The temperature of the atmosphere changes predictably as elevation increases, governing conditions for weather patterns and mountain climbing safety. Within the troposphere, the lowest layer of the atmosphere where nearly all weather occurs, the air temperature decreases consistently with height. This vertical temperature gradient is a fundamental concept in atmospheric science. Understanding this relationship is important for meteorologists, pilots calculating air density, and anyone venturing into higher altitudes.
The Average Rate of Temperature Change
The typical rate at which the surrounding air temperature drops with altitude is defined by the Environmental Lapse Rate (ELR). This value represents the average observed condition of the atmosphere, not a fixed physical law. On average, the temperature decreases by approximately 3.5°F for every 1,000 feet of ascent.
The corresponding metric value is a decrease of about 6.5°C per 1,000 meters (or one kilometer) of elevation gained. This rate is widely used as the standard approximation in the International Standard Atmosphere model for aviation and atmospheric calculations. The actual temperature change encountered on any given day can vary significantly from this average.
The Mechanism of Adiabatic Cooling
The primary physical explanation for air cooling with altitude is adiabatic cooling. This term describes a temperature change occurring within an air parcel due to expansion or compression, without heat being added or removed by the surrounding environment. This mechanism is rooted in the first law of thermodynamics.
When an air parcel rises, it moves into a region of lower atmospheric pressure and begins to expand. This expansion requires energy, which the air parcel draws from its own internal thermal energy. The expenditure of internal energy results in a drop in the air parcel’s temperature.
If this rising air is dry, the theoretical cooling rate is known as the Dry Adiabatic Lapse Rate (DALR). The DALR is a constant value of approximately 5.4°F per 1,000 feet, or 9.8°C per 1,000 meters. This theoretical rate is nearly always faster than the observed Environmental Lapse Rate.
Conditions That Alter the Standard Rate
The Environmental Lapse Rate often deviates from the standard 3.5°F per 1,000 feet due to external atmospheric conditions, most notably the presence of moisture. When a rising air parcel cools to its dew point, water vapor begins to condense, forming clouds and releasing latent heat. This release of heat energy into the air parcel slows the rate of cooling.
The cooling rate of this moisture-saturated air is called the Moist or Saturated Adiabatic Lapse Rate (MALR or SALR). Because of the warming effect from the latent heat release, the MALR is always less than the DALR. It typically falls between 2°F and 5°F per 1,000 feet (about 4°C to 6°C per 1,000 meters).
The relationship between the Environmental Lapse Rate and the adiabatic rates determines the stability of the atmosphere. If the ELR is lower than the MALR, the air is considered absolutely stable, suppressing vertical motion. Conversely, if the ELR is greater than the DALR, the atmosphere is considered absolutely unstable, leading to strong convection and the formation of thunderstorms.
An extreme variation occurs during a temperature inversion, where the temperature actually increases with altitude instead of decreasing. This happens when a layer of warmer air settles above cooler air near the surface, often trapping pollutants and reversing the expected temperature gradient.