Lapse Rate

In the dead of winter, you feel toasty sitting right next to a space heater. Walk to the far corner of the room, and it gets chilly fast. The same thing happens in Earth's atmosphere every day.

The real heater that warms Earth's air is not the sun far away in the sky. It is the ground, soaked with the sun's heat. Sunlight passes straight through clear air and heats the ground first. Then the warmed surface works like a giant heated floor mat, passing warmth along to the air above it, layer by layer.

So climbing a tall mountain or rising into the sky means moving away from that floor heater. Getting a few kilometers closer to the sun makes almost no difference. Moving away from the warm ground matters far more, so it gets chillier as you go up. Warmed air does rise, though, so the reason the upper air stays cold also has to be read together with adiabatic expansion, which comes next.

That is why, even on a sweltering midsummer day at the beach or downtown, the summit of a high mountain can be so much colder that you need to pull on a long-sleeved jacket.

Pump up a bike tire with a hand pump, and after a few hard pushes the pump barrel gets pretty warm. Air squeezed into a narrow tube heats up. The reverse is also true: when squeezed air is released into a bigger space and expands, its temperature drops sharply.

High in the sky, the weight pressing down on the air, which is air pressure, is much lower than at ground level. So when a blob of air warmed near the surface is carried over a mountain or floats upward, the squeeze on it weakens and it swells on its own, like a balloon.

To make room for itself, the blob has to push the surrounding air out of the way, and that uses up some of its own heat energy. Air that cools down by spending its own energy, with no ice added from outside, is called adiabatic expansion.

Air that grows bigger and loses heat this way gets colder and colder as it rises. The lapse rate exists partly because you are moving away from the heater, but this self-cooling by expansion plays an equally important role.

Meteorologists usually say the temperature drops about 0.65°C for every 100 meters you climb. Look a little closer, though, and how fast it drops depends a lot on how much water vapor is mixed into the air.

Here is a catch. The values below describe how fast a rising blob of air itself cools (the adiabatic lapse rate). That is a different thing from the actual temperature difference between heights that a thermometer shows as you climb a mountain (the environmental lapse rate). A dry blob of air that has not formed a cloud yet cools a full 1°C per 100 meters as it rises. This is called the dry adiabatic lapse rate. Once the blob climbs higher, its water vapor becomes saturated, and a cloud starts to form. It then cools more slowly, about 0.4 to 0.7°C per 100 meters (the warmer the air, the smaller the number). This is called the moist adiabatic lapse rate.

The reason for the difference is that when water vapor in the cold upper air turns into water droplets and forms a cloud, it releases the hidden heat (latent heat) it was holding. That warmth pitches in against the cooling, so the temperature falls less.

The 0.65°C we talk about in daily life is not calculated from those two values. It is the reference value of the International Standard Atmosphere, set from averages of real atmospheric observations over many years. In reality it varies with the weather. On a foggy or rainy mountain, the temperature change with height feels a bit gentler than on a clear day.