Arcus - Nature’s Wind Bag

Arcus is a low dense horizontal cloud that forms along the leading edge of some thunderstorms. Arcus come in two distinct forms; a shelf cloud or a horizontal roll cloud. When an entire arcus is observed it is often curved or a partial ring shape is visible - like an arc or segment of a circle. They form where the leading edge of cool air descending from the thunderstorm interacts with warm moist air streaming into the storm.

Arcus usually look very menacing. They are associated with strong straight-line winds rushing out of an approaching thunderstorm. Wind speeds can be too weak to cause damage in some situations or sometimes more than 100 mph in extreme cases. Winds of 45 to 70 mph are most common.

Arcus do not produce tornadoes but turbulence can create very chaotic conditions as winds rapidly change direction and speed. Circular motion is usually visible from under the cloud, as you can see in the video above. However the rotation is not caused by a tornado. Tornadoes are attached to the parent thunderstorm not an arcus cloud. However, it is possible for vertical rotation to occur along an arcus, and it could cause damage, but it is not a tornado. It is a good idea to be in shelter as the storm approaches.

The video above was recorded from underneath an arcus. It shows the turbulent flow. This cloud passed over with strong winds that littered the ground with small twigs and branches. It did not produce structural damage. This is typical of arcus. However, strong storms may produce downdrafts that do cause damage and for that reason it is a good idea to seek shelter when arcus approaches. The turbulence is very evident in this video.

In this video you will hear the wind blowing through the trees and into the microphone and you will also hear the call of a cardinal in the background.