Capping inversion

A capping inversion is an elevated inversion layer that caps a convective boundary layer. The boundary layer is that which is closest to the ground. Normally, the sun heats the ground, which in turn heats the air just above it.  Thermals form when this warm air rises (warm air is less dense than cold air) into the cold air, a process known as convection. A convective layer such as this has the potential for cloud formation, since as the warm air rises and cools condensation occurs.
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A layer of relatively warm air aloft (usually several thousand feet above the ground) which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability . However if the cap is removed or weakened, then explosive thunderstorm development can occur. See CIN 

The cap is an important ingredient in most severe thunderstorm episodes, as it serves to separate warm, moist air below and cooler, drier air above. With the cap in place, air below it can continue to warm and/or moisten, thus increasing the amount of potential instability. Or, air above it can cool, which also increases potential instability. But without a cap, either process (warming/moistening at low levels or cooling aloft) results in a faster release of available instability - often before instability levels become large enough to support severe weather development.