Heavy and dense cloud , with a considerablevertical extent, in the form of a mountain or huge towers. At least part of its upper portion is usually smooth, or fibrous or striated, and nearly always flattened; this part often spreads out in the shape of an anvil or vast plume. Under the base of this c;oud which is often very dark, there are frequently low ragged clouds either merged with it or not, and precipitation sometimes in the form of virga.