There is a significant drop in temperature. The day temperature registers a decline of 5 °C to 8 °C, between mid-June and mid-July. After this it remains almost constant throughout the rainy season. The night temperature is more uniform than the day temperature.
The highest temperature is experienced in the Thar Desert. Places west of the Aravalis may have temperature higher than 40 °C. Along the Western Ghats, the windward side has lower temperature than the leeward side.
The low pressure conditions over the north-western part of the Northern Plains get intensified further. It becomes powerful enough to attract the Trade Winds of the Southern Hemisphere, which are of oceanic origin.
After crossing the equator, the Southeast Trade Winds blow from southwest to northeast. These winds maintain their direction in the Peninsular India, but they blow from east to west in the Northern Plains of India.
The Southwest Monsoon winds, as they blow from the Indian Ocean, are moisture laden. India receives bulk of its rainfall during the Southwest Monsoon season. The normal date of onset in the Andaman and Nicobar Islands is 20th May, while in Kerala it is 1st June every year.
The Southwest Monsoon winds are rain-bearing strong winds and blow at an average speed of 30 km per hour. They usually cause thunder, lightning and heavy downpour. This sudden onset of monsoons is known as the burst of the monsoons.
The progress of the monsoon winds beyond Kerala is in the form of two branches the Arabian Sea branch and the Bay of Bengal branch. The Arabian Sea branch advances slowly, while the Bay of Bengal branch advances rather rapidly. On reaching the foothills of the Himalayas, the Bay of Bengal branch is deflected westwards and advances to the Northern Plain.
By June end, the monsoon winds are usually established over most parts of India. The Arabian Sea branch of the Southwest Monsoons is more powerful than the Bay of Bengal branch.
The Arabian Sea branch of the Southwest Monsoon winds is obstructed by the wall-like mountains of the Western Ghats. The windward side of the Sahyadris receives very heavy rain. The leeward side gets lesser rain. Mumbai on the west coast records about 200 cm of rain, while Pune on the leeward side receives only 50 cm rainfall during this season
A part of the Arabian Sea branch reaches Narmada-Tapi basin and moves further to central India. Absence of orographic obstacles causes less rain near the coast. Another part strikes the Saurashtra Peninsula and the Kachchh. Then it moves in a north-easterly direction parallel to the Aravali Range in the Western India.
The Aravali Range lies parallel to the direction of the monsoon winds. Thus, in the absence of an obstruction, the monsoon winds move further north without causing much rainfall in the west. In Punjab, Haryana and other north-western parts, the Arabian Sea branch of the Southwest Monsoons joins the Bay of Bengal branch.
These two branches together cause rains in the Western Himalayan region. The Bay of Bengal branch of the southwest monsoon is directed towards the coast of Myanmar and parts of southeast Bangladesh.
The Arakan Hills along the coast of Myanmar deflect these winds towards India and Bangladesh. Thus, the monsoon winds enter West Bengal and Bangladesh from southeast instead of southwest direction. After reaching the Himalayan foothills, it is again deflected towards the west and causes heavy rains in the Northern Plains of India.
A part of the Bay of Bengal branch moves up the Brahmaputra Valley and causes widespread rains in the north-eastern India. Mawsynram, about 16 km west of Cherrapunji and located on the crest of the Khasi Hills, receives the highest average rainfall in the world. The rainfall in the Northern Plains of India decreases from east to west.
The eastern coastal belt, particularly in Tamil Nadu, remains dry during this season. The Tamil Nadu coast lies in the rain shadow area of the Arabian Sea branch and is parallel to the Bay of Bengal branch.
During the months of July and August, there is a tendency to have spells of dry weather. It is called a ‘break’ in the monsoons. The rainy season comes to an end by the middle of September in the major parts of India.
The heavy rains from the monsoon winds can cause devastating floods responsible for damage to life and property in the Northern Plains of India. The monsoon winds are known for their vagaries and uncertainties. The dry and wet spells cause heavy floods in one part and may be responsible for droughts in the other.
The monsoon winds are often found to be irregular in their arrival as well as retreat, thus dislocating the farming schedule of the millions of farmers in India.