Seasons in India

There are four seasons in India viz. Winter (December-February), Hot weather summer (March-May), Rainy south-western monsoon (June-September) and Post-monsoon, also known as north-east monsoon in the southern Peninsula (October-November).

Winter Season

During the winter season, the temperature decreases with increasing latitude in India from 25°C in South to near zero temperature on north. This season is characterised by Fog and Frost in North and North-West India. There is light rainfall in this region due to Western disturbances. There is a sustained snowfall on the higher slopes of the Himalayas.

North East Monsoon

In India, rains occur in winter due to the North East Monsoon. During the winter season, North-East trade winds prevail over India. They blow from land to sea and that is why that for most part of the country, it is a dry season. A part of North-East trade winds blow over Bay of Bengal. They gather moisture which causes rainfall in the Coromandal coast while the rest of the country remains dry.  In the northern part of the country the weather is marked by clear sky, low temperatures and low humidity. The winter rainfall is very important for the cultivation of ‘Rabi’ crops.

Impact of Jet streams in Winter

Jet streams are fast flowing, narrow air currents located near the Tropopause, the transition between the troposphere and the stratosphere. The major jet streams on Earth are westerly winds (flowing west to east). Their paths typically have a meandering shape; jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including the opposite direction of most of the jet.

The strongest jet streams are the polar jets, at around 7–12 km above sea level, and the higher and somewhat weaker subtropical jets at around 10–16 km. The Northern Hemisphere and the Southern Hemisphere each have both a polar jet and a subtropical jet. The northern hemisphere polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the southern hemisphere polar jet mostly circles Antarctica all year round.

The Jet streams are upper level, irregular, concentrated, meandering bands of westerly winds that travel at speeds of 300 to 400 kmph and come to India from the Mediterranean side in winter. This jet stream is bifurcated due to the physical obstruction of the Himalayas and Tibetan Plateau. One branch is located to the south of the Himalayas, while the second branch is positioned to the north of the Tibetan Plateau. The  southern branch  blows  eastwards  south  of the  Himalayan  ranges  along  25°  N  latitude (Rajasthan., MP, Chhattisgarh etc.) .  These winds tend to descend over the north-western parts of India, resulting into the development of atmospheric stability and dry conditions.  It  is  believed  that  this  branch  of  jet  stream  exercises  a  significant  influence  on  the  winter weather conditions in India. This jet stream is responsible for bringing western disturbances from the Mediterranean region into Indian sub-continent. Winter rain and hail storms in north western plains and occasional heavy snowfall in hilly regions are caused by these disturbances. These are generally followed by cold waves in whole of northern plains.

Western Disturbances

Western Disturbances are basically the temperate cyclones that originate in the Mediterranean Sea and west Asia and happen to reach Afghanistan and Pakistan.  In winters, they cross the North West borders of India and reach up to Central India. These disturbances bring small winter rains in India which are locally called Mahavat (Rajasthan, Haryana, Punjab) and are beneficial for the Rabi Crops. They also bring cold waves and snowfall in the higher altitudes of the Jammu and Kashmir and Himachal Pradesh.

Summer Season

By the end of February the temperature starts rising and there is a hot weather season in India from March to May. During these months the central part of peninsular India expiries extreme hot weather and an elongated low pressure belt which is called monsoonal trough created, which extends from Jaisalmer in western Rajasthan to Jharkhand and parts of Odisha to the East.

However, over Indian Ocean south of the equator high pressure belt begins to develop in this season. In North-West India, afternoon dust storms are common. During summer, very hot and dry winds blow over North Indian plains. They are locally called ‘Loo’. At the same time, localized thunderstorms, associated with violent winds, torrential downpours, often accompanied by hail occur in many parts of India.

• In West Bengal, these storms are known as the ‘Kaal Baisakhi’ (calamity for the month of Baisakh).
• Towards the close of the summer season, pre-monsoon showers are common, especially in Kerala and Karnataka, which help in the early ripening of mangoes, and are often referred to as ‘mango showers’.
• The thunderstorms which occur during summer to bring some rainfall in Karnataka are also known as Cherry Blossom Showers.

Rainy south-western monsoon (June-September)

June to September are the months of advancing South-West monsoon season. By the end of May, the monsoon trough further intensifies over north India due to high temperature. The General direction of the wind during this season is from South-West to north-east. These winds are strong and blow at an average velocity of 30 km per hour. These moisture laden winds first hit at Andaman and Nicobar Islands in the last week of May and Kerala coast in the first week of June with violent thunder and lightning. This South-West monsoon that flows in to India brings about a major change in its weather. Two branches of south-west monsoon originate from:

• Arabian Sea
• Bay of Bengal.

The Arabian Sea Branch obstructed by Western Ghats gives heavy rainfall on the Western side of Western Ghats. It reaches Mumbai by 10th June. When this branch crosses the Western Ghats and reaches the Deccan Plateau and parts of Madhya Pradesh, it gives less rainfall as it is a rain shadow region. Further, this branch reaches in Northern Plain by 20th June.

Rains in Bangalore versus Mangalore

Bangalore receives less rainfall in comparison to Mangalore because Bangalore is located in the rain shadow (Leeward Side) of Western Ghats and when the wind blows from the west, it gets obstructed by the hills. Thus the moisture laden wind blows to the windward side of the ghats, causing heavy rainfall in the coastal region and ghat areas and the rainfall in Bangalore is limited. However, you must note here that during weak monsoon condition when there is ample sunshine, the lower levels of the atmosphere are warm which gives rise to convection current. The air goes up leading to the formation of clouds, resulting in rainfall in the city.

Similarly, Pune receives less rainfall because it is also located on lewardside of western Ghats. Other examples are Shillong and Hyderabad. Shillong lies on the northern leeward slopes of the Khasi Hills and therefore receives less rain.

The monsoon winds that move from Bay of Bengal strike Andaman and Nicobar Islands North-Eastern states and coastal areas of West Bengal and cover the whole of India by the 15th of July. They cause heavy rainfall in the region. However, quantity of rainfall decreases as they move towards West over the Northern plains. For examples rainfall at Kolkata is 120 cm, Allahabad 91 cm and Delhi 56cm.

Post Monsoon Season

October and November are the months of post (or retreating) monsoon season. The temperatures during September-October start decreasing in north India. Monsoonal trough also becomes weak over North-West India. This is gradually replaced by a high pressure system. The South-West monsoon winds weaken and start withdrawing gradually from North Indian Plains by November. In October the weather remains humid and warm due to continuing high temperature and moist land in month of October. In Northern plains hot and humid weather becomes oppressive at this time. It is commonly called ‘October Heat’. However, towards the end of October, temperature starts decreasing, making nights pleasant.

By the month of November, the low pressure of North India shifts to Bay of Bengal and this is the time of cyclonic storms which develop in the Bay of Bengal. These storms create havoc in coastal areas of Odisha, Andhra Pradesh and Tamil Nadu, especially in the deltas of Mahanadi, Godavari and Krishna rivers.