Clouds That Spell Severe Weather

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Shady Clouds

Man looking up stormy sky
James Jordan Photography / Getty Images

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 they're linked to could give you a head start to finding shelter!

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Cumulonimbus

Thunder
Cumulonimbus is the quintessential thunderstorm cloud. KHH 1971 / Getty Images

Cumulonimbus clouds are thunderstorm clouds. 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 there's 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.

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Anvil Clouds

Super Cell Thunderstorm
Anvil clouds are named for their anvil-like appearance. Skyhobo / Getty Images

An anvil cloud isn't 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! 

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Mammatus

Burwell Mammatus Landscape
Ryan McGinnis / Getty Images

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 aren't dangerous -- they simply signal that a storm may be nearby. 

When seen in association with thunderstorm clouds, they're typically found on the underside of anvils.

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Wall Clouds

Large Wall Cloud
Watch wall clouds carefully -- they're where tornadoes form!. NZP Chasers / Getty Images

Wall clouds form under the rain-free base (bottom) of cumulonimbus clouds. It takes it's 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. In other words, it is the cloud from which a tornado spins.      

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. 

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Shelf Clouds

Sedgewick Shelf Cloud II
Ryan McGinnis / Getty Images

Like wall clouds, shelf clouds also form underneath thunderstorm clouds. As you can imagine, this fact doesn't help observers differentiate between the two. While one is easily mistaken for the other to the untrained eye, cloud spotters know that a shelf cloud is associated with thunderstorm outflow (not inflow like wall clouds) and can be found in the storm's precipitation area (not rain-free area like walls clouds). 

Another hack to telling a shelf cloud and wall cloud apart is to think of rain "sitting" on the shelf and a tornado funnel "coming down" from the wall. 

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Funnel Clouds

Dramatic funnel cloud created in dark storm clouds with silhouetted evergreen trees
Tornadoes start as funnel clouds in the sky. Michael Interisano / Design Pics / Getty Images

One of the most feared and easily recognized storm clouds is the funnel cloud. Produced when a rotating column of air condenses, funnel clouds are the visible part of tornadoes that extend downward from the parent thunderstorm cloud.  

But remember, not until the funnel reaches the ground or "touches down" is it called a tornado!  

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Scud Clouds

Scenic View Of Sea Against Cloudy Sky
Julia Jung / EyeEm / Getty Images

Scud clouds aren't dangerous clouds in and of themselves, but because they form when warm air from outside of a thunderstorm is lifted up by its updraft, seeing scud clouds is a good indication that a cumulonimbus cloud (and hence, a thunderstorm) is nearby. 

Their low height above ground, ragged appearance, and presence beneath cumulonimbus and nimbostratus clouds mean scud clouds are often mistaken for funnel clouds. But there's one way to tell the two apart -- look for rotation. Scud do move when caught in the outflow (downdraft) or inflow (updraft) regions but that motion typically isn't rotation. 

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Roll Clouds

Arcus Roll Cloud, Eastern Argentinian Coast
Donovan Reese / Getty Images

Roll or arcus clouds are tube-shaped clouds that literally look like they've been rolled up into a horizontal band across the sky. They appear low in the sky and are one of the few severe weather clouds that are actually detached from the storm cloud base. (This is one trick for telling them apart from shelf clouds.) Spotting one is rare, but will tell you where a thunderstorm's gust front or other weather boundary, like cold fronts or sea breezes lay, since these clouds are formed by outflows of cold air.

Those in aviation may recognize roll clouds by another name—Morning Glorys.

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Wave Clouds

Wave clouds Brixham, Devon
Wave clouds occur when vertical wind shear and stable air are great. Moorefam / Getty Images

Wave, or Kelvin-Helmholtz clouds, resemble breaking ocean waves in the sky. Wave clouds are created when air is stable and winds at the top of a cloud layer are moving faster across it than those below it, causing the top clouds to be whipped around in a downward curling motion after hitting the stable layer of air above.

While wave clouds aren't related to storms, they are a visual cue for aviators that a large amount of vertical wind shear and turbulence are in the area.  

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Asperitas Clouds

Asperitas Clouds
Asperitas clouds are the newest cloud type, proposed in 2009. J&L Images / Getty Images

Asperitas are another cloud type that resemble a roughened sea surface. They appear as if you were underwater looking upward toward the surface when the sea is particularly roughened and chaotic. 

Although they look like dark and storm-like doomsday clouds, asperitas tend to develop after convective thunderstorm activity has developed. Much is still unknown about this cloud type, as it is the newest species to be added to the World Meteorological Organization's International Cloud Atlas in over 50 years. 

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Spotting Clouds That Could Mean Danger

Father and son watching storm coming in
Ambre Haller / Getty Images

Now that you know which clouds are related to severe weather and what they look like, you're one step closer to becoming a storm spotter

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Means, Tiffany. "Clouds That Spell Severe Weather." ThoughtCo, Apr. 13, 2017, thoughtco.com/clouds-that-spell-severe-weather-4089934. Means, Tiffany. (2017, April 13). Clouds That Spell Severe Weather. Retrieved from https://www.thoughtco.com/clouds-that-spell-severe-weather-4089934 Means, Tiffany. "Clouds That Spell Severe Weather." ThoughtCo. https://www.thoughtco.com/clouds-that-spell-severe-weather-4089934 (accessed January 24, 2018).