A lapse rate is the rate at which an atmospheric variable, usually temperature, decreases with height through a specified layer, most often expressed in degrees Celsius per kilometer. A larger positive value means the temperature falls off faster with height, which corresponds to a more unstable environmental profile for a lifted parcel.
The reference values
Two reference lapse rates set the physical scale. The dry adiabatic lapse rate is close to 9.8 degrees Celsius per kilometer and describes how an unsaturated parcel cools as it rises. The moist adiabatic lapse rate is variable but generally in the range of 4 to 7 degrees per kilometer in the mid-troposphere, because latent heat released by condensation slows the cooling.
The environmental lapse rate is the actual observed rate in the sounding. When it lies between the dry and moist adiabatic rates, the layer is conditionally unstable. When it exceeds the dry adiabatic rate, the layer is absolutely unstable and mixing removes the excess almost immediately. When it is smaller than the moist adiabatic rate, the layer is absolutely stable and resists both saturated and unsaturated ascent.
How forecasters use it
Reading lapse rates layer by layer is one of the first steps in a convective forecast. Steep mid-level lapse rates, usually evaluated in the 700 to 500 mb layer, favor stronger updrafts by giving the parcel a warmer-than-environment path higher in the column. Steep low-level lapse rates favor rapid boundary-layer mixing during the day and often show up ahead of dryline setups.
Composite parameters and stability indices are built on lapse rates. The lifted index, K index, and total-totals index all reduce specific layer differences to a scalar. Downdraft CAPE relies on the low-level lapse rate to describe how easily a descending parcel can cool through the subcloud layer.
Where it misleads
Lapse rate averages hide inversions. A 700 to 500 mb lapse rate of 8 degrees per kilometer can describe a smoothly cooling column, or a strongly cooling upper part beneath a warm spike near the top of the layer that would not be evident from the single number.
Lapse rate also says nothing about moisture. A steep lapse rate over a bone-dry boundary layer produces little more than a well-mixed subsident afternoon and no storms, because there is no fuel for the parcels to release once lifted. Reading lapse rates alongside moisture and forcing is the way they become useful.