Science Puzzle
Why Is It Colder on Top of a Mountain?
Kilimanjaro stands just south of the equator in Tanzania. The plains at its foot are hot all year round, yet its summit, 5,895 metres up, is cold enough to keep patches of glacier ice. Climbing to the top takes you about 6 kilometres closer to the Sun.
Why does the air get colder as you climb, even though you're getting closer to the Sun?
The Answer
The atmosphere is heated from the bottom, by the ground; air also cools as it rises, because it expands.
First, forget the distance to the Sun. Six kilometres out of 150 million is about one part in 25 million, far too little to matter. Sunlight actually reaches a mountain top stronger than at sea level, because there's less air above to filter it, which is why climbers get sunburnt so easily.
Sunlight passes through clear air without warming it much. It heats the ground instead. The ground then warms the air touching it. So the warmest air sits at the bottom, near the source of the heat. Air also gets thinner and its pressure falls the higher you go. When a pocket of warm air rises, it moves into lower pressure and expands. An expanding gas uses some of its own energy pushing the surrounding air out of the way, so it cools. Dry rising air cools by nearly 10 °C for every kilometre it climbs.
Moisture and mixing soften that a little; on average the lower atmosphere gets about 6.5 °C colder for every kilometre of height. Six kilometres up, that's nearly 40 degrees colder than at sea level, plenty to keep ice on a mountain on the equator. You can feel the same effect in reverse when you pump up a bike tyre: squeezing air into the pump warms it.
The principle: The lapse rate. The lower atmosphere is heated from the ground up, and rising air cools as it expands into lower pressure, so the temperature falls by about 6.5 °C for every kilometre of height.