Breakdown: Why ‘roll clouds’ are unique formations

Why ‘roll clouds’ are unique formations

MEMPHIS, Tenn. (WMC) - Rounds of rain and storms have impacted the Mid-South the past few days. In the wake of these storms, we have been receiving pictures that look like UFOs in the sky.

While the clouds look ominous, in reality a process in the sky is occurring creating what is known as a “roll cloud.” In this episode of the Breakdown, we will explain how these clouds form and why they are not dangerous.

Roll Cloud photo by Misty Turner in Collierville, Tennessee
Roll Cloud photo by Misty Turner in Collierville, Tennessee (Source: Misty Turner)

A roll cloud is a low, horizontal, tube-shaped cloud formation, a rolling solitary wave.

There are two types of arcus clouds -- roll clouds and shelf clouds.

Shelf clouds are typically found along the leading edge of a super-cell thunderstorm resulting from the downdraft of the storm.

Roll clouds can sometimes form in advance of the shelf cloud when a horizontal vortex forms in the atmosphere. It can then become detached from the structure as the storm decays and continue moving along appearing to be rolling horizontally across the sky.

They are typically found along coastal regions resulting from a sea breeze or cold front. The picture we received was a result of storms that had been moving through the area the day the photo was taken.

As the storms decayed, the downdraft combined with forward movement and gravity to produce the giant roll cloud that moved through the Mid-South making for a uniquely beautiful yet, somewhat shocking sight for those who saw it.

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