Air has weights and it exerts pressure which varies from place to place. These variations are specified ion pressure belts. The pressure at different location of the world also varies in different seasons. This variation causes the movement of air called winds.
These winds are divided into different types as per their features and specifications. Air masses and cyclones are also formed due to such steep difference in air pressure.
Measurement of Air Pressure
A column of air exerts weight pressure on the surface of the earth. The weight of the column of air at a given place and time is called air pressure or atmospheric pressure. Atmospheric pressure is measured unit area by an instrument called barometer and marked in millibar (mb).
Distribution of Air Pressure
Vertical Distribution
Air pressure decreases with increase in altitude but it does not always decrease at the same rate. The columnar distribution of atmospheric pressure is known as vertical distribution of pressure.
Temperature of the air, amount of water vapour present in the air and gravitational pull of the earth determine the air pressure of a given place and at a given time. Since these factors are variable with change in height, there is a variation in the rate of decrease in air pressure with increase in altitude.
Horizontal Distribution
The distribution of atmospheric pressure over the globe is known as horizontal distribution of pressure and shown on maps through isobars. An isobar is a line connecting points that have equal values of pressure.
Factors responsible for variation:
- Air Temperature
- The Earth’s Rotation
- Presence of Water Vapour
Pressure Belt
The horizontal distribution of air pressure across the latitudes is characterized by high or low pressure belts. These are:
- Equatorial Low Pressure Belt
- Sub tropic High Pressure Belts
- Sub-polar Low Pressure Betts
- Polar High Pressure Belts
1. Equatorial Low Pressure Belt: The sun shines almost vertically on the equator throughout the year resulting in warm rises over the equatorial equatorial low pressure. It extends from equator to 10°N and 10°S latitudes.
Due to excessive heating horizontal movement of air (wind) is absent here. This belt is called doldrums (the zone of calm). It is also known as Inter Tropical Convergence Zone (ITCZ).
2. Sub tropic High Pressure Belts: It extends from the tropics to about 35° latitudes in both the Hemispheres. This pressure belts is formed because the rising air of the equatorial region is deflected towards poles due to the earth's rotation.
After becoming cold and heavy, it descends in these regions and get piled up resulting in high pressure.
3. Sub-polar Low Pressure Belts: It extends between 45°N and the Arctic Circle in the northern hemisphere and between 45°S and the Antarctic Circle in the southern hemisphere.
Winds coming from the sub-tropical and the polar high belts converge here to produce cyclonic storms or low pressure conditions. This zone of convergence is also known as polar front.
4. Polar High Pressure Belts: Slanting sun rays resulting in low temperatures causing compressed air with increased density. Hence, high pressure is found here. In northern hemisphere it called the North polar high pressure belt and South polar high pressure belt in the southern hemisphere.
Wind
Horizontal movement of air in response to difference in pressure is termed as wind while vertical or nearly vertical moving air is called air current.
Pressure Gradient and Wind
The greater the difference in air pressure between the two points, the steeper is the pressure gradient and greater is the speed of the wind. The gentler the pressure gradient slower is the speed of the wind.
Coriolis Effect and Wind
They get deflected from their original paths due to the earth's rotation on its axis, known as the Coriolis effect or Coriolis force. In northern hemisphere winds are deflected towards their right, and in the southern hemisphere towards their left known as Farrel’s law.
Type of Winds
Winds are generalized under three categories:
1. Planetary winds or Permanent winds: Planetary or permanent winds blow from high pressure belts to low pressure belts in the same direction throughout the year. They are easterly and westerlies and polar easterlies.
2. Periodic winds (Monsoon Winds): The direction of these winds changes with the change of seasons i.e. Monsoon winds.
3. Local winds (Including Hot and Cold wind): Local winds usually affect small areas and found in the lower levels of the troposphere.
Hot Winds
Loo: Loo are hot and dry winds, which blow very strongly over the northern plains of India and Pakistan in the months of May and June. Their direction is from west to east and they are usually experienced in the afternoons. Their temperature varies between 45°C to 50°C.
Foehn: Foehn is strong, dusty, dry and warm local wind which develops on the leeward side of the Alps mountain ranges. The temperature of the winds vary from 15°C to 20°C which help in melting snow. Thus making pasture land ready for animal grazing and help the grapes to ripe early.
Chinook: Hot and dry local wind which moves down the eastern slopes of the Rockies in U.S.A. and Canada. They keep the grasslands clear of snow. Hence they are very helpful to ranchers.
Cold Winds
Mistral: Mistrals originate on the Alps and move over France towards the Mediterranean Sea through the Rhone valley. They are very cold, dry and high velocity winds. They bring down temperature below freezing point in areas of their influence.