Monsoons refer to a system of winds in the tropical regions under which the direction of winds is reversed completely between the summer and the winter seasons. Under this system, the winds blow from land to sea in winter and from sea to land in summer. Therefore, most of the rainfall in the regions influenced by the monsoons is received in the summer season while winter season is generally dry.
Due to a higher temperature over the land in summer, air in the region gets heated. Hot air rises, low pressure area is created under it. This low pressure is also known as monsoonal trough. On the other hand temperature over Indian Ocean is relatively low, as water needs more time to get heated as compared to land. So a relatively high pressure region is created over the sea.
Thus, there is a difference of temperature and resultant pressure over North Central Indian Plains and Indian Ocean. Due to this difference, air from high pressure region of the sea starts moving towards the low pressure region of North India. Thus, by mid June the general movement of air is from equatorial region of Indian Ocean to the Indian subcontinent and the direction of these winds in general is from South-West to North-East. This direction is exactly opposite to that of the trade winds (North-East to South-West) prevailing during winter in India. This complete reversal of wind direction from North-East to South West and vice-versa is known as monsoons.
In winter, the continents become colder than the neighboring oceans. As a result a high pressure area is developed over the continents. Therefore, winds blow from land to sea in winter. These winds, being of continental origin, are dry and do not cause rain.
During winter, the weather conditions is influenced by high pressure developed over Northwestern part of the subcontinent. This results in the blowing of cold dry winds from this regions towards southern low pressure areas lying over water bodies surrounding peninsular India.
During summer, the northwestern parts of India become very hot due to very high temperature. This is ascribed to the apparent shift of the sun in northern hemisphere. This results in the reversal of air pressure conditions not only in northwestern India but also on water bodies surrounding the peninsular. As a result, Northeast Trade winds are replaced by Southwest monsoon winds.
Since these winds are sea bearing and blow over warm water bodies before reaching land, they are moisture laden, causing wide spread rain over the most parts of India. This period of southwest monsoon from June to September, is known as the rainy season for most parts of the country.