemisphere is found only over the oceans.
The low temperature in Northern hemisphere produces a continuous high pressure system.
In July, the Equatorial low pressure zone extends too much into the Northern hemisphere linking with the low pressure system of India and Tibet.
The Sub-Tropical high pressure belt in Northern hemisphere is existing only over the oceans whereas in Southern hemisphere forms a continuous and single belt of high pressure.
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Note - This is my Vision IAS Notes (Vision IAS Class Notes) and Ashutosh Pandey Sir's Public Administration Class notes. I've also added some of the information on my own.
Hope! It will help you to achieve your dream of getting selected in Civil Services Examination 👍
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