India is a large country where has the diversity and changes in weather from one end of the country to the other. And Tamil Nadu is located in the South-East and is hottest part of India on average. Temperatures here rarely below 30ºC and can reach to the high 37/38ºC in May and June. Rainy season is shorter, from beginning from October to December, however, it can rain from July to October as well.
North India(Rajasthan, Agra, Delhi, Varanasi)
The best time to travel here is between October and April/May. Temperatures are generally between 20 ºC to the low 30ºC. Rain fall usually ranges from very low to none during this time.
May to September temperatures can reach as high as 40ºC/45ºC with high rain fall as this is when the monsoon hits.
November to May is the best time to visit South India. Kerala is in the South-West where temperatures are usually around 30ºC all year. Monsoon season is June until September/October.
Tamil Nadu in the South-East, on average is the hottest part of India. Temperatures are rarely below 30ºC and can rise to the high 37/38ºC in May and June. Rainy season is shorter; usually running from October to December, however there may be rain from July to October as well.
North East- (Darjeeling, Sikkim) North West India- (Dharamsala, Manali, Ladakh)
North East India and North West India, get extremely cold in the winter, due to their high altitudes temperatures are usually around 7 or 8ºC at best. The rainy seasons are different though, the North-East begins in June and end in October, while the North-West runs from July until the end of August. Although rain is likely to occur in both regions throughout the year.
Bombay and Goa
30ºC all year. The main monsoon season is June until September; however even then the temperatures remain constant.This area of India, central and south are generally hot, with temperatures around the low. (Kerala and Tamil Nadu)
For more information and great ideas about where to travel in India take a look at India Travel Plan.