The Cycle of Seasons in India

India has four seasons as mentioned below.

• Winter season commences in January and remains till the end of February.
• During this period, due to the sun’s apparent movement towards the south, vertical rays of the sun fall over the Southern Hemisphere. That is why the temperature starts to decrease in the Northern Hemisphere, and pressure belts along with Inter Tropical Convergence Zone (ITCZ) start to move southwards.  
• As the temperature over Indian Subcontinent starts to decrease in winter, the pressure starts to increase. The high pressure replaces the low pressure over Northern India.
• The Inter-Tropical Convergence Zone (ITCZ) starts to move southwards and gradually moves out of India. As a result, monsoon winds also start to retreat, moving towards ITCZ, i.e. from North-West to South-East. These winds are dry as they move from land towards the sea and have already shed moisture. These are known as retreating monsoon or North East Monsoon.

Cold Waves

• During this period, high-pressure areas develop over central and western Asia. Dry and cold continental winds from the Caspian Sea and Turkmenistan enter the Northern Plains, which cause a fall of many degrees in the temperature of the great plains. These north-western cold winds, moving at a speed of 3 to 5 km per hour, are termed as the Cold Waves.

Jet Stream and Upper Air Circulation

• Along with that, Sub-Tropical Westerly Jet (STWJ), flowing over Tibet Plateau in the summers, shifts southwards and creates high pressure over North India. STWJ extends from the Mediterranean sea to Western Asia, the Persian Gulf, Iran and Pakistan, crosses the great plains of India and reaches the China sea. 

Rainfall due to North East Monsoon over Coromandal Coast in Winter

• Rain occurs in winter due to the North-East monsoon as well. 
• When Inter-Tropical Convergence Zone (ITCZ) moves out of India, dry continental winds start to blow from North East to South-West.  
• But a part of North-East winds pass over the Bay of Bengal & cause rainfall on the Coromandal Coast. It is known as North-East Monsoon and precipitates over Coromandal Coast only.

Western Disturbances

• Sub-Tropical Westerly Jet reaches Northern India after passing over the Mediterranean Sea, where it rains during winter. These winds bring cyclonic disturbances formed over the Mediterranean Sea to north India. 
• Due to Sub-Tropical Westerly Jet,  
  1. Punjab, Haryana, northern Rajasthan, Jammu and Kashmir and Western Uttar Pradesh receive an average of 20 to 50 cm of cyclonic rainfall, which is very useful for the Rabi crops. 
  2. Neighbouring Himachal Pradesh, Kashmir and Kumaon Hills experience snowfall.
• Precipitation during the winter season is good for Rabi crops, especially wheat. 
• But excessive rainfall can also cause crop damage, landslides and avalanches. 

Weather in Cold Season

• The weather is pleasant in winter. 
  1. Days are pleasantly warm (moderate)
  2. Nights are cold. 
  3. Skies are clear with dry weather.
• Temperature decreases from south to north. It falls from 31° C in Trivandrum, 26° C in Calicut, 24° C in Chennai, 16° C in Varanasi, and Drass Valley has a temperature of -45° C
• Due to the sharp fall in night temperature, sometimes dense or heavy fog appears.
• Due to the cold waves in the plains, a severe frost occurs.

• At the end of February, the temperature rises as sun-rays fall vertically over areas north of the equator. Hence, the summer season remains from March till the end of May, and it is the longest season in India.
• Interior areas record the maximum daily temperatures. E.g., The maximum daily temperature recorded is 
  1. 38° C in Nagpur  
  2. 40° C in Madhya Pradesh  
  3. 45° C in the North-Western parts
  4. 48° C in the Barmer (Rajasthan)
• But coastal lands have moderate climates due to moderating effect of ocean and sea breeze. The temperature remains between 30 to 32° C in South India.

Pressure Systems 

• With an increase in temperature, Inter-Tropical Convergence Zone (ITCZ) starts shifting towards the north.
• In May and June, due to high temperatures, a low-pressure Thermal Depression develops in the north–western parts of the country. Additionally, the Sub-Tropical ‘Jet’ stream shifts towards the north of the Himalayas. In this way, a special situation develops in the north–west, in which till the height of 6–7 Km from the surface of the land, the low-pressure cycles join each other and attract the monsoon winds fastly towards themselves.

Loo

In summer, North India comes under the influence of Loo. Loo is a strong, dusty, hot, and dry summer wind blowing from the west over the western Indo-Gangetic plains region of Northern India. It is especially strong in May and June. Due to its very high temperatures, exposure to it leads to fatal heat strokes. Due to its high temperature and dryness, it has severe drying effects on vegetation leading to browning in the areas it affects.

Norwesters or Kalbaisakhi

• The Loo ends in late summer due to the arrival of the monsoon. Loo (dry winds at high temperature) from the North-West and warm & moist winds from the Bay of Bengal converge around the Chotanagpur plateau. It causes thunderstorms and lightning accompanied by strong winds and heavy rainfall, mainly in the states of Assam and West Bengal. 
• These local winds are known as ‘Norwesters’ or ‘Kalbaisakhi,’ i.e. calamity of the month of Baisakh. They bring rainfall in Assam, West Bengal and Odisha. This rainfall is quite beneficial for the cultivation of Jute and Rice in West Bengal and Tea in Assam.  

Blossom and Mango Showers

• The coastal areas of Kerala and Karnataka receive rainfall close to the end of summer. These showers are extremely useful in the cultivation of tea, coffee and mango. They are also known as Mango Showers in Kerala (as they help in the early ripening of mangoes) and ‘Blossom Showers‘ in Karnataka, as with these showers, coffee flowers blossom in these areas.  

• Rain happening in October and November is known as Retreating Monsoons. 
• The Inter-Tropical Convergence Zone begins moving southward in response to the sun’s southward movement towards the end of September, which weakens the southwest monsoon. Hence, the temperature in the Northern Plains starts decreasing, and consequently, pressure starts increasing. In response to this, Monsoon winds start to retreat from northern India, and this is also known as North-East monsoon as the winds blow from North-East to South-West.

• These monsoon winds retreat in almost the same sequence as they arrive in the country. 
  • They first leave Punjab till 15 September, 
  • Uttar Pradesh till 1 October, 
  • Ganges Delta till 15 October 
  • Till 15 November, these leave southern parts of Karnataka and Andhra Pradesh.

• The retreat of the monsoon is marked by clear skies and a drop in the night temperature. But the land still remains moist. The combination of high temperature and humidity gives rise to oppressive weather and is known as ‘October Heat.’

• The North-East monsoons are generally dry as they originate on land except on the coastal areas of Tamil Nadu, i.e. Coromandal coast because when they pass over the Bay of Bengal, they collect moisture and shed that moisture on the Tamil Nadu coast.

Cyclones on the Eastern Coast during retreating monsoon

• Due to the shifting of ITCZ over the Bay of Bengal and the convergence of winds on both sides of ITCZ, cyclonic depressions are created in the Bay of Bengal, whose water is already at high temperature due to continuous heating throughout the summers. These cyclones move towards the west or north-west. 
• These cyclones frequently hit the eastern coastal region and cause significant damage to life and property. The areas most vulnerable to these storms include the coastal belt of Tamil Nadu, Andhra, Odisha and West Bengal. 
• The number of devastating severe Cyclones hitting the Indian coasts has been increasing owing to the increase in sea surface temperature in the northern Indian Ocean.