As air rises, the pressure decreases. It is this lower pressure at higher altitudes that causes the temperature to be colder on top of a mountain than at sea level.

Why it is cold in high altitude?

As altitude increases, the amount of gas molecules in the air decreases—the air becomes less dense than air nearer to sea level. ... High-altitude locations are usually much colder than areas closer to sea level. This is due to the low air pressure.

Why are mountains cold when they are closer to the sun?

When the air is high on the mountain, it spreads out. This is like your big square. The same amount of heat is now in a bigger space, so it is more spread out. So the air is colder on top of mountains because there is less air pressure.