Seasons: The Cycle of Seasons in India (with map and diagrams)!
The Indian monsoons may be studied in the framework of four distinct seasons over an annual cycle. The winter season lasts for the months of December, January and February. The season is mostly dry as the northeast trade winds blow from the land to the sea.
The cold is severe in the northern parts of the country and only mild in the southern regions. In the northern plains, January temperature may be around 10°C while in the south mean temperatures could be 25°C. The days are generally warm. Nights are cold. Snowfall and frost formation take place at places situated on high altitudes.
Winters are the ideal season, specially for the tourists from foreign countries. The days are sunny with cool breezes. Humidity is low and practically rainless months make travelling easy and uninterrupted. Western disturbances cause light rainfall during the winter months. These showers greatly benefit the winter crops like wheat, sugarcane, mustard and gram. Western disturbances are caused by shallow cyclonic depressions originating over the east Mediterranean Sea.
Travelling eastward, they reach the north-western parts of India. On their way, they are loaded with more moisture from the Caspian Sea and the Persian Gulf. Apart from light rains over north-western parts of the country, the western disturbances also lead to snowfall in the western Himalayan regions. This snow melts during the summer months and feeds the river systems originating from Himalayas.
Winter rains are often followed by cold waves. There is sudden drop of temperature during the period of the cold wave. The north-east trade winds also known as north-east monsoons bring winter rains to Tamil Nadu. That is the only part of India that gets winter rains.
The summer season lasts from March to May. The heat belt keeps on shifting from south to the northern parts of the country. In the Deccan plateau, the highest temperatures may be recorded in March. Gujarat and Madhya Pradesh experience the hot season during April. By May, summer sets in covering west parts of India. Maximum temperatures shoot up to 48°C in these areas.
Rising temperatures lead to creation of low pressure in the northern regions of the country. By the end of May, a low pressure trough develops extending from the desert regions in the north-west to east and south-east regions covering Bihar and the plateau of Chota Nagpur.
May is the season of loo, the hot and dry winds that blow during the day and often even at night. Heat strokes are common with direct exposure to these winds. Intense heat during the day brings dust storms by the evening. Occasionally, the storms are accompanied by light showers. The showers are a great relief from the scorching heat.
But often these showers bring with them hailstorms. This is the time when winter crops lie in the fields waiting to be shifted to the farmers, house or the mandi. Torrential rains or hailstorms cause heavy damage in such cases.
Well before the onset of the monsoon season, coastal areas of Kerala and Karnataka experience pre-monsoon showers. These however remain localized. A high pressure belt over the Deccan plateau checks their further advance to the north.
Cyclones and Anti-cyclones:
These are winds that keep on changing their directions. The cyclone has a low pressure at its centre and a circular wind motion. The motion is counter clockwise in the northern hemisphere and clockwise in the southern. A cyclone is often accompanied by heavy rain.
Tropical cyclones are violent and are also known as hurricanes or typhoons. Developing over water bodies, they cause wide spread destruction. Anticyclone has a high pressure area at its centre surrounded by a low pressure all around. Wind blows out from the centre to all directions.
These winds blow permanently towards equator from the sub-tropical high pressure belts. Winds are named after the direction from which they blow. The north-east trade winds actually blow in the south-west direction. The nomenclature is with respect to the direction from which wind is blowing.
Winds moving from high pressure to low pressure belts do not blow directly north to south or otherwise. They are deflected to the right in the northern hemisphere due to rotation of earth and vice versa in the southern hemisphere [Ferral’s Law].
Precipitation and Rainfall:
Precipitation refers to moisture that is deposited from the atmosphere in the form of dew, frost, rain, sleet, hail or snow. Winds loaded with moisture often come across hills and mountains in their way. These obstructions make them rise up.
In this process, the winds get cooled forming clouds. As the cold increases, condensation forms droplets which come together and form bigger drops. The bigger drops cannot float and fall down as rain. In their downward journey, these falling drops pick up many more small droplets.
