The main cause of air movement is the differences caused by pressure and temperature. The air in warm temperature rises in an upward direction, whereas the air in cold temperature is denser and moves downward and replaces the warm air. The phenomenon is known as wind.
Wind due to Differences in Pressure
The movement of air that occurs due to the difference in temperature and pressure is due to wind. When the difference of pressure occurs between two places, a pressure gradient is created which enables the air to move from the high-pressure region to the low-pressure region. The movement of air does not occur in the quickest straight-line path and follows a spiralling route, which is outwards from high pressure and inwards towards low pressure.
The main reason for this pressure is due to the rotation of the earth beneath the moving air causing an apparent deflection of the wind to the right in the northern hemisphere and left in the southern hemisphere. On the other hand, air blows in an anticlockwise direction through a low-pressure centre and clockwise around a high-pressure centre in the northern hemisphere.
Wind due to Differences in Temperature
The difference in temperature is known as convection or advection and this is the reason for wind. The transfer of heat energy from warmer regions present near the earth’s surface to the regions higher up in the atmosphere distant from the heating influence of the earth’s surface is known as convection. Advection involves the horizontal movement of air and heat energy transference. On the other hand, convection is the vertical movement of air.
Temperature differences present at the earth’s surface occur when there are differences in surface substances. As an example, a dark tarmacked surface will heat up more quickly on a sunny day when compared to a grassy field. In the same way, along the coast, large areas of land heat up more quickly than adjacent seawater. Air near the land surface is heated by radiation and conduction, and this starts to expand and rise, turning to be lighter than the surrounding air. The above process is known as convection.
Air temperature is more at the ground level due to the heat emitted by the sun, and it decreases with altitude. Since warmer air nearer the surface is lighter than the colder air, the vertical temperature difference creates a significant uplift of air. And the vertical uplift of air can create clouds and rain. Sometimes air from the warmer regions of the world collides with air from the cold region.
Reasons for the Movement of Air
The weather and climate of any place are decided by the movement of air across the earth. The land, sea and air are heated up by the sun’s radiation. The land and water bodies also heat up the air, becoming dense. Hot air is increased, which creates low pressure over that area and cold air sinks and creates a region of high pressure. Air also likes to maintain equal pressure like other fluids.
Air carries water vapour. The amount of water it carries and the temperatures of air determine how much water can exist as vapour in the air. If more vapour is added, the air cannot hold any more water, and as a result, it begins to rain. The less water vapour it carries, the lower the temperature of the air. When air cools down, the vapour condenses and causes rain. This whole cycle of evaporation and condensation is called the Water Cycle.
How does the Wind react due to the Difference in Pressure?
During the summertime, in India, the land heats up much faster than the sea, and a region of low pressure is created over the Indian subcontinent and the Tibetan Plateau. By June, there is sufficient low pressure for cooler air from the sea to blow towards the land. When the vapour from the Indian Ocean blows towards India, it picks up moisture and water vapour. As it reaches India, they begin to cool, causing rain.
Most of the rain from the monsoon is dumped over hills and mountains, especially the Western Ghats and the Himalayas. This is known as the Advancing Monsoon climate. As the air hits these mountains, it tries to move towards the lower pressure areas by rising and gaining altitude.
When the air rises, the temperature falls, and the air cools. Unable to hold vapour, the water becomes rain. Since the Himalayas are so tall, by the time the rain reaches the Tibetan Plateau and Ladakh, it has cooled sufficiently to release almost all the moisture it can hold. Therefore, Tibet and Ladakh only receive dry winds, which equalise the low pressure but bring very little rain.
How does the Wind react due to the Difference in Temperature?
As Winter approaches, the reverse happens. However, the air is dried as it has lost all the moisture over land. Air coming from the North East of India and Bengal passes over the Bay of Bengal before reaching Tamilnadu. This monsoon is called the Returning Monsoon.