In meteorology, convection refers primarily to atmospheric motions in the vertical direction.
An air parcel will rise naturally if the air within the parcel is warmer than the surrounding air (like a hot air balloon). Therefore, if cool air is present aloft with warm air at lower levels, thermals can rise to great heights before losing their buoyancy.
Successive thermals following the same path usually rise higher than previous ones, and if a thermal is able to rise high enough to cool to its saturation point, the moisture within condenses and becomes visible as a cloud.
Photograph by: Holle
When a deep stable layer exists just above the cloud base, continued vertical growth is restricted and only fair weather cumulus are able to form. However, if a deep unstable layer (cold air aloft) is present, continued vertical growth is likely, leading to the development of a cumulonimbus cloud, which contains raindrops. Once the supply of thermals is cut off, the cloud begins to dissipate and eventually disappears. Convective clouds are typically much more vertically developed than those clouds generated by convergence lifting.