Low-Level Lapse Rates (C km-1)
A lapse rate is the rate of temperature change with height. The faster the temperature decreases with height, the "steeper" the lapse rate and the more "unstable" the atmosphere becomes.
Lapse rates are shown in terms of degrees Celcius change per kilometer in height. Values less than 5.5-6.0 C km-1("moist" adiabatic) represent "stable" conditions, while values greater than 9.8 C km-1 ("dry" adiabatic) are considered "absolutely unstable." In between these two values, lapse rates are considered "conditionally unstable." Conditional instability means that if enough moisture is present, lifted air parcels could have a negative LI (lifted index) or positive CAPE.
The 0-3 km lapse rates, also referred to as low-level lapse rates, are meant to identify regions of deeper mixing (e.g., steeper lapse rates) that often result in weakening convective inhibition that precedes surface-based thunderstorm development, as well as the potential for strong downdrafts in the low levels.