What does it mean when roll clouds appear in the sky?
Anvil clouds (cumulonimbus incus), feather clouds, rainbow clouds—there are many types of amazing clouds. Today, we will tell you about another rare type of clouds that will help you predict the weather.
Where in the sky do roll clouds come from?
Everything starts as usual: water evaporates, turning into vapor, which, along with the rising currents of warm air from the heated earth or ocean, rises into the sky and cools down.
As it cools, the vapor molecules «stick» to small particles of dust and soot floating in the air — droplets form, and they, in turn, form clouds.
Morning gloria is the best-known example of roll clouds. The phenomenon is noticed in the Gulf of Carpentaria in northern Australia, most often from September to mid-November. Photo: Mick Petroff
But the atmosphere is not a homogeneous medium. The air mass cools unevenly, leading to pressure and temperature differences. And where there are differences, there is movement: wind. After all, differences in pressure cause denser air (cold) to replace less dense air (warm).
Sometimes, different factors—the heating of the land and bodies of water, the landscape, the tides, and the rotation of the Earth—combine so that the air does not move in one direction, but instead, swirls.
Have you ever noticed how the wind can twist leaves and bags in the same spot? Sometimes, the same thing happens at the cloud level, and then the clouds twist with the wind. That’s how roll clouds are formed.
When the rolls in the sky are a bad sign
Roll clouds are a type of arcus clouds in the shape of a separate tube from the rest of the clouds. There is often only one «pipe» in the sky; you rarely see several roll clouds.
Rollclouds sometimes appear at the boundary of atmospheric fronts, and accumulate enough water to make it rain. Photo: Eazydp
They usually appear at an altitude of 2,000 to 7,000 meters, and continue to curl visibly. They are more often seen in areas with complex terrain and changeable weather—both in the mountains and on the coasts of the oceans. For example, in the Alps, around Japan, Scotland, and northern Australia.
Roll clouds are helpful to meteorologists because their appearance sometimes heralds bad weather—strong winds, thunderstorms, storms, or even tornadoes. A sharp, rapid roll of clouds can indicate a storm is about to intensify, while a smooth movement can mean calmer weather.
Roll clouds alone don’t guarantee a change in weather, but if other factors are present, such as squall winds as a front moves in: it’s worth considering that rain or something more significant could be on the way.
Are you a cloud watcher? Share your photos with others in the Windy Community!
Text: Jason Bright, a journalist, and a traveler
Cover photo: Daniela Mirner Eberl
Read more about clouds:
What are wave clouds and how do they form
Cumulus clouds — a precursor of stormy weather
Explore the different types of clouds. They will help you predict the weather
