Those winds which blow in a certain direction and a certain order through out the year, are called permanent winds. These are also known as prevailing, planetary or global winds.
Types of Permanent Winds:
Permanent Winds are mainly divided into the following types:
1. Trade winds: Winds blowing in both the hemispheres, from the sub-tropical high air pressure belt, to the equatorial low air pressure belt are known as trade winds. These winds do not blow straight and are deflected to their right side in the northern hemisphere and to their left side in the southern hemisphere as per Ferrel’s Law.
Thus, these winds are called the North Eastern trade winds in the northern hemisphere and the South Eastern trade winds in the southern hemisphere. These winds used to facilitate trading through ships in ancient times, hence they are called ‘trade winds’. These winds have different characteristics in different regions.
These winds are dry and calm as winds descend near the sub tropical high air pressure region. These winds absorb water vapor from the sea surface as they proceed ahead. Reaching near the equator, these winds get almost saturated with water vapor, and become unstable and cause rainfall. The trade winds of the both the hemispheres collide near the equator with each other, and rise up and cause heavy rainfall.
2. Westerlies Winds: Winds flowing towards sub-polar low air pressure belts from the sub-tropical high air pressure regions in both the hemispheres are called Westerlies winds. Their direction is from the south – west to the north – east in the northern hemisphere. Due to the high intensity of latitude and seasonal changes in the Northern Hemisphere, the western flow of these winds becomes depleted.
These winds are more regular and permanent due to the oceanic expansion in the Southern Hemisphere. Their velocity in the Southern Hemisphere is also high. Due to the intensity of these winds, they are called ‘Roaring Forties’ at 40° – 50° latitudes in the southern hemisphere, ‘Furious Fifties’ near 50° south latitude and ‘Shrieking/Screaming Sixties’ near 60° south latitude. The range of westerlies winds towards the poles is unstable. These winds generate instability in the weather.
3. Polar Winds: In both the hemispheres, the winds blowing from Polar high air pressure belts to sub – polar low air pressure belts are called polar winds. In the northern hemisphere, their direction is from the north-east to the south – west and in the southern hemisphere it is from south – east to north – west. These winds are extremely cold and dry as they flow from the polar frigid areas.
Due to low temperature, their ability to absorb water vapor is also low. The polar winds that blow rapidly in the northern hemisphere are called ‘Nor’esters’. whose highest impact is felt in North Eastern Canada and the USA.
