skyeye
A Roll Cloud Over Uruguay

What kind of cloud is this? A roll cloud. These rare long clouds
may form near advancing cold fronts. In particular, a downdraft
from an advancing storm front can cause moist warm air to rise,
cool below its dew point, and so form a cloud. When this happens
uniformly along an extended front, a roll cloud may form. Roll
clouds may actually have air circulating along the long horizontal
axis of the cloud. A roll cloud is not thought to be able to morph
into a tornado. Unlike a similar shelf cloud, a roll cloud, a type of
Arcus cloud, is completely detached from their parent cumulonimbus
cloud. Pictured above, a roll cloud extends far into the distance in
2009 January above Las Olas Beach in Maldonado, Uruguay.