The Concept and Mechanism of Monsoon, Cycle of Seasons in India Part 2

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The Concept and Mechanism of Monsoon

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.

The monsoon is caused by the differential heating of land and sea. Due to a higher temperature over the land in summer, a low-pressure area develops over the continents and the winds blow from neighbouring oceans towards the land. These winds are of maritime origin and hence cause ample rainfall in summer. The continents become colder than the neighbouring oceans in winter. Hence, 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. This traditional theory of monsoon has been criticized by the German meteorologist Flohn. He argues that the differential heating of land and sea is not enough to cause a seasonal reversal of winds at a global scale. He has explained the origin of the monsoon on the basis of seasonal shift of the pressure and wind belts under the influence of the shift of the vertical rays of the sun.

According to this theory, as the vertical rays of the sun shift northwards over the Tropic of Cancer in summer season, the Inter Tropical Convergence Zone (ITCZ) also shifts to the north. This results in the formation of a low-pressure area over the north-western parts of India. This low pressure is further intensified by high temperatures in this region. This low-pressure area sucks the air from the Indian Ocean towards the Indian landmass in the form of southwest monsoons. In winter season, the ITCZ shifts southwards and a mild high pressure is produced over northern parts of India. This high pressure is further intensified by the equatorward shift of the sub-tropical high-pressure belt. Due to high pressure over northern India, the winds start blowing from northeast as retreating monsoons.

According to recent observations, the origin of Indian monsoon is influenced by a number of other factors, besides the differential heating of land and sea and the seasonal shifts of pressure and wind belts. One of the most important factors among these is the system of subtropical westerly and tropical easterly jet streams. The subtropical westerly jet streams blowing over India in winter cause a high pressure over northern India. It thus intensifies the northeast monsoons. This jet stream shifts northwards beyond India in summer season and tropical easterly jets develop over India in this season. The behaviour of this jet streams is partly responsible for the variations in the time of onset of southwest monsoons over India.

Cycle of Seasons in India

The complete reversal of direction of winds is the most striking feature of the monsoons. These changing monsoon winds result in the change of seasons over the year. Climatically, the year is divided into following four seasons in India:

The Cold Weather Season

This season usually begins with late November in northern India. January is the coldest month over most parts of the country as sun shines vertically over the Tropic of Capricorn in the southern hemisphere. During these months, the mean daily temperatures remain below 21°C over northern plains and northern mountain regions. The night temperatures sometimes fall below freezing point resulting in widespread damage to the standing crops due to frost. The temperature increases as one moves from north to south. As a result of low temperatures, a feeble high-pressure area develops over northern parts of India. This mild high pressure causes the off-shore northeast monsoon winds. Their direction in the Northern Plains is westerly owing to the relief. These land bearing winds being cold and dry don’t give rain over most parts of the country. However, these winds cause rain along the Coromandel coast since they collect moisture on their way over the Bay of Bengal.

Mean Temperature and Pressure (January)

Mean Temperature and Pressure (January)

Mean Temperature and Pressure (January)

The succession of depressions is another feature of this season. These low-pressure systems are called western disturbances as they originate in the Mediterranean region. These depressions move with the westerly jet streams. Covering a long distance over Iraq, Iran and Pakistan, they reach India around mid-December. Their arrival results in the increase of temperature and in light rains over northern plains. They cause wide spread snowfall over western Himalayas and the adjoining ranges.

The peninsular India has no well-defined winter season. The mean monthly temperatures in the month of January is above 20°C. Moreover, the coastal plains hardly experience any seasonal change as is clear from the mean monthly temperature of above 27°C at Thiruvananthapuram. But Chennai records a temperature of 25°C during December and early January owing to the rains caused by Northeast monsoon winds.

The Hot Weather Season

The apparent movement of the sun towards the north increases the temperatures in the northern plains. As a result, the spring sets in soon giving way to the hot weather season which lasts till end of June in this region. The temperatures increase northwards and reach around 45°C in mid-May in most parts of the northern plains. The characteristic features of this season are afternoon dust storms and ‘Loo’ which is a hot dry wind which blows during the months of May and June mainly over the northern plains. These winds cause heat stroke resulting in deaths of hundreds of people every year. The day temperatures at times rise above 45°C in some north-western parts of the country.

Mean Temperature and Pressure (April)

Mean Temperature and Pressure (April)

Mean Temperature and Pressure (April)

The wind direction is variable during this season. The weather conditions are generally hot and dry throughout the country. However, dust storms cause drizzle in Northern Plains. Light showers are also experienced in Kerala, West Bengal, and Assam. In Kerala, these pre-monsoon showers are popularly known as Mango Showers. In West Bengal and Assam, they are called Northwesters or Kal Baisakhi. Sometimes, due to high velocity of winds these Northwesters cause heavy loss of life and property.