Lapse Rate
The rate at which the air gets colder the farther you move away from the Earth's heated floor.
Definition The lapse rate is the rate at which air temperature decreases with an increase in altitude. On average, the temperature drops by about 0.65°C for every 100 meters (or roughly 3.5°F per 1,000 feet) you climb, varying depending on atmospheric moisture and weather conditions.
Why does it get colder at higher altitudes?
In the dead of winter, sitting right next to a space heater feels nice and toasty, but walk away toward the far corner of the room and you will quickly feel the chill. The exact same thing happens in Earth's atmosphere every single day.
The real heater warming our air is not the distant Sun high in the sky, but Earth's surface, which absorbs and radiates solar heat. Sunlight passes straight through transparent air to heat the ground first. Like a massive heated floor, the warm ground then transfers its warmth to the surrounding air layer by layer.
Climbing a high mountain or ascending into the sky means moving farther away from this ground heater. The cooling effect of leaving that warm ground beneath your feet far outweighs the tiny difference of getting a few miles closer to the Sun.
That is why even during the sweltering heat of midsummer in low-lying valleys or coastal cities, reaching the summit of a high mountain will have you reaching for a warm jacket.
Air cools down on its own as it expands
If you have ever held down the nozzle on a can of compressed air or spray paint, you might have noticed the can turning ice-cold in your hand. When compressed gas escapes into a larger space and rapidly expands, its temperature plummets.
High up in the sky, atmospheric pressure—the weight of the air pressing down—is much lower than at sea level. So when a warm parcel of air rises over a mountain or floats upward, the decreasing pressure allows it to expand just like a balloon.
To expand, the air parcel must push outward against the surrounding air, using up its own internal thermal energy in the process. This drop in temperature caused by an expanding gas doing work without absorbing outside heat is called adiabatic expansion.
As the expanding air loses heat, it gets colder and colder the higher it climbs. Moving away from the ground heater is only part of the story; this self-cooling effect from expanding air plays an equally crucial role in creating the lapse rate.
A closer look: Humidity changes how fast air cools
Meteorologists generally state that temperature drops by about 0.65°C for every 100 meters gained. Look closer, though, and you will find that the cooling rate varies significantly depending on how much water vapor the air contains.
On a clear, dry day, dry air cools rapidly at about 1°C per 100 meters (the dry adiabatic lapse rate). In contrast, humid air saturated with water vapor cools more slowly, at roughly 0.5°C per 100 meters (the moist adiabatic lapse rate).
This difference occurs because when water vapor condenses into water droplets to form clouds in the chilly upper air, it releases latent heat into its surroundings. This released warmth helps offset the cooling of the rising air, slowing down the overall temperature drop.
The standard average of 0.65°C per 100 meters reflects a typical mix of dry and moist conditions in our atmosphere. This is why hiking up a foggy, rainy mountain often brings a gentler temperature drop than climbing on a crisp, clear day.
🤔 Common misconceptions
Mountain summits are closer to the Sun, so they should be warmer than flat land.
The Sun is about 93 million miles away, so gaining a few miles in altitude has zero noticeable effect. Instead, moving farther from the heated ground and lower atmospheric pressure causing air expansion make high altitudes far colder.
🧺 Where you meet it
The lapse rate is the rate at which air temperature decreases with altitude, caused by moving farther away from the heated ground and the cooling effect of expanding air.