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This amazing picture shows a rare phenomenon called a roll cloud which tend to form ahead of a cold front and can stretch for miles.
Photographer Daniela Eberl took this snap at Las Olas Beach in Maldonado, Uruguay. Photo: NATIONAL NEWS
They are most common when an advancing storm front causes moist air to rise, then cool to the point where it becomes a cloud known as the dew point.
When this happens along a front, a roll cloud can form, often with air actually circulating along the horizontal axis of the cloud.
Roll cloud A roll cloud in Uruguay. 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 the thunderstorm base or other cloud features. Roll clouds usually appear to be "rolling" about a horizontal axis. They can be a sign of possible microburst activity.