Anyone who lives in tornado alley has heard of the dreaded “wall cloud”. We know it can accompany severe weather and even lead to devastating tornadoes, but what exactly is it? A wall cloud is a cloud that is lowered from a thunderstorm, forming when rapidly rising air causes lower pressure below the storm’s main updraft. In layman’s terms – a big scary cloud.

As the storm builds, the updraft pulls in low-level air from several miles around which is then drawn into the updraft. This rain-cooled air is very humid, causing the moisture to condense below the rain-free base to form the wall cloud.

Wall clouds are part of supercell thunderstorms that produce a rotating updraft caused by wind shear. Supercells can be violent thunderstorms that involve hail, lightning and pounding rain. Not all wall clouds rotate, but if there is rotation, this is the first sign that a tornado may be in the works

Persistent wall clouds have the best chance of forming tornadoes and can exist for up to 20 minutes before a tornado actually forms. According to the National Weather Service, the diameter of a wall cloud can range from a fraction of a mile to nearly five miles.

Weather spotters stay focused on wall clouds and their rotation to determine if there needs to be a tornado warning issued. Wall clouds have the terrifying ability to “rain wrap” tornadoes in torrential rain, making them difficult to spot. Fortunately, only a small percentage of wall clouds produce tornadoes. With advanced warning from meteorologists and available safety measures, you can keep yourself safe from these beautiful but deadly displays of nature.