by neufer » Wed Jun 05, 2013 12:51 pm
Cloudgazer wrote:
Hi, with a little luck I can share with you these 3 images of the roll cloud I sighted mid-May in South Africa.
The sunset lighting was not conducive to great photos with a small digi cam, the reduction in size to upload even takes more away from them, but anyway: I hope someone enjoys looking at them ...
Very pretty, Cloudgazer. But this seem to be Kelvin–Helmholtz instability due to velocity shear and/or lee waves:
http://en.wikipedia.org/wiki/Kelvin-Helmholtz_instability wrote:
<<The Kelvin–Helmholtz instability (after Lord Kelvin and Hermann von Helmholtz) can occur when there is velocity shear in a single continuous fluid, or where there is a velocity difference across the interface between two fluids. An example is wind blowing over water: The instability manifests in waves on the water surface. More generally, clouds, the ocean, Saturn's bands, Jupiter's Red Spot, and the sun's corona show this instability.>>
http://en.wikipedia.org/wiki/Mountain_waves wrote:
<<In meteorology, lee waves are atmospheric standing waves. The most common form is mountain waves, which are atmospheric internal gravity waves. These were discovered in 1933 by two German glider pilots, Hans Deutschmann and Wolf Hirth, above the Krkonoše. They are periodic changes of atmospheric pressure, temperature and orthometric height in a current of air caused by vertical displacement, for example orographic lift when the wind blows over a mountain or mountain range. They can also be caused by the surface wind blowing over an escarpment or plateau, or even by upper winds deflected over a thermal updraft or cloud street.>>
http://en.wikipedia.org/wiki/Arcus_cloud wrote:
An arcus cloud is a low, horizontal cloud formation. Roll clouds and shelf clouds are the two types of arcus clouds. A shelf cloud is usually associated with the leading edge of thunderstorm outflow; roll clouds are usually formed by outflows of cold air from sea breezes or cold fronts in the absence of thunderstorms.
A roll cloud 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. One of the most famous frequent occurrences is the Morning Glory cloud in Queensland, Australia. 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. However, similar features can be created by downdrafts from thunderstorms and are not exclusively associated with coastal regions.
Coastal roll clouds have been seen over California, the English Channel, Shetland Islands, Lithuania, Eastern Russia, other maritime regions of Australia, off the Mexican coast in the Sea of Cortez, Uruguay, in the Canadian provinces of Nova Scotia and Ontario, and Campos dos Goytacazes and Coronel Vivida bay in Brazil.>>
[quote="Cloudgazer"]
Hi, with a little luck I can share with you these 3 images of the roll cloud I sighted mid-May in South Africa.
The sunset lighting was not conducive to great photos with a small digi cam, the reduction in size to upload even takes more away from them, but anyway: I hope someone enjoys looking at them ...[/quote]
Very pretty, Cloudgazer. But this seem to be Kelvin–Helmholtz instability due to velocity shear and/or lee waves:
[quote=" http://en.wikipedia.org/wiki/Kelvin-Helmholtz_instability"]
<<The Kelvin–Helmholtz instability (after Lord Kelvin and Hermann von Helmholtz) can occur when there is velocity shear in a single continuous fluid, or where there is a velocity difference across the interface between two fluids. An example is wind blowing over water: The instability manifests in waves on the water surface. More generally, clouds, the ocean, Saturn's bands, Jupiter's Red Spot, and the sun's corona show this instability.>>[/quote][quote=" http://en.wikipedia.org/wiki/Mountain_waves"]
<<In meteorology, lee waves are atmospheric standing waves. The most common form is mountain waves, which are atmospheric internal gravity waves. These were discovered in 1933 by two German glider pilots, Hans Deutschmann and Wolf Hirth, above the Krkonoše. They are periodic changes of atmospheric pressure, temperature and orthometric height in a current of air caused by vertical displacement, for example orographic lift when the wind blows over a mountain or mountain range. They can also be caused by the surface wind blowing over an escarpment or plateau, or even by upper winds deflected over a thermal updraft or cloud street.>>[/quote][quote=" http://en.wikipedia.org/wiki/Arcus_cloud"]
An arcus cloud is a low, horizontal cloud formation. Roll clouds and shelf clouds are the two types of arcus clouds. A shelf cloud is usually associated with the leading edge of thunderstorm outflow; roll clouds are usually formed by outflows of cold air from sea breezes or cold fronts in the absence of thunderstorms.
[size=150]A roll cloud is [b][u][color=#FF0000]a low[/color][/u][/b], horizontal, tube-shaped, and relatively rare type of arcus cloud. They differ from shelf clouds by being [b][u][color=#0000FF]completely detached from other cloud features[/color][/u][/b]. Roll clouds usually appear to be "rolling" about a horizontal axis.[/size]
[b][u][color=#0000FF]They are a solitary wave[/color][/u][/b] called a soliton, which is a wave that has a single crest and moves without changing speed or shape. One of the most famous frequent occurrences is the Morning Glory cloud in Queensland, Australia. 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. However, similar features can be created by downdrafts from thunderstorms and are not exclusively associated with coastal regions.
Coastal roll clouds have been seen over California, the English Channel, Shetland Islands, Lithuania, Eastern Russia, other maritime regions of Australia, off the Mexican coast in the Sea of Cortez, Uruguay, in the Canadian provinces of Nova Scotia and Ontario, and Campos dos Goytacazes and Coronel Vivida bay in Brazil.>>[/quote]