The Environmental Lapse Rate (ELR) is a fundamental measurement in atmospheric science describing how the ambient temperature changes with increasing altitude at a specific time and location. This rate quantifies the observation that air generally becomes colder as one moves higher above the Earth’s surface. The ELR is a dynamic variable, reflecting the current thermal structure of the atmosphere, and is crucial for predicting vertical air movement and weather phenomena.
Defining the Environmental Lapse Rate
The Environmental Lapse Rate represents the actual, measured temperature gradient of the stationary atmosphere. It is the decrease in temperature observed in the surrounding air as elevation increases, typically expressed in degrees Celsius per kilometer or degrees Fahrenheit per thousand feet. Meteorologists routinely determine the ELR using specialized instruments, such as radiosondes, which are weather balloon packages that transmit temperature data back to the ground as they ascend.
While the ELR is highly variable, an international standard atmosphere is often cited for a baseline reference. This average value is approximately 6.5°C per 1,000 meters, or about 3.5°F for every 1,000 feet of ascent in the lower atmosphere (troposphere). The actual value fluctuates based on factors like the time of day, geographic region, and the season. For instance, intense daytime solar heating can cause the lower atmosphere’s temperature to drop much faster with height than the average, leading to a steeper ELR.
Adiabatic Lapse Rates: The Crucial Distinction
To interpret the ELR, it must be compared against theoretical rates known as adiabatic lapse rates, which describe the temperature change of a vertically moving air parcel. An adiabatic process occurs when an air parcel rises or sinks rapidly enough that it does not exchange heat with the surrounding air. When a parcel rises, decreasing pressure causes it to expand and cool; conversely, sinking air is compressed, causing it to warm.
The Dry Adiabatic Lapse Rate (DALR) is the constant rate at which an unsaturated air parcel cools as it rises, approximately 9.8°C per 1,000 meters (5.5°F per 1,000 feet). The Moist or Saturated Adiabatic Lapse Rate (MALR/SALR) applies when the air parcel is saturated with water vapor and condensation is occurring. This condensation releases latent heat into the rising parcel, which slows the rate of cooling. Because of this warming effect, the MALR is always less than the DALR, typically ranging from about 3.6°C to 9.2°C per 1,000 meters.
Determining Atmospheric Stability
Meteorologists determine the atmosphere’s vertical stability by comparing the measured ELR to the theoretical adiabatic rates. This comparison reveals whether a vertically displaced air parcel will continue to rise, sink, or return to its original position. Stability depends on buoyancy: a parcel warmer and less dense than its environment will rise, while a cooler, denser parcel will sink.
In Absolute Instability, the ELR is greater than both the DALR and MALR. The ambient air temperature drops so rapidly with height that any air parcel cools slower than the environment, remaining warmer and continuing its upward acceleration.
Absolute Stability occurs when the ELR is less than both adiabatic rates. A rising parcel cools much faster than the surrounding air, becoming colder and denser, which forces it to sink back down. This resistance to vertical motion suppresses air currents.
Conditional Instability exists when the ELR is less than the DALR but greater than the MALR. An unsaturated air parcel will initially resist rising. However, if the parcel is lifted high enough to become saturated, it cools at the slower MALR. Once saturated, the parcel becomes warmer than the surrounding air and accelerates upward, making its stability conditional upon water vapor and an initial lifting mechanism.
Real-World Impacts of Stability
The stability determined by comparing the lapse rates dictates the type of weather and air quality experienced at the surface. Unstable air, characterized by a rapidly decreasing ELR, encourages strong vertical air currents and convection. This upward movement drives the formation of towering cumulus clouds, heavy precipitation, and severe weather like thunderstorms, which promote the rapid dispersion of pollutants.
Conversely, stable atmospheric conditions, where the ELR is very small or even positive, suppress vertical motion. A positive ELR, where temperature increases with height, is known as a temperature inversion. Stable air often leads to clear skies, but it traps moisture and pollutants near the surface, contributing to the formation of fog, haze, and elevated smog concentrations. The lack of turbulence in stable air also influences aviation, making the atmosphere calmer for flight.