Earth’s general circulation system (Surface wind)
The earth’s surface wind system or earth’s general circulation of wind can be represented by a simple model shown in fig. In this model, the earth surface been considered uniform, means either all and or all water and the effects of local systems have been ignored, therefore the actual wind system is much more complicated than the described in the model.
It is to be noted that unequal heating of the earth’s surface generate pressure gradient which give rise to wind. There are three latitudinal circulations and there are also important longitudinal variations around each hemisphere.
1. Trade winds:
The condition of greatest heating and expansion at the equator causes rising of air and creating low pressure belt (50 N to 50S latitude) known as doldrums or equatorial low or calm. The rising of air from equator causes increase in pressure at 350 N 350S which is known as sub-tropical high or Horse latitude belt. The winds therefore flow from horse latitude belt. The winds therefore flow from horse latitude to the equatorial region called “Trade winds” While moving these winds, they are deflected by corollas force to the right in the and nor them hemisphere and to left in southern hemisphere and become North – East trades and south –East Trades in northern and southern hemispheres respectively.
The blow of air from equator and accumulation of air over 25-35 0 latitudes giving rise to high pressure belt region of descending air is known as Hadley cell.
2. Westerlies wind:
There situated at about 600 – 650 latitudes a low pressure area in both the hemisphere is known as sub-polar low or polar front. The winds that flow from sub-tropical high pressure area (Located at 250 – 350 latitude in both the hemisphere ) to the low pressure area, situated at about 600 – 650 latitude in both the hemisphere, are known as Westerlies or prevailing wisterias ( anti trade winds) In the upper atmosphere the reverse air movement takes place. This circulation is known as feral cell. These winds instead flowing in straight line are deflected due to corollas force. In northern hemisphere their direction is North – West and in southern hemisphere it is South-West.
3. Polar winds or Polar Easterlies winds :
Near the poles due to shrinkage of air and due to cooling, there exists permanent high pressure on the poles. Therefore winds flow from the polar high to sub-polar low pressure area at about 60-650 latitude. The wind flow in North-East direction in northern hemisphere and in south-East direction in southern hemisphere. These winds consist of clod air. The air circulation is known as polar cell.
These winds are generated due to local condition and hence influence over very small area, therefore such winds are called local wind.
1. Land and sea breeze :
An interchange of air between the sea and coastal land due to unequal heating and cooling is known as land and sea breezes. They are local in nature. During day time the coastal land and sea breezes.. They are local in nature. During day time the coastal land as heated very fast as compared to sea water causing low pressure over the land. Therefore the surface air blows from sea to land and this is known as sea breeze. While during night time, the land cools faster than the sea, causing high pressure area over land as compared to sea. Therefore air blows from land to sea and this is known as land breeze.
2. Mountain and valley breeze:
An interchange of air between the mountain and valley due to unequal heating and cooling of the two places is known as mountain and valley breeze. During daytime, the valley breeze. During daytime, the valley floors become more heated; the air over it expands and rises. This rising air slides up the mountain slope and is known as valley breeze. During night reverse process takes place. Due to cooling of the air in the valley contracts and consequently augmented by air from the neighboring hills and mountains. The air on the mountain slopes also cools and slides down into the valley which is known as mountain breeze.
3. Ketabatic winds:
A mass of cold air over an elevated plateau during the winter tends to become more dense through radioactive cooling and then will drain down the slopes into the valleys below. The resulting down slope, drainage type winds are called Ketabatic winds. Most are relatively gentle breezes, not exceeding 4 to 5 m/s occasionally how ever, the cold dense air may be set in motion by a migrating cyclone or anticyclone and the Ketabatic winds may then attain destructive violence.