Why is it colder at the top of the mountain when the top is closer to the sun?

The top of a mountain is closer to the sun than the land around during the day it, but given that the sun is more than 90 million miles away, that difference is negligible. The higher you go up in the atmosphere, the colder it gets. This is because the air at high altitudes is less compressed than the air at low altitudes. When a gas is decompressed, it cools.