The south-west monsoon begins to r from northern India by the second or third wed September with the southward migration of the and consequent weakening of the low pressure over north-western India.
By the end of September the south-west monsoon retreats from the Pure and adjacent regions, after which clear and weather sets in over that area. However, unlike sudden burst the retreat is highly gradual, By mid-October the southerly branch of the stream returns to its winter position south of Himalayas, indicating once more that Northern sphere polar dynamics are in command of the site: The return of the jet stream is accompanied by restoration of light north-east trade winds to the Temperature-the temperature during this season is uniformly high, about 26°C in the beginning of October.
The day temperature is high, but nights are pleasant with the mean minimum temperature going down to 20°C or even lower. The diurnal range of temperature is, however, more pronounced. The temperature begins to decline in November and by December the cold weather sets in with about 16°C average temperature in the north and north-west, 20°C in the interior parts of the Peninsula and 26°C along the coasts.
Exhibits average temperature in October. A look at the figure shows that major parts of the country experience average temperature between 25°C and 27.5°C. High temperatures (more than 27.5°C) are observed in southern Rajasthan, Gujarat and coastal plains of the east coast while Jammu and Kashmir, Himachal Pradesh, Uttaranchal and interior parts of Madhya Pradesh and Karnataka record lower temperature (less than 25°C).
Pressure and Winds-with the beginning of October the low pressure area over the north-west India is dissipated and its place is taken by a low pressure cell located over the northern parts of the Bay of Bengal. By the beginning of December it moves further southwards and by the month-end it merges with the equatorial low.- The winds are westerly in the north-western parts of the country and in the Ganga Plain, north-easterly in the Peninsular region and north westerly in the east coast.
Moisture and Rainfall-during this season winds are generally continental and, hence, the cloudiness and moisture are low except in the southern parts of the Peninsula. The relative humidity is 48 per cent in Jaipur, 59 in Ganganagar, 59 in Nagpur,
68 in Hyderabad, 73 in Mumbai, 83 in Chennai and 87 in Thiruvananthapuram in the month of November.
The retreating south-west monsoon causes some rains in the coastal areas of Tamil Nadu. The amount is more than 25 cm in the south Coromandel Coast and south Malabar; below 12.5 cm in a region to the east of a line connecting Mangalore with Dibrugarh; and less than 2.5 cm in north-west India.
Weather during this season is also influenced by a number of tropical cyclones which are very violent and destructive. Their impact becomes pronounced at the head of the Bay of Bengal when these are combined with a ‘hurricane wave’ and wind driven seas. The wave height sometimes reaches as high as 9 m when these are funneled up a confined estuary. These cyclones cause immense damage to coastal areas of the east coast and Saurashtra besides producing heavy rainfall.