What are cumulonimbus clouds?  Cumulonimbus clouds are menacing looking multi-level clouds, extending high into the sky in towers or plumes. More commonly known as thunderclouds, cumulonimbus are the only cloud type that can produce hail, thunder and lighting. The base of the cloud is often flat with a very dark wall like feature hanging underneath, and may only lie a few hundred feet above the Earth's surface.   How do cumulonimbus clouds form?  Cumulonimbus clouds are born through convection, often growing from small cumulus clouds over a hot surface. They get taller and taller until they represent huge powerhouses, storing the same amount of energy as 10 Hiroshima-sized atom bombs. They can also form along cold fronts as a result of forced convection, where milder air is forced to rise over the incoming cold air.  What weather is associated with cumulonimbus clouds?  Cumulonimbus clouds are associated with extreme weather such as heavy torrential downpours, hail storms, lightning and even tornadoes. Individual cumulonimbus cells will usually dissipate within an hour once showers start falling, making for short-lived, heavy rain. However multicell or supercell storms contain many cumulonimbus clouds and the intense rainfall may last much longer.  If there is thunder, lightning or hail, the cloud is a cumulonimbus, rather than nimbostratus.  Source: https://www.metoffice.gov.uk/learning/clouds/low-level-clouds/cumulonimbus

What are cumulonimbus clouds?

Cumulonimbus clouds are menacing looking multi-level clouds, extending high into the sky in towers or plumes. More commonly known as thunderclouds, cumulonimbus are the only cloud type that can produce hail, thunder and lighting. The base of the cloud is often flat with a very dark wall like feature hanging underneath, and may only lie a few hundred feet above the Earth's surface. 

How do cumulonimbus clouds form?

Cumulonimbus clouds are born through convection, often growing from small cumulus clouds over a hot surface. They get taller and taller until they represent huge powerhouses, storing the same amount of energy as 10 Hiroshima-sized atom bombs. They can also form along cold fronts as a result of forced convection, where milder air is forced to rise over the incoming cold air.

What weather is associated with cumulonimbus clouds?

Cumulonimbus clouds are associated with extreme weather such as heavy torrential downpours, hail storms, lightning and even tornadoes. Individual cumulonimbus cells will usually dissipate within an hour once showers start falling, making for short-lived, heavy rain. However multicell or supercell storms contain many cumulonimbus clouds and the intense rainfall may last much longer.

If there is thunder, lightning or hail, the cloud is a cumulonimbus, rather than nimbostratus.

Source: https://www.metoffice.gov.uk/learning/clouds/low-level-clouds/cumulonimbus

A shelf cloud is a low, horizontal wedge-shaped cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). A rising cloud motion often can be seen in the leading part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.  Most false tornado and false funnel cloud reports are associated with shelf clouds. Shelf clouds are low-hanging, horizontal cloud features attached to the front side of lines of storms or even a single storm. Usually there isn't any persistent rotation on a vertical axis within shelf clouds or within individual cloud fragments that extend downward from the shelf cloud, therefore they are just another scary-looking cloud.  Shelf clouds often resemble snow plows, big waves or tsunamis and can be very scary-looking since they are usually low-hanging. Sometimes they may found only a couple hundred feet above the ground. There are two other phenomena that might resemble tornadoes or funnel clouds but are not 1) dark rain shafts or narrow columns of heavy rain, and 2) the white color of a hail shaft, a column of hail extending from the ground to the cloud base, may generate a light-dark contrast with surrounding rain, resulting in what might appear to be a funnel cloud or a tornado to the untrained eye.    Source:  NOAA   .

A shelf cloud is a low, horizontal wedge-shaped cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). A rising cloud motion often can be seen in the leading part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.

Most false tornado and false funnel cloud reports are associated with shelf clouds. Shelf clouds are low-hanging, horizontal cloud features attached to the front side of lines of storms or even a single storm. Usually there isn't any persistent rotation on a vertical axis within shelf clouds or within individual cloud fragments that extend downward from the shelf cloud, therefore they are just another scary-looking cloud.

Shelf clouds often resemble snow plows, big waves or tsunamis and can be very scary-looking since they are usually low-hanging. Sometimes they may found only a couple hundred feet above the ground. There are two other phenomena that might resemble tornadoes or funnel clouds but are not 1) dark rain shafts or narrow columns of heavy rain, and 2) the white color of a hail shaft, a column of hail extending from the ground to the cloud base, may generate a light-dark contrast with surrounding rain, resulting in what might appear to be a funnel cloud or a tornado to the untrained eye.

Source: NOAA

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What Are Scud Clouds?  Scud clouds are low, ragged and wind-torn cloud fragments, usually not attached to the thunderstorm base. They are often seen in association with, and behind, gust fronts.  Scud clouds DO NOT produce severe weather. Scud clouds are often mistaken for wall clouds and tornadoes, especially when attached to the thunderstorm base.  A way to differentiate scud clouds from wall clouds is to watch their relative position with respect to the rain area: scud clouds move away from the rain area while wall clouds maintain the same relative distance.    Source:  NOAA

What Are Scud Clouds?

Scud clouds are low, ragged and wind-torn cloud fragments, usually not attached to the thunderstorm base. They are often seen in association with, and behind, gust fronts.

Scud clouds DO NOT produce severe weather. Scud clouds are often mistaken for wall clouds and tornadoes, especially when attached to the thunderstorm base.

A way to differentiate scud clouds from wall clouds is to watch their relative position with respect to the rain area: scud clouds move away from the rain area while wall clouds maintain the same relative distance.

