Capping inversion

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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.

An inversion layer is when the normal temperature (warm air below, cold air above) profile is reversed, creating a stable configuration of dense, cold air sitting below lighter, warm air.

An elevated inversion layer is thus a region of warm air above a region of cold air, but higher in the atmosphere (generally not touching the surface).

A capping inversion occurs when there is a boundary layer with a normal temperature profile (warm air rising into cooler air) and the layer above that is an inversion layer (cooler air below warm air). Cloud formation from the lower layer is capped by the inversion layer, which can lead to severe thunderstorms. If the capping inversion layer or "cap" is too strong (too close to the surface), it will prevent thunderstorms from developing. This can result in fog.