The two factors affecting the climate of India are:
The Himalayas: They form a climatic barrier separating the Indian subcontinent from the rest of Asia. They not only prevent the cold Siberian winds from entering the Indian region and from India becoming a cold desert but they also force the moisture laden South West Monsoon winds to shed rainfall in India.
Altitude: There is a decrease of 1o for every 166 m rise in height, because temperature decreases as you move to higher altitudes. So, the mountains are cooler than the plains.