Heavy masses of dense cloud whose cumuliform summits rise in the forms of towers, the upper parts having a fibrous texture and often spreading out into the shape of an anvil; these clouds generally produce showers of rain and sometimes of snow, hail or soft hail, and often develop into thunderstorms; up to 12000 m; summits may be as cold as -50°C; strong convective motions with vertical velocities of 3 to greater than 30 m s-1.