As pointed out earlier, the characteristics of monsoon climate are to be found mainly in the Indian subcontinent. The Indian Meteorological Department divides the annual changes into the following four seasons:
(a) Winter season, January-February
(b) Hot weather season, March-May
(c) Rainy season, June-September
(d) Post-monsoon season, October-December.
(a) Winter season:
In winter the anticyclonic subsident air prevails over the subcontinent. The cold weather season is characterized by the out-flowing winds, dry and stable air, and cleat skies. Since there is relatively high pressure over northwest India, a northwesterly flow prevails down the Indus and Ganges Valleys.
During this season the southern branch of subtropical jet stream is positioned over northern India. The middle latitude westerlies reach down to the surface north of about 25°N. South of this latitude the general movement of air is from the northeast.
This northeasterly wind is called the winter monsoon. Since the northerly wind blowing over central India is not the result of differential heating of land and water, it is not monsoon in the strict sense of the term.
In Peninsular India the general direction of wind is from east to west. Because of its trajectory over the Bay of Bengal the easterlies are full of moisture and yield some precipitation along south-east coastal regions.
In northwestern region of the sub-continent winter precipitation is caused by the depressions that are associated with the westerly disturbances moving out from the Mediterranean Sea.
The cold-weather precipitation, though moderate to scanty, is beneficial to ‘rabi’ crops. Besides, snowfall from these moving depressions feed the western Himalayan glaciers.
(b) Hot weather season:
The winter season gradually gives way to the hot and dry season as the sun is nearly overhead in April and May. Anticyclonic subsidence still prevails and the weather is generally dry. The skies are almost cloudless.
It is during this period that highest temperatures are recorded in the northern part of the sub-continent. On the contrary, South India has a much more equable temperature regime. Except the extreme south, the northeast and coastal areas, rest of the Indian sub-continent remains practically dry.
Precipitation in the southern extremity of Indian Peninsula is brought about by inter-tropical convergence and equatorial westerlies which have moved towards north. The coastal areas receive some rainfall from the intensified sea breeze full of moisture.
The northeastern regions are characterized by abundant precipitation associated with strong convective activity. West Bengal, Assam, Bangladesh and Burma receive convectional precipitation during this season lasting from March to May. The spring-time rains are referred to as ‘mango showers.’
In the northern plains, vigorous convective systems occasionally produce thundershowers. In northwestern part of the sub-continent the rainfall is light, but the skies are covered with cumulonimbus clouds produced by strong convection.
Other climatic characteristics of this season are strong squall winds and dust storms. These violent dust storms are designated as andhis or nor’ westers in different localities.
Sometimes they are accompanied by heavy downpours. Most of these dust storms are confined to the afternoon or late evenings. They generally travel form northwest to southeast. However, there is a lot of controversy about their origin.
(c) Rainy season:
The abruptness with which south-west monsoon arrives is the most peculiar feature of rainy season extending from June to September. Even though the monsoon rains begin in Burma in May or even late April, they arrive in India several weeks later.
First of all the monsoon arrives in southern part of India, but soon afterwards it rapidly spreads over the whole country. The northward advance of south-west rain-bearing monsoon is invariably associated with thunderstorms and squall winds.
However, once the monsoon is well established, the weather is not so turbulent and the number of thunderstorms decreases. The weather during rainy season is labelled as muggy.
There is, however, a marked fall in temperature because of heavy rains. But if the rains stop, the weather during the months of July and August becomes hot and enervating.
(d) Post-monsoon season:
This season is called autumn in India and lasts from October to December. This is the period of retreating monsoon. The inter-tropical convergence zone now moves towards the equator. The temperature starts falling gradually.
There are occasional showers. Wind direction undergoes a change. During post-monsoon period winds blow from westerly direction in northern India, while in the south the northeasterlies prevail, as the so-called winter monsoon gradually establishes itself.
The most important climatic characteristic of this season is the weakening of low pressure centre over the subcontinent as a whole. But the low pressure centre is found over the Bay of Bengal where water surface temperatures reach a maximum in this season.
A number of cyclones form over the southern part of the Bay. Most of these cyclones curve round to the north of central low and reach the east coast from west. Sometimes these cyclones become very violent.
The northern coastal districts of Tamilnadu receive a number of heavy showers from these depressions during October and November. The southern districts are lashed by such cyclonic rainfall during the months of November and December.
According to Spate, cold weather precipitation of the south eastern pat of India is in fact brought about by the retreating southwest monsoon and not by the northeast monsoon. This fact is contrary to the general belief.
The close of rainy season is the most uncomfortable part of the year. In northern India, however, there is a sudden drop of temperature during the month of November.