The atmospheric lapse rate describes how the temperature of the air changes as altitude increases. This rate of temperature decrease is a primary factor in determining weather patterns, atmospheric stability, and cloud formation. Understanding the lapse rate is also important to aviation, as it forms the baseline for calibrating aircraft instruments and calculating performance. The atmosphere generally cools with height because decreasing air pressure affects the behavior of air masses as they move vertically.
Defining the Standard Atmospheric Lapse Rate
The standard lapse rate is a theoretical value established to create a consistent reference point for atmospheric conditions. This model is formally known as the International Standard Atmosphere (ISA) and is used globally by engineers, meteorologists, and aviators. The ISA specifies that, on average, the temperature decreases by 6.5 degrees Celsius for every kilometer of altitude gained within the troposphere (up to about 11 kilometers). This value converts to approximately 3.56 degrees Fahrenheit per 1,000 feet.
This standard rate is a simplified average of countless real-world observations. It is a hypothetical profile that assumes specific conditions, such as completely dry air and a sea-level temperature of 15 degrees Celsius (59 degrees Fahrenheit). Aircraft performance charts and altimeter calibrations are built upon this standardized profile, making it a powerful tool for consistency in flight planning.
The Physics Behind Temperature Change with Altitude
The decrease in temperature with altitude is not primarily due to distance from the sun, but rather a physical process called adiabatic cooling. This process describes the temperature change of an air parcel when it expands or compresses without exchanging heat with the surrounding air. As a parcel of air rises, it encounters lower surrounding pressure. This reduction in external pressure allows the air parcel to expand in volume.
The expansion requires the gas molecules to push against the lower-pressure environment, which is a form of work. To perform this work, the air parcel must expend its own internal energy, causing its temperature to drop, resulting in adiabatic cooling.
Conversely, when an air parcel sinks, it enters regions of higher pressure and is compressed. The surrounding air does work on the parcel, which increases its internal energy and causes its temperature to rise, known as adiabatic heating. Because air is a poor conductor of heat, these vertical movements occur too quickly for significant heat transfer, which is why the adiabatic approximation is used. This constant cooling and warming of air parcels drives much of the vertical motion and weather in the lower atmosphere.
Real-World Lapse Rates and Atmospheric Stability
While the standard lapse rate is a useful average, the actual decrease in temperature at any given time and place is known as the Environmental Lapse Rate (ELR). The ELR is the rate measured by weather instruments like radiosondes carried aloft by weather balloons. This actual rate is constantly changing due to factors like solar radiation, wind, and the moisture content of the air.
Meteorologists also use two other specific rates that describe the behavior of a vertically moving air parcel. The Dry Adiabatic Lapse Rate (DALR) applies to unsaturated air and is a constant value of about 9.8 degrees Celsius per kilometer (5.4 degrees Fahrenheit per 1,000 feet). Since no heat is being added or removed, this rate is faster than the standard lapse rate.
The Moist or Saturated Adiabatic Lapse Rate (MALR or SALR) applies when the air parcel has cooled enough to reach its dew point, causing water vapor to condense. This condensation process releases latent heat into the air parcel, which offsets some of the adiabatic cooling. Because of this added heat, the MALR is slower and varies significantly, typically ranging between 4 and 7 degrees Celsius per kilometer, depending on the air’s temperature and pressure.
The relationship between the ELR and the adiabatic rates determines atmospheric stability, which predicts vertical air movement and weather phenomena.
Absolutely Unstable
If the ELR is greater than the DALR, the atmosphere is considered absolutely unstable. A rising air parcel will remain warmer and less dense than its surroundings and continue to accelerate upward, often leading to strong convection and thunderstorms.
Absolutely Stable
Conversely, if the ELR is less than the MALR, the air is absolutely stable. This suppresses vertical movement and results in calmer conditions.
Conditionally Unstable
If the ELR falls between the two adiabatic rates, the atmosphere is conditionally unstable, meaning only a saturated air parcel can continue to rise.