The lapse rate is a fundamental concept in atmospheric science describing how air temperature changes with altitude. It is typically expressed as the rate at which atmospheric temperature decreases for every increase in height. Understanding this vertical temperature gradient is foundational to predicting weather phenomena, such as cloud formation and thunderstorms. Meteorologists rely on different types of lapse rates to gauge the potential for vertical air movement.
The Underlying Principle: Adiabatic Temperature Change
The reason air temperature changes with height is due to adiabatic temperature change. This process describes the cooling or warming of an air parcel without exchanging heat with the surrounding air. When a parcel of air rises, it moves into an area of lower atmospheric pressure. The reduced pressure allows the gas inside to expand, requiring air molecules to use internal energy to work against the external pressure. This expansion causes the air parcel to cool, a process called adiabatic cooling.
The reverse occurs when an air parcel descends through the atmosphere into an area of higher pressure. The surrounding air compresses the parcel, resulting in the outside air doing work on it. This compression increases the internal energy of the molecules, causing the air parcel to warm without adding heat from an outside source. This principle of compression leading to warming and expansion leading to cooling drives the specific temperature change rates used in meteorology.
The Theoretical Rates: Dry vs. Moist Adiabatic Lapse Rates
The two primary theoretical lapse rates are the Dry Adiabatic Lapse Rate (DALR) and the Moist Adiabatic Lapse Rate (MALR), which are determined by the air’s saturation level. The DALR applies only to air that is considered unsaturated, meaning its relative humidity is less than 100%. For this dry air, the rate of temperature change is a constant value of approximately \(9.8^\circ C\) per kilometer of ascent, or about \(5.5^\circ F\) per 1,000 feet. This fixed rate is a consequence of the thermodynamic properties of dry air expanding and cooling.
This rate changes once the air parcel cools enough to reach its dew point, becoming saturated with water vapor. The air then begins to condense its moisture, forming clouds, and the MALR takes effect. The MALR (or Saturated Adiabatic Lapse Rate, SALR) is always lower than the dry rate because of the heat released during condensation. When water vapor changes phase into liquid water droplets, it releases latent heat energy into the air parcel, partially offsetting the cooling due to expansion.
Because the amount of water vapor—and thus the amount of latent heat released—varies widely, the MALR is not a constant value like the DALR. The moist rate ranges from about \(4^\circ C\) to \(9^\circ C\) per kilometer, depending on the air’s temperature and pressure. A parcel of warm, tropical air with a high moisture content will have a much lower MALR due to the greater amount of latent heat released. In contrast, a colder air parcel with less moisture will have an MALR closer to the dry rate.
Determining Weather Patterns: Environmental Lapse Rate and Atmospheric Stability
While the DALR and MALR describe how a rising air parcel should cool, the Environmental Lapse Rate (ELR) is the actual, measured temperature profile of the surrounding atmosphere. The ELR is routinely measured by weather balloons and varies greatly. Meteorologists compare the ELR to the theoretical adiabatic rates to determine the atmosphere’s stability, which dictates whether air will rise or sink vertically.
The atmosphere is considered absolutely unstable when the ELR is greater than the DALR. Any rising air parcel, whether saturated or unsaturated, will cool slower than the air around it, remaining warmer and more buoyant than its environment. This leads to rapid, unimpeded vertical movement, often resulting in towering cumulus clouds and the formation of severe thunderstorms.
Conversely, the atmosphere is considered absolutely stable when the ELR is less than the MALR. A rising air parcel cools faster than the surrounding air, quickly becoming colder and heavier than its environment. This suppresses all vertical motion, causing the air parcel to sink back down and preventing the formation of significant clouds or convection.
The third condition is conditional instability, which occurs when the ELR falls between the DALR and the MALR. In this state, the air is stable if unsaturated, but becomes unstable if forced upward until it reaches saturation. If the air is lifted high enough to condense its moisture and begin releasing latent heat, the parcel can become warmer than its environment and continue rising. This condition explains why mountains or weather fronts are often necessary to trigger the upward motion leading to cloud and precipitation development.