Cumuliform clouds are a group of clouds characterised by their vertical development, puffy or cauliflower-like upper portion, and relatively flat, horizontal base. They form by convection: solar heating of the surface generates rising currents of warm air that, as they ascend and cool, reach the condensation level (cloud base) and continue rising as long as they remain warmer than the surrounding air, creating increasingly tall towers. The cumuliform family includes a progression from lesser to greater vertical development: cumulus humilis (fair-weather cumulus, wider than tall), cumulus mediocris (moderate development), cumulus congestus (tall, vigorous towers, sometimes with pileus), and finally the cumulonimbus, the quintessential storm cloud reaching the tropopause at 12–18 km altitude. Altocumulus castellanus (convective towers at mid-levels) are also considered cumuliform. The presence and evolution of cumuliform clouds are key indicators of atmospheric state and tendency: small, scattered cumulus indicate fair weather; rapidly growing vertical cumulus signal increasing instability and possible thunderstorm development. In local forecasting, observing cumuliform evolution through the morning allows fairly reliable anticipation of afternoon storm probability. Mammatus on the underside of a cumulonimbus anvil indicate severe turbulence.