I know it is colder at higher elevations (there is snow on the top of mountains when no snow is left on the bottom). When I was hiking this summer in Colorado and it started snowing I thought about why it was colder in higher elevations. My guess was that it was mainly due to lower air pressure and being higher up in the atmosphere where air was cooler than is was closer to sea level.

So I did some research online and the main explanations seem to be that at higher elevations the air pressure is lower (molecules and atoms under less pressure move more slowly which means the temperature is less).

Hot air does rise, but the amount of hot air is minor compared to the existing cold air in the atmosphere. So when hot air rises from the ground it is cooled down before getting far off the earth’s surface. And as it rises the pressure decreases, which cools it down.

Mountain Environments report, United Nations Environment Programme:

Photo of John Hunter at Hurricane Ridge in Olympic National Park.

Related: Why is the air cooler at higher altitudes? - Why is the Sky Blue? - scientific explanations for what we experience - Flint and Steel: What Causes the Sparks? - Mount Rainier National Park Photos - Low air pressure decreases temps at high elevation

How does elevation effect climate? Wendell Bechtold, Meteorologist, Forecaster, National Weather Service

Air is mainly heated from below (from the heat from the ground). The Sun provides much of the heat, but not by heating the air directly, but by heating the ground which then radiates heat into the air.