The monsoon is a system of winds in which the direction of the winds is reversed between the summer and the winter season. According to the traditional thermal theory of monsoon they are a result of the differential heating of land masses and the oceans. In summer the land area gets heated more than the sea and hence low pressure develops over the interior parts of the land masses. Thus in summer season winds blow from sea to the land. During winter season a high pressure is formed over the continents as the temperature there is very low. On the other hand the pressure over the oceans is comparatively lower and the winds start blowing from land to the sea. This forms the monsoon system. Monsoon winds are prominent in India, Pakistan and other Southeast Asian countries.
However, the modern meteorologists do not explain the monsoon in terms of the differential heating of the land and sea. They explain it in terms of the dynamic theory of monsoon. According to the dynamic theory of monsoon, this system of winds is a result of the poleward shift of the Inter-tropical Convergence (ITC) under the influence of the vertical sun during the summer season. During the northern summer, in the months of May and June the sun shines vertically near the Tropic of Cancer and the ITC shifts northward of the equator. The ITC is the convergence zone of the Trade winds blowing from northeast in the northern hemisphere and from the southeast in the southern hemisphere. As the ITC shifts northward of the equator, the southeast trade winds start blowing north of the equator to reach the ITC and as they cross the equator, their direction is altered due to the influence of the Coriollis force, i.e. they are deflected towards their right and thus it gives rise to the equatorial westerlies blowing between the equator and the ITC. These westerlies in the months of May and June blow from the equator towards the ITC from southwest to northeast and they are called the southwest monsoon.
During the winter season the ITC again moves southward and the areas north of the equator which experienced the equatorial westerlies during the summer season, now come under the influence of the northeast trade winds. These northeasterly winds are called the northeast monsoon. During this very season the ITC shifts southward of the equator and the northeast trades blowing towards ITC, get deflected upon crossing the equator southward. Here they give rise to the equatorial westerlies blowing from northwest to southeast, replacing the trade winds of the southern hemisphere between the ITC and the equator. Thus the areas situated in the tropical zone come under the influence of the trade winds during the respective winter and the equatorial westerlies during the respective summer season. Thus the direction of the winds is reversed seasonally and it makes up the monsoon system of these regions.
Local winds owe their characteristics to the local topography or the local temperature conditions. One of the local winds blowing in the northern plains of India in the summer season are the loo winds. They blow from the Thar Desert into the Ganga Plains and cause the temperature to rise in these plains. Similarly a warm dry wind blows on the eastern side of the Rockey mountains in North America. The warm character of this wind is result of the descending current of air on the eastern slopes of the mountain range. This wind is called Chinook. The very cold winds of polar origin blowing into the temperate regions are called northers. Local winds blowing in the Arctic and Antarctic regions are called blizzards. In fact the local winds blow in many parts of the world and they are known by different names. Fohn, mistral, sirocco and khamsin are some of the other local winds blowing in various parts of the world. Winds owing their character to the surface features and ascent or descent over them are also called the katabatic winds. Fohn and Chinook are examples of katabatic winds.
Another important group of local winds are the land breeze and sea breeze. They blow in the coastal regions and their direction changes on a daily basis. Due to a higher temperature and lower pressure over the land during the day the winds in the coastal regions blow from sea to land. At night the continent cools rapidly and the oceans is comparatively warmer and has a lower pressure. Therefore the winds blow from land to sea at night. This system of winds is called the system of land and sea breeze. A similar reversal of direction of winds occurs in the mountains also where it produces the valley and mountain winds. The valley winds blow from lower valleys to the upper slopes during the day as the temperature over the upper slopes is higher and the warmer air there rises upwards. At night the rapid cooling of the upper slopes makes the air above them heavier and denser which flows down-slope into the valleys. These winds are called the valley winds.
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