June to September is the period for monsoon rains. About 75 to 90% of the total rainfall takes place during these months. The rainfall is, however, not evenly distributed. The north-west areas get rains for practically two months only. The rainy season is relatively longer in the eastern regions. A similar variation occurs between regions in the south and those in the north. The duration is longer in the south than in the north.
The continued low pressure over the north-west region attracts trade winds from the Southern Hemisphere. These winds come from the India Ocean. As they cross over the Bay of Bengal and the Arabian Sea, they are trapped by air circulation over India. These winds are loaded with abundant moisture. They replace the north-east trade winds that blow during the winter months.
The moisture loaded winds blow at a very fast speed. Their approach is marked by thunder and lightning. This is referred to as ‘breaking of monsoon’. The actual amount of rain from the south-west monsoons is not uniform. The relief features influence it to a large degree. Areas of the Western Ghats that are situated in the direction of the wind receive maximum rainfall. The other side gets much less.
The variation is between over 250 cm to 50 cm. The hilly ranges of the north-east cause heavy rainfall in the north-eastern states. There is a consistent decline in the amount of rainfall from east to west. Delhi gets rain that is less than half of what Kolkata gets. In between, Allahabad gets lesser rain than the eastward situated Patna.
Unlike the trade winds, the monsoons are not regular. There are rainy days followed by rainless days during the monsoon months. The cyclonic depressions that are formed at the head of the Bay of Bengal are responsible for the uneven and uncertain distribution of rainfall. These depressions travel towards the low pressure monsoon trough. The trough is not steadily placed. It moves northwards or southwards for a variety of reasons.
The plains would have rains or not depends upon the monsoon trough. If the axis of the trough shifts close to the Himalayas, there are dry spells in the plains but heavy damage to the plains year after year. The monsoons are also affected by changes in the pressure conditions over the southern oceans. While when high pressure prevails over eastern South Pacific Ocean, low pressure is formed in the tropical eastern South Indian Ocean.
In certain years, these pressure conditions are reversed. The intensity of the monsoons gets affected by the Southern Oscillation. If the pressure differences over Pacific Ocean and Indian Ocean are negative, the monsoons could be late and below average.
Another development that affects the intensity of the trade winds is the periodic development of a warm ocean current along the coast of Perty. The Peruvian current is cold but every 2.5 years, during Christmas time, it is replaced by the warm ocean current named El Nino. The worm ocean current increases sea surface temperatures.
Indian farmers still depend upon monsoons for their irrigation needs. A dry spell causes as much ruin as the flooded rivers. There is nothing certain about the monsoons. Their arrival, stay and departure are uncontrolled. When and where they would cause prosperity or devastation is unpredictable. No wonder Indian farmers continue to have faith in the whims and mercy of Indra-the God of rains.
Retreating Monsoon Season:
The months of October and November are known for the retreating monsoons. As the monsoon trough becomes weaker with the approach of winter months, it starts retreating. There is reversal in the direction of the surface winds. As October sets in, the monsoons withdraw from the northern plains.
The sky now becomes clear due to the absence of clouds. This results in oppressive heat as it is combined with humidity arising out of moist land. There is now a shift in the low pressure conditions from the north-western plains to the Bay of Bengal.
During middle of November, this shift results in occurrence of cyclones. Tropical cyclones cause rains on the Coromondal coast. The deltas of Krishna and Kaveri along with Bangladesh have to hear the fury of these cyclones every year.
Distribution of Average Annual Rainfall:
The average annual rainfall varies from year to year and from region to region. The areas of low rainfall include Rajasthan, parts of Gujarat, interior of Deccan plateau and areas around Leh. Most other areas receive moderate rainfall. The states in the north-east are recipient of rainfall that may exceed 400 cm.
Monsoons as an Integrating Force:
Monsoons, despite their uncertainties, act as a unifying force so far the climatic unity of India is concerned. July and August are wet all over the country. Not with standing their vagaries, the monsoons have a very important place in the cultural life of the people. There is so much music and poetry that centre’s round the rains.
Festivals like Varsha Mangal, have their equivalents in a number of north Indian regions. For the farmers, rains bring prosperity. For others it is the season for romantic creations. Either way, the monsoons act as an integrating force.