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Monsoon – India’s Life line

Monsoon is basically a periodic wind, especially in the Indian Ocean and southern Asia. Monsoon has become the synonym of the season in which wind blows from the southwest in India and adjacent areas which brings rainfall to the Indian Sub-continent.

The word "monsoon" is known to have originated from the
Arabic word "mausem", which means season.

It is often applied to the seasonal movements wind along the shores of the Indian Ocean, especially in the Arabian Sea and the Bay of Bengal, that blow from the southwest during June-September and from the northeast during December to March.

The annual monsoon cycle can be physically described as a result of the annual variation of incoming solar radiation and the differential heating at the surface of land and water. Simply stated, sections of the earth's surface heat and cool at different rates depending on their ability to absorb solar radiation and the time of year. Bodies of water, which can absorb sunlight at varying depths (and consequently reflect less back to the atmosphere), store energy more efficiently than land and therefore retain heat longer than a land mass. Land surfaces gain or lose heat at a quicker rate due to the shallowness of their absorbing surfaces. To maintain an energy balance, heat is transferred from areas of surplus to deficit, and in the case of a land-water differential, this is accomplished through a phenomenon known as the "land-sea breeze". For example, on a sunny day at the beach, the land warms more quickly than the ocean. As the hot air rises over the land, it is replaced by the cooler air over the water. At night, however, the land cools at a quicker rate than the water, so the wind shifts, blowing from the land to the warmer water.

On a larger scale, such as a continent surrounded by oceans, heat build up on land over time results in lower density air masses, or areas of low pressure. On the contrary, denser air associated with high pressure dominates ocean surfaces. Wind and ocean currents that result from air flowing from high tolow pressure mix areas of warmer and colder air and water, contributing to the global energy balance. This exchange is evident at different levels of the atmosphere. Air converging into alow pressure center at the surface rises, leading to moisture condensation and the subsequent release of heat into the upper atmosphere. Diverging air at the surface in a high pressure center is associated with subsiding air from the upper atmosphere and evaporation, a mechanism for energy storage.

Just as energy imbalances develop between land and water surfaces, the variation in space and time of solar heating due to the earth's tilt create seasonal hemispheric energy imbalances. The hemisphere receiving the most direct radiation (during the summer months) experiences a net radiative heating (more energy is gained from the sun than is lost to space). The winter hemisphere is at the same time experiencing net radiative cooling. As part of a global compensation, heat is transported from warmer to cooler areas by ocean and wind currents. Since the areas of heat surplus and deficit change throughout the year, as in the sea breeze example, the direction of transport must change as well. As noted earlier, climates dominated by monsoons experience the most pronounced seasonal wind shifts, indicative of a pronounced land-sea effect. In the South Asian example, the rainy season, typically beginning in June, is preceded by nearly two months of scorching temperatures, cooled only with the commencement of the summer rains brought by the south-westerly winds and themonsoon. January is the peak of the dry season, which is marked by cool, dry northeasterly flow over most of the region.

Indian economy which is still mainly an agricultural economy – is dependant on the amount of monsoon rains as a large part of the agricultural produce comes from the monsoon fed crops. Good monsoon always means a good harvest and brings in cheers all around India. A weak or bad monsoon is always considered as a big set back to India’s economy and always results in a big loss in the country GDP levels.

To under monsoon –it’s definition has to be been broadened to include the phenomena associated with the annual weather cycle within the tropical and subtropical continents of Asia, Australia, and Africa and the adjacent seas and oceans.

It is within these regions that the most significant, vigorous and dramatic cycles of weather events on the globe takes place.

Monsoon has two phases

  1. North-East Winter Monsoon
  2. South-West Summer Monsoon

North-east Winter Monsoon

The NE winter monsoons take place between December to early March. During this season low temperature over central Asia , creates a zone of high pressure. The jet stream splits into two, the southern sub-tropical jet and the polar jet. The sub-tropical jet forces north-easterly winds to blow across south Asia which creates dry air streams which also produces clear skies over India during the months of November to May. Meanwhile, alow pressure system develops over northern Australia and winds are directed towards Australia.

During the NE winter monsoon, Eastern coastal parts of Southern India, Australia and South-east Asia receive good amount of rainfall.

South-West Summer Monsoon

The South-western summer monsoon occurs between June to September. After shifting of winds during the months of March and May- an intenselow pressure system develops over central Asia, and the jet stream starts blowing over this area.

The summer, monsoon is caused by the low pressure that develops over S Asia as the landmass warms during the summer season. Moisture-laden air over the oceans is drawn toward this center oflow pressure . The air cools as it ascends the slopes of mountain barriers and it can no longer retain moisture, which results in heavy rainfall.

The south-eastern winds blow towards the area of low pressure over Asia, passing over south-east Asia, and this brings in large amounts of rainfall in this period. Meanwhile, the south-westmonsoon is drawn towards the Himalayas, creating winds and blowing rain clouds towards India. This brings substantial rains and some areas receive up to 10,000 mm of rain.

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