Mountains are colder than valleys because thin air at altitude holds less heat.

The longer version

Air high on a mountain is under less pressure, and a parcel of air that rises and expands always cools. Even though the sun can feel strong at altitude, the thinner air contains fewer molecules to absorb and hold heat, so warmth escapes readily.

The ground also radiates its heat away quickly into clear, dry air. Temperatures typically fall by a few degrees for every kilometre of height gained, so a summit can stay snow-covered while the valley below is mild. Slope direction, exposure, and the way cold air pools in hollows overnight all change the picture locally.

Where this entry sits

Clouds, wind, rain, and storms shape daily life in ways that most people barely notice. This collection explains snow, lightning, rainbows, the turning seasons, temperature, the instruments forecasters rely on, and scraps of old weather lore. The full list sits on the Weather and Climate page, where every entry is listed in order.