The Lifted Condensation Level, or LCL, is the height at which a lifted air parcel first cools to its dewpoint and becomes saturated, so it is the pressure level where condensation and the base of a parcel-driven cloud can begin. On a Skew-T, it is the point where the dry adiabat from the parcel's starting temperature and the mixing ratio line from its starting dewpoint cross.
The physics
As a parcel rises, it expands into lower pressure and cools at the dry adiabatic lapse rate of about 9.8 degrees Celsius per kilometer. Its water vapor content stays fixed, so as the temperature falls the relative humidity climbs. When the temperature reaches the dewpoint, the air is saturated and any further cooling produces condensation.
That level of first saturation is the LCL. Above it, the parcel follows a moist adiabat because condensation releases latent heat. Below it, the parcel is dry and unsaturated. The LCL is therefore the natural cloud base for an ascent driven only by parcel physics.
How forecasters use it
The most common use is estimating cloud base height. A low LCL means a moist boundary layer and a cloud base near the ground. A high LCL means a drier boundary layer and a cloud base thousands of feet aloft. Craven, Jewell, and Brooks compared observed convective cloud-base heights with LCL calculations and found the mean-layer LCL, using the lowest 100 hPa, matched observed bases much more closely than the surface-based LCL.
In severe weather forecasting, low LCLs contribute to significant tornado composites because moister low-level environments tend to produce cooler, less negatively buoyant rear-flank downdrafts, which some studies associate with a more favorable environment for near-ground rotation. High LCLs, by contrast, are more common in supercell environments that produce large hail and damaging wind but fewer significant tornadoes.
Important limits
The LCL height depends on which parcel is chosen. A surface-based LCL uses the actual surface temperature and dewpoint and can be biased low over a strongly heated afternoon with a shallow moist layer. A mean-layer LCL averages the lowest hundred millibars and is generally closer to the observed cloud base.
The LCL also assumes the parcel actually gets lifted. A capped environment may show a very favorable LCL and never produce a single cloud from parcel ascent. And even when storms form, the visible cloud base can differ from the calculated LCL because storm-scale processes, such as forced lift, rain-cooled inflow, and mixing, all modify the parcel before it reaches saturation.