Whether it is attached. A roll cloud is a low-level, horizontal, tube-shaped cloud associated with the of a convective storm, or occasionally a cold front. What differentiates it from the more familiar shelf cloud is that it is completely detached from the storm’s cloud base. Roll clouds are relatively rare.
The rolling look is real rather than an illusion. These clouds appear to be rolling about a horizontal axis because of the shearing effects and the provided by the differing air masses meeting along the gust front. Cold outflow undercutting warmer air sets up the rotation, and the cloud marks where that boundary runs.
Roll cloud is the popular term for one kind of arcus, the family of low horizontal clouds that form along the leading edge of a storm’s outflow. Specifically it is a low-level, horizontal, tube-shaped arcus cloud associated with the gust front of a convective storm, or occasionally with a cold front.
The distinction that matters is attachment. Roll clouds are completely detached from the convective storm’s cloud base, which is what differentiates them from the more familiar . A shelf cloud is a wedge joined to the storm behind it; a roll cloud is a tube standing on its own in clear air, with the storm some distance away.
They are also relatively rare, which is worth saying because photographs of shelf clouds are frequently captioned as roll clouds. The two are produced along the same kind of boundary, and telling them apart is a question of looking at whether the cloud connects to anything.
The rotation is not an artifact of the shape. Roll clouds appear to be rolling about a horizontal axis because of the shearing effects and horizontal vorticity provided by the differing air masses. That happens along the gust front, where cold outflow undercuts warmer air. The glossary points to rotor clouds and the morning glory as related phenomena. One caution: a definition like this gives the form and the mechanism and no numbers at all.
wat. If the tube of cloud is joined to the storm it is a shelf; standing alone, it is a roll.
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The detachment is the diagnostic. A tube of cloud standing clear of the storm behind it is marking the leading edge of the outflow, which is where the wind arrives before the rain does.
How do we know?
The American Meteorological Society’s Glossary of Meteorology defines the roll cloud as a form of arcus and gives the boundary it accompanies. It distinguishes it from shelf clouds by attachment, and names the mechanism behind its rotation.
What is still unknown
A glossary entry, so it defines the form and its mechanism without quantifying anything: no typical dimensions, wind speeds or lead times before the gust front passes.
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Where to next
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