Weather Education: What is the Cap?

The cap or capping inversion is a layer of relatively warm air aloft (Typically several thousand feet above the ground) which suppresses or delays the development of thunderstorms. Air parcels (Pink arrow) rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further and they fall back to the surface (Orange arrow). 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 (Blue arrow). Pictured is a thunderstorm in Texas shot by Zach that has broken the cap. You can see the explosive nature of growth. The cap is an important ingredient in most severe thunderstorms, 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.

Example of a Cap Inversion(2)d