The Equilibrium Level, or EL, is the height at which a rising parcel cools back to the temperature of its environment near the top of its positively buoyant layer, so parcel theory predicts the parcel decelerates and stops rising on its own from that level. On a Skew-T, the EL is the upper crossing of the parcel path and the environmental temperature curve, above the LFC.
The physics
Between the LFC and the EL, the parcel is warmer than the environment and accelerating upward. As it rises into the upper troposphere, the environmental temperature curve typically bends warmer near the tropopause. At the EL, the parcel and environment temperatures match again, buoyancy goes to zero, and the parcel decelerates.
The height of the EL therefore depends on both how warm the parcel is when it started and how the environmental temperature profile behaves near the tropopause. A hotter, moister surface parcel reaches a higher EL. A colder tropopause allows the parcel to remain buoyant to a greater height. A low, warm tropopause caps the EL lower in the column.
How forecasters use it
EL height is a common proxy for expected anvil altitude and storm top. Satellite infrared brightness temperatures near the anvil can be compared with the EL temperature to get a first estimate of how much the storm is overshooting.
EL is also a term in some composite indices and enters the calculation of the effective inflow layer, because the effective bulk wind difference is measured from the effective inflow base to about half the depth between that base and the EL. A shallow EL therefore produces a shallow effective layer, which changes how deep-layer shear should be interpreted.
Important limits
Parcel theory ignores vertical momentum. A parcel that reaches the EL still has its upward velocity, and inertia carries it above that level before drag and negative buoyancy bring it back down. The overshooting top on strong storms is exactly this behavior, and its height above the EL scales roughly with updraft strength.
EL height also depends on parcel choice. A surface parcel from a moist boundary layer gives one EL. A most-unstable parcel from an elevated moist layer above a stable near-surface layer gives another. And like every parcel diagnostic, the EL from a sounding describes the environment, not any particular storm, so its usefulness rests on the sounding being representative of the air the storm ingests.