Why is India cooler in December than in July?

The Tropic of Cancer passes mid-way across India. On June 21, the rays of the Sun falls directly over the Tropic of Cancer as the North Pole get tilted towards the Sun. The southern hemisphere is tilted away from the Sun during this time. Hence, July is hot.

On December 22nd, the Southern Hemisphere is tilted towards the Sun and the Sun rays fall vertically over the Tropic of Capricorn, while the Northern Hemisphere is tilted away from the Sun. So, India, which lies in the Northern Hemisphere experience winters in December.