When a mass of air shifts vertically in the atmosphere, its thermal properties are immediately affected. Unlike heating with a flame, the temperature change experienced by a moving air parcel is a purely mechanical process. This change is driven by the varying atmospheric pressure at different altitudes. As air descends toward the Earth’s surface, this process leads to predictable warming.
The Physics of Compression Heating
As a parcel of air descends through the atmosphere, it moves from a region of lower pressure aloft into a region of higher atmospheric pressure near the ground. This increasing external pressure acts on the air parcel, forcing its volume to decrease. This mechanical process is known as compression heating.
The air parcel undergoes an adiabatic process, meaning the temperature change occurs without significant heat exchange with the surrounding environment. The process happens quickly, preventing heat conduction or radiation from warming or cooling the parcel from the outside. Instead, the external pressure compresses the gas molecules within the parcel, forcing them into a smaller space.
This compression directly increases the kinetic energy of the air molecules, causing them to move faster and collide more frequently. This rise in kinetic energy is registered as an increase in the parcel’s internal energy. This internal energy increase is observable as a temperature rise.
Quantifying the Temperature Increase: The Dry Adiabatic Lapse Rate
The warming of descending air happens at a fixed, predictable rate consistent throughout the atmosphere. This rate is known as the Dry Adiabatic Lapse Rate (DALR). The DALR quantifies the temperature change of an unsaturated air parcel as it moves vertically. An air parcel is considered unsaturated if its relative humidity is less than 100% and no condensation is occurring.
The numerical value of the DALR is approximately 10°C for every 1,000 meters of descent (or 5.5°F per 1,000 feet). This constant rate is a consequence of the fundamental laws of physics governing gas compression. For every unit of vertical distance the air descends, the pressure increases by a set amount, leading to a fixed, corresponding temperature increase.
The DALR describes the temperature change of the moving air parcel itself. This is separate from the Environmental Lapse Rate (ELR), which describes the temperature structure of the surrounding, stationary atmosphere. The DALR also differs from the Moist or Saturated Adiabatic Lapse Rate (MALR). The MALR is a lower rate of temperature change for rising, saturated air due to the release of latent heat from condensation. Since descending air is typically dry, the DALR is the rate used to calculate its warming.
Meteorological Impacts of Descending Air
The principle of adiabatic warming profoundly affects local weather and climate, especially in mountainous regions. The most recognizable examples are downslope winds, such as the Chinook wind in the North American Rocky Mountains and the Foehn wind in the European Alps. These winds are known for bringing about rapid temperature increases.
This effect occurs when airflow is forced up and over a mountain range. The air cools and drops its moisture as precipitation on the windward side. The now-dry air descends on the leeward side, warming at the higher DALR. This results in a warm, dry wind that can melt snow quickly, leading to the Chinook’s nickname of “snow eater.” These winds have been observed to raise winter temperatures by as much as 30°C or more in a matter of hours.
Subsidence and High-Pressure Systems
Descending air masses also play a significant role in high-pressure systems. In these systems, air sinks over a broad region, a process called subsidence. This large-scale sinking motion compresses the air column below it, causing widespread adiabatic warming near the surface. Subsidence contributes to stable atmospheric conditions, leading to clear skies and warmer temperatures over a large area.