This map from Wikimedia provides information on the average annual temperature figures across the country over a range of below 20 Deg C and above 27.5 Deg C. The map reveals that:

• Most of the country lies in the range of 25-27.5 Deg C, followed by 22.5-25 Deg C.
• A small patch below the Himalayan range and in the Western Ghats falls in the range 20-22.5 Deg C.
• The Himalayan range covering Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Sikkim and Arunachal Pradesh and parts of Meghalaya are the coolest areas and fall in the below 20 Deg C range
• A small region surrounding Gandhinagar in Gujarat, most of Coastal Andhra, Rayalseema and Coastal Tamil Nadu, are the hottest areas with a temperature range of above 27.5 Deg C.