A wall cloud is an accessory cloud formation that appears as a localised, often rotating lowering of the base of a cumulonimbus or supercell. It forms in the area of strongest updraft, where warm, moist low-level air is drawn upward so rapidly that condensation occurs below the general cloud base, creating a menacing-looking descending protrusion. Its identification is of vital importance in operational meteorology because the wall cloud is the most reliable precursor to tornado formation. When the wall cloud shows persistent rotation (visible as slow spinning to the naked eye), it is called a mesocyclone and tornado risk rises dramatically. A funnel descending from the wall cloud to the ground constitutes a tornado. Not all wall clouds produce tornadoes, but nearly all significant tornadoes are born from one. A typical wall cloud has a diameter of 1–5 km and is generally located on the southern or southwestern flank of the storm (in the Northern Hemisphere). Visually, it is distinguished from other formations by its smooth upper surface (versus the rough texture of the cumulonimbus), its position lower than the surrounding base, and the absence of precipitation in its immediate interior (rain falls in adjacent areas). Storm chasers look for it as the primary sign of severe activity.