Northern India, which includes Rajasthan, has a warm and dry climate with cold nights during the winter and extremely high temperatures in May and June (temperatures during the day can rise above 40 degrees). Besides the monsoon season (July-September) there is little rain. In Rajasthan, it can go years without raining.
South India encompasses a large area and hence has a variety of tropical climates. The most important monsoon is the wet south-west monsoon, which reaches the southernmost point of the subcontinent in the beginning of June, and the rest of South India a month later. The monsoon rains take the form of downpours in the afternoon and early evening. During and after the monsoon, the country is a lush green all over.
Best time to travel: Spring (February and March) and Autumn (mid-September until mid-December) is a good time to visit Rajasthan. The desert nights in January can be extremely cold. February and March are very characteristic of spring months, while April onwards can see the temperatures rise to uncomfortable levels. In October and November, many festivals and celebrations take place. The summer months are hot and usually clammy. The nice thing about the summer months is that most people are awake in the evenings and nights, holding long siestas during the tropical daytime. If you can adjust to this body-clock, it becomes endurable.
The best time to travel to South India is from October to March. From April to June, the interior of Tamil Nadu gets very hot. Monsoon time is actually not a bad period to visit South India. The landscape is beautiful with a fresh green colour and an abundance of water. The rain showers last a few hours a day at most and bring a welcome coolness. The weather is spectacular with enormous clouds and thunder and lightning. Streets can change into swirling rivers within minutes. However, the monsoon can make travelling slow as roads become flooded.