A cap is a layer of the troposphere that prevents convective uplift from turning into deep convection. Thus, in a thunderstorm set-up, a cap will prevent storms from developing. What creates the stability of the cap is a temperature profile that causes parcels of air that try to penetrate it to become stable and sink back down. Often the cap will have relatively warm temperatures. An inversion could be in place or the cooling with height is not significant enough to allow for convective uplift to penetrate it. A parcel of air will rise when the surrounding air is denser (colder) than the parcel air. A cap causes the reverse situation since the surrounding air will be less dense (warmer) than the parcel air.

The pedagogical diagram below shows an analogy of a cap. On the diagram, from the surface to the LFC (Level of Free Convection), the basketball is denser than the surrounding air. Thus, the basketball will not rise since it is denser than the surrounding air. The capped basketball just sits on the ground and no convection takes place. However, if the basketball could find a way to lift up to the LFC, then it would explosively rise on its own since the basketball is less dense than the surrounding more dense water. The basketball would continue to rise up to the EL (Equilibrium Level) until it reaches the elevation in which only air surrounds the basketball again. The basketball will actually overshoot the EL somewhat (overshooting top) since the momentum of the rising basketball takes time to slow down before it stops rising.

A cap is weakened by solar heating of the surface, dynamic uplift and mixing of the stable air within the cap with more unstable air. These three mechanisms can be thought of as processes that would move the basketball from the surface to closer to the LFC. Once the basketball reaches the LFC then it is said that the cap is “broken”, and convective uplift throughout the troposphere can commence.