Wind is air in motion. It is usually the natural horizontal motion of the atmosphere. Winds are produced by differences in atmospheric pressure, which are primarily attributable to differences in temperature. Variations in the distribution of pressure and temperature are caused largely by unequal distribution of heat from the sun, together with differences in the thermal properties of land and ocean surfaces. When the temperatures of adjacent regions become unequal, the warmer air tends to rise and flow over the colder, heavier air. Winds initiated in this way are usually greatly modified by the earth's rotation. Winds may be classified into four major types:
In the northern hemisphere, the northerly wind blowing toward the equator is deflected by the rotation of the earth to become north-easterly and is known as the North east trade wind. In the southern hemisphere, the southerly wind, which is similarly deflected, becomes south-easterly and is known as the Southeast trade wind.On the polar side of the horse latitudes in either hemisphere, the atmospheric pressure diminishes toward low-pressure centres in middle and high latitudes. The winds set in motion poleward by these pressure systems are deflected toward the East by the earth's rotation.
Because winds are known by the direction from which they blow, the winds in middle latitudes are known as the prevailing westerlies. These westerlies are greatly affected by raveling cyclonic and anticyclonic disturbances that cause their actual direction to change greatly from day to day. The colder regions at the poles tend to be high-pressure centres, particularly in the southern hemisphere, and winds spreading out from these areas are deflected to become the polar easterlies. Considerably stronger winds, however, occur near the centres of tornadoes. With increasing elevation above the surface of the earth, the prevailing westerlies increase in speed, and their range of latitudes expands toward the equator and the pole. The trade winds and polar easterlies are thus relatively shallow, and are generally replaced by westerlies above a few thousand meters. The strongest westerlies occur at heights of about 10 to 20 km, and tend to be concentrated in a rather narrow belt called the jet stream, where wind speeds as high as 550 km/h have been measured.
In winter, most parts of Asia create a high pressure system because of the cooling of the air. On the other hand, in the southern hemisphere, conditions are reversed and the summer season creates a low pressure system over Australia. Winds blow from the Asian high pressure system as the North east monsoon, crosses the equator and is deflected to the left to reach Australia as the north west monsoon.
Similar daily changes in temperature occur over irregular terrain and cause mountain and valley breezes. Other winds induced by local phenomena include whirlwinds and winds associated with thunderstorms.
At the rear of the bulge, the colder air sinks under the warm air forming the cold front. The passing of the cold front is accompanied by very strong winds from the North west, falling temperature and rising pressure. Thick cumulonimbus clouds are formed when the cold air lifts the warm air over the cold air aggressively. Heavy rain falls but it will not last long. Eventually, the cold front catches up with the warm front and lifts it off the ground. It then becomes an occluded front and the cyclone soon dies out. Weather associated is drizzle, thick clouds followed by heavy rain then showers. The depression then slowly fades away.
The tropical cyclone fades away when it reaches land because there is no large supply of warm moist air. The tropical cyclone consists of the eye in the centre where the pressure is the lowest, the front vortex and the back vortex where the winds spiral rapidly upwards to form cumulonimbus clouds and heavy rain. The eye of the cyclone is calm and dry as the centrifugal force prevents the winds from entering the eye.