The LCL is the Lifting Condensation Level. Simply put it is the cloud base height.
When you take a dry parcel of air at the surface and allow it to rise it will cool. Once the parcel cools to the same temperature as the dewpoint water vapor will condense and form a cloud. The level at which that happens is called the LCL. That's why clouds usually have their bases all at the same level.
The drier the air is the higher the LCL will be. That is when we talk about high-based convection, with cloud bases several thousand feet up. On more humid days the LCL will be much closer to the ground.
Originally posted by B Ozanne
The drier the air is the higher the LCL will be. That is when we talk about elevated convection, with cloud bases several thousand feet up. On more humid days the LCL will be much closer to the ground.
Just a minor note. "Elevated" convection usually refers to convection that is drawing its parcels from ABOVE the surface. For example, suppose there is a cap at 850mb. A storm could still develop, but it's updraft parcel source would be above 850mb -- say, at 825-775mb. In other words, the storm updraft is drawing it's air from a region above the surface. This is a rather common occurrence during the spring/summer when the nocturnal strengthening of the low-level jet initiates elevated convection above a nocturnally-strengthening capping inversion.
With large T-Td deficits, the LCLs are high, and the convection is termed "high-based". The parcel source layer can still be from the surface, thus the storm is still "surface-based", not elevated.