For an agrarian country like India, the rainy season (monsoon) is the most important season. During this season, which stretches from June to September, the country gets about 80% of its rain. The agriculture in India depends very much on this rain.
→ Rain during the monsoon season is very much dependent on the south-west monsoon winds blowing towards India. This rainy season is also called as the season of 'North-West monsoon winds'.
→ In the north-western plains, a low pressure condition develops due to high temperature by the end of May. At this time, high pressure prevails over the Indian Ocean situated to the south of India. So winds blow from the Indian Ocean towards the low pressure in north India. These winds bring enough moisture with them. So India gets rain during June to September.
-These winds are not as regular and permanent as the Trade winds. They originate and blow during a specific season only, and that is why these winds are known as 'Seasonal (Mausami) winds'.
The south-west monsoon winds bifurcate into two branches due to the Indian peninsula: 1. The Arabian Sea Current and 2. The Bay of Bengal Current.
1. The Arabian Sea Current: These winds enter India through Kerala situated on the western coast of south India. The Western Ghats obstruct them as soon as they enter the coast. This gives more rain on the windward side of Western Ghats After crossing the Western Ghats when these winds reach Deccan plateau, there is little humidity left in them. So the amount of rain decreases. The Deccan plateau lies on the leeward side of the Western Ghats so it becomes rain shadow area. Mumbai and Pune are not far from each other, however Mumbai which is situated on the windward side of Western Ghats gets more than 200 cm rain, while Pune on the leeward side gets less than 75 cm rain.
→ On the western coast of India, rain decreases from south to north from Kerala to Karnataka, Goa and Maharashtra.
One branch of the monsoon winds advances through Narmada valley in Madhya Pradesh. When these winds advance. the winds from the Bay of Bengal merge with them.
→ One weak branch of Arabian Sea goes towards Gujarat. Saurashtra and Rajasthan. It contains less humidity. Moreover, there are no high mountains or dense forests in Gujarat. So there is little possibility of condensation of the humidity. As a result. Gujarat gets less rain. When these winds reach Rajasthan. there is very little humidity within them, so there is less rain. The deserts of Rajasthan get even less than 10 cm rain.
2. The Bay of Bengal Current: These winds first enter West Bengal and reach up to Meghalaya. These winds are full of moisture. These winds are obstructed by Garo. Khasi and Jaintia hills in Meghalaya. So the windward slopes of these hills get torrential rain. Cherrapunji. situated on the slopes of Khast hills. gets about 1200 cm rain annually which is highest in the world. Mawsynram, situated 16 km far from Cherrapunji. is also famous for maximum rainfall received during 24 hours.
→ When the direction of the winds blowing towards west changes, their direction becomes south-east. These winds pass over West Bengal, Bihar. Uttar Pradesh and Haryana and reach Punjab The moisture in them eventually decreases as they advance towards west. So the rain in the northern plain decreases while going from east to west. Kolkata gets 160 cm rain. Allahabad 100 cm and Delhi gets 65 cm rain.
→ When the monsoon winds coming from the Arabian Sea and from the Bay of Bengal merge they give good amount of rain in northern Himalayan area. Sometimes the rain here is in the form of snow.
→ Most of the rain in north India is brought by winds coming from the Bay of Bengal. The cyclones originating in the Bay of Bengal also play an important part in bringing rain. In the northern part of the Bay of Bengal a low pressure is developed and five to six cyclones are formed during the monsoon. These cyclones advance towards west along with the monsoon winds and become much useful to bring rain. Sometimes the impact of these cyclones reach even up to Gujarat, which gets rain.