Why is it colder on a mountaintop if it's closer to the Sun?

Distance from the Sun barely matters here. The Sun is about 150 million kilometres away, and standing on Everest brings you only around 9 kilometres closer, far too little to feel warmer. What matters is the air. Higher up there is less atmosphere, pressure is lower, and the air is colder. On average the temperature drops by roughly 0.65°C for every 100 metres of climb, according to MeteoSwiss, though other sources give figures close to 1°C per 100 metres. [1] [2]

A mountain is tiny next to the gap between Earth and Sun. A physics page from the Chinese University of Hong Kong notes that more than ten million of the highest mountains could be stacked between Earth and the Sun. Caroline Steel, speaking on The Naked Scientists, makes the same point: the change in distance is negligible against 150 million kilometres, so it should have no real effect on temperature. [3] [4]

Air pressure is greatest near sea level and falls with height, because the column of air above you gets shorter and has less mass. When air rises it meets lower pressure and expands. Expanding takes energy, which the air draws from its own heat, so the molecules have less average energy and the temperature falls. MeteoSwiss and the Naked Scientists both describe this process. Washington State University's Ask Dr. Universe adds that sinking air does the opposite: it is compressed and warms, and this mixing is a driver of weather. [2] [4] [5]

The atmosphere is mostly warmed from below. Sunlight heats the ground, and the ground radiates heat upward. Water vapour absorbs some of that heat and sends part of it back down, which is part of the natural greenhouse effect. Water vapour content falls with altitude, so higher air absorbs and returns less heat, and more escapes to space. Colorado State University makes a related point: thinner air has fewer molecules to trap and hold heat, even though it lets more of the Sun's radiation through. [2] [6]

Colorado State also notes that colder air makes snow more likely than rain, and snow-covered ground reflects much sunlight back to space, which helps keep high places cold. The sources explain the mechanism with different emphasis, but they agree on the overall picture. Colorado State's added claim that fewer molecular collisions means less heat is offered as one of two reasons and is not echoed by the other sources. [6]

The cooling rate varies. MeteoSwiss says very dry air under high pressure can cool by almost 1°C per 100 metres. The cooling continues up to the tropopause, about 12 kilometres up, where Science Focus gives a temperature around -55°C. Above that, in the stratosphere, it warms again because ozone absorbs ultraviolet light, and the thermosphere has a very high nominal temperature but would not feel hot because it is almost a vacuum. [2] [1] [4]