Source: NOAA

A funnel cloud is a vortex of condensated water vapor and air spinning at high velocity. Funnel clouds can form under cumulus clouds if there is enough humidity and vorticity in the air. Landspouts and waterspouts are funnel clouds which reach to ground level, but are usually weak, as opposed to tornadoes. A tornado by definition is a funnel cloud created by mesoscale storm rotation rather than the small vorticity in the air which is responsible for funnel clouds.  Source: http://www.weatherscapes.com/album.php?cat=clouds&subcat=funnel_clouds

A funnel cloud is a vortex of condensated water vapor and air spinning at high velocity. Funnel clouds can form under cumulus clouds if there is enough humidity and vorticity in the air. Landspouts and waterspouts are funnel clouds which reach to ground level, but are usually weak, as opposed to tornadoes. A tornado by definition is a funnel cloud created by mesoscale storm rotation rather than the small vorticity in the air which is responsible for funnel clouds.

Source: http://www.weatherscapes.com/album.php?cat=clouds&subcat=funnel_clouds

A  wall cloud  is an isolated cloud lowering attached to the rain-free base of the thunderstorm. The wall cloud is usually to the rear of the visible precipitation area.  A wall cloud that may produce a tornado usually exists for 10–20 minutes before a tornado appears. A wall cloud may also persistently rotate (often visibly), have strong surface winds flowing into it, and may have rapid vertical motion indicated by small cloud elements quickly rising into the rain-free base.  As the storm intensifies, the updraft draws in low-level air from several miles around. Some low-level air is pulled into the updraft from the rain area. This rain-cooled air is very humid; the moisture in the rain-cooled air quickly condenses below the  rain-free base  to form the wall cloud.   Source: https://www.nssl.noaa.gov/education/svrwx101/tornadoes/

A wall cloud is an isolated cloud lowering attached to the rain-free base of the thunderstorm. The wall cloud is usually to the rear of the visible precipitation area.

A wall cloud that may produce a tornado usually exists for 10–20 minutes before a tornado appears. A wall cloud may also persistently rotate (often visibly), have strong surface winds flowing into it, and may have rapid vertical motion indicated by small cloud elements quickly rising into the rain-free base.

As the storm intensifies, the updraft draws in low-level air from several miles around. Some low-level air is pulled into the updraft from the rain area. This rain-cooled air is very humid; the moisture in the rain-cooled air quickly condenses below the rain-free base to form the wall cloud. 

Source: https://www.nssl.noaa.gov/education/svrwx101/tornadoes/

the whale's mouth-effect is the weird-looking sky sometimes appearing when the first gust front of a storm is passing over. The cold downdraft of air of the storm rushes outward and forward along the surface, and lifts the warmer air in the direct vicinity of the storm over its condensation level. One gets to see the back side of this mini cold front, and the inside of the gust front cloud. It looks somewhat like mammatus and other forms of turbulent cloud masses, sometimes showing very neat ordering.  Source: http://www.weatherscapes.com/album.php?cat=clouds&subcat=whales_mouth

the whale's mouth-effect is the weird-looking sky sometimes appearing when the first gust front of a storm is passing over. The cold downdraft of air of the storm rushes outward and forward along the surface, and lifts the warmer air in the direct vicinity of the storm over its condensation level. One gets to see the back side of this mini cold front, and the inside of the gust front cloud. It looks somewhat like mammatus and other forms of turbulent cloud masses, sometimes showing very neat ordering.

Source: http://www.weatherscapes.com/album.php?cat=clouds&subcat=whales_mouth

What Are Mammatus Clouds?   Share  |  Mammatus are pouch-like cloud structures. They're also a rare example of clouds in sinking air-- most clouds form in rising air. Although mammatus most frequently form on the underside of a cumulonimbus, they can develop underneath cirrocumulus, altostratus, altocumulus and stratocumulus.  For a mammatus to form, the sinking air must be cooler than the air around it and have high liquid water or ice content. They derive their name from their appearance, like the bag-like sacs that hang beneath the cloud resemble cow's udders.  Mammatus are long-lived if the sinking air contains large drops and snow crystals since larger particles require greater amounts of energy for evaporation to occur. Over time, the cloud droplets do eventually evaporate and the mammatus dissolve.  Despite popular misconception, mammatus clouds are not a sign that a tornado is about to form. While associated with thunderstorms, mammatus clouds are not necessarily an indicator of severe weather. Mammatus result from the sinking of moist air into dry air. They are in essence upside-down clouds. The sharp boundary of mammatus is much like the sharp boundary of a rising cumulonimbus cloud before an anvil has formed.    Source:  NOAA    https://www.accuweather.com/en/weather-glossary/what-are-mammatus-clouds/5506130       

What Are Mammatus Clouds?

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Mammatus are pouch-like cloud structures. They're also a rare example of clouds in sinking air-- most clouds form in rising air. Although mammatus most frequently form on the underside of a cumulonimbus, they can develop underneath cirrocumulus, altostratus, altocumulus and stratocumulus.

For a mammatus to form, the sinking air must be cooler than the air around it and have high liquid water or ice content. They derive their name from their appearance, like the bag-like sacs that hang beneath the cloud resemble cow's udders.

Mammatus are long-lived if the sinking air contains large drops and snow crystals since larger particles require greater amounts of energy for evaporation to occur. Over time, the cloud droplets do eventually evaporate and the mammatus dissolve.

Despite popular misconception, mammatus clouds are not a sign that a tornado is about to form. While associated with thunderstorms, mammatus clouds are not necessarily an indicator of severe weather. Mammatus result from the sinking of moist air into dry air. They are in essence upside-down clouds. The sharp boundary of mammatus is much like the sharp boundary of a rising cumulonimbus cloud before an anvil has formed.

Source: NOAA

https://www.accuweather.com/en/weather-glossary/what-are-mammatus-clouds/5506130