The experience of climbing a mountain or flying a plane often confirms a simple atmospheric rule: the higher the elevation, the colder the temperature. This common observation results from fundamental physical processes governing how air behaves. While the surface might be warm, the air temperature generally decreases steadily as altitude increases. This predictable relationship is a core concept in meteorology, and understanding it affects everything from weather patterns to the types of ecosystems that survive on mountain slopes.
Why Air Cools at Higher Elevations
The primary mechanism responsible for cooling air at higher elevations is adiabatic cooling. This process describes a temperature change within a parcel of air due to expansion or compression, without heat exchange with the surrounding environment. The key lies in the relationship between atmospheric pressure and air density.
As a parcel of air rises, the weight of the atmosphere above it decreases significantly. This reduction in atmospheric pressure allows the air molecules within the rising parcel to spread out and occupy a greater volume. This expansion requires energy, which is drawn directly from the internal thermal energy of the air molecules themselves.
The energy, once expressed as heat, is converted into the work needed to push against the lower external pressure, causing the air parcel to cool. This cooling defines the adiabatic process. Conversely, when air descends, it is compressed by increasing pressure, causing its temperature to rise through adiabatic heating.
Air density also plays a role because the air is much thinner at altitude. At sea level, air molecules are tightly packed, retaining thermal energy. At higher elevations, the molecules are much farther apart, resulting in fewer collisions and less capacity to store heat energy from the ground or the sun.
Calculating the Rate of Temperature Change
The rate at which the temperature decreases with altitude is quantified by the lapse rate, a measurement central to atmospheric science. This rate is not a single, fixed value but varies depending on the moisture content of the air and whether the air is rising or is simply the ambient surrounding air. The average temperature decrease in the non-rising, ambient atmosphere (the environmental lapse rate) is approximately 6.5 degrees Celsius for every kilometer (18.8 degrees Fahrenheit per mile) of ascent in the lower atmosphere.
When considering a vertically moving parcel of air, two specific rates apply: the Dry Adiabatic Lapse Rate (DALR) and the Moist Adiabatic Lapse Rate (MALR). The DALR applies to unsaturated air, meaning its relative humidity is less than 100%. This rate is a constant 9.8 degrees Celsius per 1,000 meters of ascent (or 5.4 degrees Fahrenheit per 1,000 feet).
The Moist Adiabatic Lapse Rate (MALR) applies once the rising air parcel cools to its dew point and becomes saturated, causing water vapor to condense. As the water changes state from gas to liquid, it releases latent heat energy into the air parcel. This heat release partially counteracts the cooling effect of expansion, causing the air to cool at a slower rate than the DALR.
The MALR is not a constant value and can vary significantly, typically ranging from 3.6 to 9.2 degrees Celsius per kilometer. The exact value depends on the temperature and pressure, as warmer air holds more water vapor and thus releases more latent heat upon condensation. Understanding the difference between the dry and moist rates is crucial for predicting cloud formation and atmospheric stability.
Local Conditions That Modify the Effect
While the lapse rate provides a standard model for temperature change, specific local conditions frequently cause deviations from this expected pattern.
Temperature Inversions
One notable exception is a temperature inversion, where the temperature temporarily increases with altitude instead of decreasing. This often occurs on clear, calm nights when the ground rapidly loses heat through radiation, cooling the air immediately above the surface. Inversions can also be caused by cold, dense air sinking into valleys or by subsidence, the slow sinking and warming of air under high-pressure systems. These events trap cooler air near the surface, reversing the standard temperature gradient.
Geographic Factors and Wind
Geographic factors also modify temperature at elevation. The aspect, or the direction a slope faces, determines the intensity of solar radiation it receives. Slopes facing the sun receive more direct energy, leading to warmer surface temperatures and localized heating. Furthermore, strong wind patterns can dramatically alter local temperatures. Downslope winds, such as the Chinook in North America, warm significantly due to adiabatic compression as they descend, raising temperatures rapidly on the leeward side of a mountain range.