Convective clouds may range from relatively innocuous showers to major supercell systems accompanied by dramatic conditions. ‘Severe and unusual weather events’ explains how convective cells generate in cumulonimbus clouds and how thunderstorms and associated lightning develop. Convective clouds may cluster together resulting in vigorous circulation and heavy precipitation. Even more active and dramatic systems are supercells that create tornadoes. These arise when a deep pool of unstable air is accompanied by a strong increase in wind speed with height, together with directional wind shear. Tropical cyclones are closed, non-frontal, low-pressure systems of high winds arising some 5–10° away from the equator, where the Coriolis acceleration promotes their overall rotation.