1. What is LFC?
The LFC (Level of Free Convection) is the lower boundary of the most significant region of CAPE in the troposphere. It is the point at which a lifted parcel of air will become equal in temperature to that of the environmental temperature. Once a parcel of air is lifted to the LFC it will rise buoyantly on its own all the way to the top of the CAPE region.
2. How is LFC determined?
Find the region of CAPE on the sounding. The pressure level at the bottom of the CAPE region that is closest to earth's surface is the LFC. The LFC on the sounding below is at 809 mb (notice at this pressure level that the parcel temperature and environmental temperature are the same).
3. Operational significance of LFC:
Tornadoes: A LFC closer to the surface is more supportive of tornadoes in a severe thunderstorm environment. This is because the region of +CAPE (region where buoyant rising occurs) is closer to the surface.