Level of free convection

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The Level of Free Convection (LFC) is any point in the atmosphere where the temperature of the environment decrease faster than the moist adiabatic lapse rate of a saturated parcel at the same level.

Diagram showing an air parcel path when raised along B-C-E compared to the surrounding airmass Temperature (T) and humidity (Tw)

The usual way of finding the LFC is to lift a parcel from a lower level along the dry adiabatic lapse rate until it crosses the mixing ratio line of the parcel: this is the Lifted condensation level (LCL). From there on, follow the moist adiabatic lapse rate until the temperature of the parcel reach the airmass temperature. If the temperature the parcel along the moist adiabat is warmer than the environment on further lift, one has found the LFC.

Since the volume of the parcel is larger than the surrounding air after LFC by the ideal gas law (PV = nRT), it is less dense and becomes buoyant rising until its temperature (in E) crosses back the airmass one. If the airmass has one or many LFC, it is potentially unstable and may lead to convective clouds like Cumulus and Thunderstorms.

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