The midlevel lapse rate is the environmental temperature decrease per kilometer through a middle-tropospheric layer, most often computed from 700 to 500 mb, and it is one of the primary controls on how much CAPE a given surface parcel produces. A larger value means the environment cools more quickly with height through that layer, so a lifted parcel remains warmer than its surroundings over a greater depth.
Why the midlevels matter
CAPE grows most quickly where the environment cools faster than the parcel does. Between the LFC and the equilibrium level, that gap is what buoys the parcel upward. A steep 700 to 500 mb lapse rate keeps the environmental curve to the left of the parcel path across a large depth of the mid-troposphere, which produces a deep, wide positive area on the Skew-T.
In the Plains, steep midlevel lapse rates typically arise when air originally mixed over the Mexican Plateau, the Rockies, or the desert Southwest is transported eastward at mid-levels and rides over cooler, more moist air near the ground. That vertical structure is the classical elevated mixed layer, and it is one of the reasons the region produces so many high-CAPE, capped-warm-sector days.
How forecasters use it
Midlevel lapse rates in the 7 to 8 degrees Celsius per kilometer range are commonly used as a rough marker of a favorable profile for deep, sustained updrafts. Values approaching 9 degrees per kilometer describe extremely steep environments where the parcel remains strongly buoyant into the mid-troposphere.
The parameter is checked on mesoanalysis maps and in individual soundings, alongside LCL, LFC, and CIN, before drawing conclusions about the day's storm mode. Combined with strong deep-layer shear, steep midlevel lapse rates favor rotating storms with the depth to produce large hail. Combined with weak shear, they can favor pulse-type storms with strong downdrafts.
Where it misleads
A steep 700 to 500 mb lapse rate does not describe what is happening above 500 mb or below 700 mb. A very warm upper troposphere or a strong cap immediately above 700 mb can undo much of what the steep midlevel value implies, so the parameter has to be read as one line of a longer sounding rather than as a standalone number.
Very steep midlevel lapse rates also often come with strong caps at the base of the elevated mixed layer, near 700 mb. On many Plains outbreak days the cap breaks explosively only along a boundary or after significant afternoon heating and moistening, and any assessment that focuses on midlevel lapse rate without also checking cap strength misses that timing question.