Closer to the sun, and colder
Why Is It Colder Up a Mountain?
Climbing a mountain takes you nearer the sun and makes you colder, which sounds wrong until you know where the heat comes from.
-
Step 1 of 5
Sunlight barely warms the air it passes through.
Air is almost transparent to sunlight. The light goes through it, warms the ground, and the ground warms the air touching it — so the heat starts at the bottom.
-
Step 2 of 5
Rising air expands, and expanding cools it.
Air pushed upward finds less pressure and spreads out. Spreading out uses energy, and that energy comes out of the air's own heat, so it cools without losing anything.
-
Step 3 of 5
The rate is remarkably steady.
Dry air cools by about ten degrees for every kilometre it rises, damp air by about six. That is why a mountain's temperature can be estimated from its height alone.
-
Step 4 of 5
Thin air holds heat badly.
There is simply less air up there to hold any warmth. Sunshine feels fierce and shade feels bitter, and the moment the sun sets the temperature falls away.
-
Step 5 of 5
Which is why one mountain has many climates.
Walking up a tall tropical mountain passes through forest, then scrub, then bare rock, then snow — the same sequence as walking from the equator to the pole.
Water boils cooler up a mountain because the air presses less on it. At high altitude an egg can be boiled for a long time and still not cook.
The short version
Air is warmed from the ground up and cools as it rises and expands, so every kilometre of height costs several degrees.
Try it yourself
Let air out of a bicycle tyre onto your hand. It comes out cold, because it expanded on the way.