A Cloud Spotters Guide to the Skys Signs of Bad Storms Ahead.B.S., Atmospheric Sciences and Meteorology, University of North Carolina.Tiffany Means is a meteorologist and member of the American Meteorological Society who has worked for CNN, the National Oceanic and Atmospheric Administration, and more.
When the threat of severe weather looms, clouds are often the first sign that skies are turning unfriendly. Look for the following types of clouds during disturbed weather; recognizing them and the severe weather theyre linked to could give you a head start to finding shelter. Once you know which clouds are related to severe weather and what they look like, youll be one step closer to becoming a storm spotter. They develop from convection the transport of heat and moisture upward into the atmosphere. But, whereas other clouds form when air currents rise several thousand feet and then condense where those currents stop, the convective air currents that create cumulonimbus are so powerful, their air rises tens of thousands of feet, condensing rapidly, and often while still journeying upward. The result is a cloud tower with bulging upper portions (that look something like cauliflower). If you see a cumulonimbus, you can be sure theres a nearby threat of severe weather, including bursts of rainfall, hail, and possibly even tornadoes. Generally, the taller the cumulonimbus cloud, the more severe the storm will be. An anvil cloud isnt a stand-alone cloud, but more of a feature that forms at the top of a cumulonimbus cloud. The anvil top of a cumulonimbus cloud is actually caused by it hitting the top of the stratosphere the second layer of the atmosphere. Since this layer acts as a cap to convection (the cooler temperatures at its top discourage thunderstorms), the tops of storm clouds have nowhere to go but outward. Strong winds high up fan this cloud moisture (so high up that it takes the form of ice particles) out over great distances, which is why anvils can extend outward for hundreds of miles from the parent storm cloud. Whoever first exclaimed The sky is falling must have seen mammatus clouds overhead. Mammatus appear as bubble-like pouches that hang on the underside of clouds. As odd as they look, mammatus arent dangerous they simply signal that a storm may be nearby. When seen in association with thunderstorm clouds, theyre typically found on the underside of anvils. Wall clouds form under the rain-free base (bottom) of cumulonimbus clouds. It takes its name from the fact that it resembles a dark gray wall (sometimes rotating) that lowers down from the base of the parent storm cloud, usually just before a tornado is about to form. Wall clouds form as the thunderstorm updraft draws in air near the ground from several miles around, including from the nearby rain shaft. This rain-cooled air is very humid and the moisture within it quickly condenses below the rain-free base to create the wall cloud. Like wall clouds, shelf clouds also form underneath thunderstorm clouds.
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