Shelf cloud A
shelf cloud is a low, horizontal, wedge-shaped arcus cloud attached to the base of the parent cloud, which is usually a thunderstorm cumulonimbus, but could form on any type of convective clouds. Rising air motion can often be seen in the leading (outer) part of the shelf cloud, while the underside can often appear as
turbulent and wind-torn. Cool, sinking air from a
storm cloud's
downdraft spreads out across the land surface, with the leading edge called a
gust front. This outflow cuts under warm air being drawn into the storm's
updraft. As the lower and cooler air lifts the warm moist air, its water condenses, creating a cloud which often rolls with the different winds above and below (
wind shear). People seeing a shelf cloud may believe they have seen a
wall cloud. This is likely to be a mistake, since an approaching shelf cloud appears to form a wall made of cloud. Shelf clouds usually appear on the leading edge of a storm, while wall clouds are usually at the rear of the storm. A sharp, strong gust front will cause the lowest part of the leading edge of a shelf cloud to be ragged and lined with rising
fractus clouds. In a severe case there will be
vortices along the edge, with twisting masses of
scud that may reach to the ground or be accompanied by rising dust. A very low shelf cloud accompanied by these signs is the best indicator that a potentially violent wind
squall is approaching. An extreme example of this phenomenon looks almost like a tornado and is known as a
gustnado. File:Shelf cloud pano oct07 ver4.jpg|Underside of a weak shelf cloud File:Shelf clouds gif.gif|A
time-lapse photography of shelf cloud just before a
thunderstorm in
Pondicherry, India File:Rolling-thunder-cloud.jpg|A shelf cloud over
Enschede, Netherlands File:Shelf cloud 1.jpg|A shelf cloud in Durango, Mexico
Roll cloud A
roll cloud (
Cloud Atlas name
volutus) is a low, horizontal, tube-shaped, and relatively rare type of arcus cloud. They differ from shelf clouds by being completely detached from other cloud features. Roll clouds usually appear to be "rolling" about a horizontal axis. They are a solitary wave called a
soliton, which is a wave that has a single crest and moves without changing speed or shape. This rolling is due to the variation in speed and direction of the winds with altitude (
wind shear). One of the most famous frequent occurrences is the
Morning Glory cloud in
Queensland,
Australia, which can occur up to four out of ten days in October. One of the main causes of the Morning Glory cloud is the
mesoscale circulation associated with sea breezes that develop over the
Cape York Peninsula and the
Gulf of Carpentaria. Such coastal roll clouds have been seen in many places, including
California, the
English Channel,
Shetland Islands, the North Sea coast, coastal regions of Australia, and Nome, Alaska. However, similar features can be created by
downdrafts from thunderstorms or advancing cold front, and are not exclusively associated with coastal regions. Roll clouds have not been associated with
funnel clouds or
tornadoes, as they are a horizontal vortex. File:Volutus Southern Ocean.jpg|A sequence of volutus clouds at sea in the
Drake Passage of the
Southern Ocean File:Rollclouds.jpg|Roll clouds over southern Brazil == See also ==