Starship Asterisk*

A Roll Cloud Over Uruguay

Explanation: What kind of cloud is this? A type of arcus cloud called 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 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.

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Always an interesting phenomenon....

Anyone for a chorus of "Roll out the Barrel"?
:---[===] *

One hears of "a cold front moving in", and this is a case of seeing that cold front moving in.

World's largest contrail. Or is it a chemtrail?

A potentially misunderstood statement in the description is that air can circulate "along" the long horizontal axis of the cloud. Actually it circulates AROUND the horizontal long axis, and the cloud appears to roll. However, these usually stay still as the air moves (rolls) through them, as they are coastal clouds related to sea-land winds and they stay in position oriented parallel to the coast.

Here is a good time lapse video

Science fiction readers will be reminded of the Eye of God storm in Larry Niven's book Ringworld.