Earth Pressure and Winds
Air moving in a particular direction is called wind. The principal cause of winds is difference in pressure. Air always moves from areas of high pressure to those with low pressure. The slope of the pressure from high to low is known as Pressure Gradient and the direction of this direction decides the direction of winds.
Wind velocity is directly related to the steepness of the pressure gradient.
In addition, the direction of winds is affected by the Coriolis Force, which is caused by the rotation of the earth. Under the influence of this effect, winds are deflected to their right in the Northern Hemisphere and to their left in the Southern Hemisphere.
This is referred to as Parrel’s Law. Coriolis force is absent at the equator and increases towards the poles. Due to this, the winds, which would blow at right angles to the isobars under the pressure gradient, blow obliquely to them.
Global Pressure Belts
Equatorial Low Pressure Belt (or Doldrums)
- From 5°N to 5°S.
- Tremendous heat, thus warm air rises creating low pressure. Also, the centrifugal force is very high at the equator, where the velocity of rotation is high. Hence, the air masses tend to be thrown out, resulting in low pressure.
- Wind speed low, that’s why called Doldrums (Belt of Calm).
Tropical High Pressure Belt (or Horse Latitudes)
- From 30° to 35° N and S.
- Apart from 2 months, usually high temperature.
- Here the pressure is high, although high temperature, because here pressure depends on the rotation and movement of air (as winds from Doldrums belt rises up and accumulate here. Also winds from Sub-Polar Low Pressure Belt accumulate here).
Sub-Polar Low Pressure Belt
- From 60° to 65°N and S
- Here the low pressure is created because of intense high pressure at the poles.
Winds and Their Types
- 3 broad categories are:
- Regular Winds/Prevailing Winds/Planetary Winds: (E.g.: Trade winds, Westerlies and Polar Easterlies).
- Periodical Winds (which blow seasonally): Monsoons
- Variable Winds: Cyclones and other local winds
- Trade in German means Track. To blow trade means to blow steadily in the same direction and in a constant course’.
- These are steady currents of air blowing from the sub-tropical high pressure belts towards the equatorial low pressure areas (doldrums). Under the influence of the Coriolis force they blow from the north-east in the northern hemisphere and from the south-east in the southern hemisphere.
- Blows from subtropical high pressure to sub-polar low pressure belt.
- In the northern hemisphere, land masses cause considerable disruption in the westerly wind belt. But between 40° and 60° S lies the almost unbroken ocean belt. Westerlies are strong and persistent here, giving rise to mariner’s expressions- ‘Roaring Forties’, Furious Fifties’ and ‘Shrieking Sixties’.
- Move from high pressure poles to sub-polar low pressure areas.
- These are deflected by the Earth’s rotation to become east winds, or the polar easterlies.
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