The relationship between elevation and temperature is a fundamental concept in atmospheric science. People often notice a distinct drop in temperature when they ascend a mountain or fly in an airplane. This change confirms that height directly influences the surrounding air temperature. Understanding this phenomenon requires examining the physical properties of the air and how it interacts with the ground.
The Standard Relationship Between Altitude and Temperature
Temperature consistently decreases as altitude increases throughout the troposphere, the lowest layer of Earth’s atmosphere. This layer extends from the surface up to an average of about 12 kilometers (7.5 miles) and is where nearly all terrestrial weather occurs. This inverse correlation exists because the primary heat source for the air is not direct solar radiation, but the Earth’s surface itself. The ground absorbs sunlight and re-radiates that energy back into the atmosphere as longwave radiation. Consequently, the air closest to the ground is the warmest, and the air farther away is progressively cooler.
The Physics of Atmospheric Cooling
The drop in temperature with altitude is governed by two interconnected physical principles: air density and expansion. Air pressure decreases significantly at higher elevations because there is less weight from the column of air pushing down from above. This results in lower air density, meaning fewer molecules are present to hold and transfer thermal energy, contributing to the colder conditions.
This lower pressure also drives the process known as adiabatic cooling. When a parcel of air rises, it moves into a region of lower pressure and expands. This expansion requires the air molecules to push outward, consuming internal energy. Because no heat is exchanged with the exterior air, the work performed by the expanding air causes its internal temperature to decrease. Conversely, descending air is compressed by increasing pressure, causing it to warm up adiabatically.
Measuring the Rate of Change
The rate at which temperature decreases with increasing altitude is quantified by the lapse rate. The Environmental Lapse Rate (ELR) describes the actual observed temperature change of the stationary atmosphere at a given time and place. On average, the ELR in the troposphere is approximately 6.5 degrees Celsius per 1,000 meters of ascent (3.5 degrees Fahrenheit per 1,000 feet). This average value serves as the standard for atmospheric modeling and meteorological predictions.
Adiabatic Lapse Rates
The rate of cooling for a vertically moving air parcel is defined by the adiabatic lapse rate. For a parcel of dry, unsaturated air, the cooling rate is constant and known as the Dry Adiabatic Lapse Rate (DALR). This rate is consistently about 9.8 degrees Celsius per 1,000 meters (5.4 degrees Fahrenheit per 1,000 feet). When an air parcel becomes saturated with moisture, it cools more slowly due to the release of latent heat from condensing water vapor. This slower cooling is described by the Moist Adiabatic Lapse Rate (MALR), which is variable but ranges between 3.6 and 9.2 degrees Celsius per 1,000 meters.
When the Pattern Reverses
While the general rule is that air gets colder with height, this pattern temporarily reverses under specific atmospheric conditions known as a temperature inversion. A temperature inversion occurs when a layer of warmer air sits on top of a layer of colder air near the surface. This creates an inverted temperature profile where air temperature increases as altitude rises, rather than decreasing.
One common cause is a radiational inversion, which happens on clear, calm nights. Without cloud cover to insulate the surface, the ground rapidly loses heat, cooling the air layer directly above it more than the air higher up. Inversions also form in valleys when cold, dense air flows down the slopes and pools at the bottom, displacing warmer air upward. This stable structure acts like a lid, trapping cooler air and pollutants close to the surface until the sun’s warmth breaks the inversion.