Why hills are cooler during summer season?

This is because of the air. When we go up on a hill, the air gets thinner. There is less air pushing down. This thin air spreads out. When air spreads, it becomes cool.

Also, the ground in plains gets very, very hot from the sun. This makes the air near ground also hot. Hills are far up from that very hot ground. The air on hills is not thick, so it cannot hold much heat like the air in the plains.

That is why in India, we all go to hill stations like Shimla or Ooty in summer. It is a simple science, but it is very nice for us to escape the heat