High-altitude locations are usually much colder than areas closer to sea level. This is due to the low air pressure. Air expands as it rises, and the fewer gas molecules—including nitrogen, oxygen, and carbon dioxide—have fewer chances to bump into each other.

Why does it get colder the higher you go in the troposphere?

In the troposphere, the temperature generally decreases with altitude. The reason is that the troposphere's gases absorb very little of the incoming solar radiation. Instead, the ground absorbs this radiation and then heats the tropospheric air by conduction and convection.

Why is it cooler in the mountains?

It has more to do with air pressure. Like all gases, the air in our atmosphere is a poor conductor—because it's not dense with particles. ... So, even though they're closer to the sun, thin air in the mountains keeps them colder than the thicker air in the lowlands surrounding them.