Cloud Index: Fractus
Ragged shreds hanging below the cloudbase
Welcome to the Cloud Index — a regular feature on Looking Up where we profile a type of cloud, explaining how they form, what they’re made of and how to use them to forecast the weather. You can find the full list of clouds we’ve covered in the Cloud Index index.
We’ve covered all the basic types of cloud now, so it’s time to get a bit more interesting. This week we’re looking at fractus clouds — a species of tropospheric cloud that tends to hang in ragged shreds below larger clouds above. They’re often left behind after atmospheric disturbance.
They can be associated with cumulus or stratus clouds, and tend to be very short-lived, appearing and disappearing very quickly, without defined tops or bottoms. They can form near the surface too, as patches of fog.
How they form
Fractus clouds tend to form or break off from a larger cloud — either as cumulus fractus or stratus fractus — you can tell the difference by looking at the colour and puffiness. Cumulus fractus tend to be taller and lighter-coloured, whereas stratus fractus clouds are flatter, dark and more dispersed.
Both types of fractus either get torn off by strong winds or appear when a larger cloud breaks apart. They can become cumulus themselves if the ground heats up enough for convection to begin, forming their own fractus clouds and repeating the cycle.
What they mean for the weather
Fractus is very unstable, and also indicates unstable air. As such, they can often signal a change coming in the weather — usually appearing at the leading and trailing edges of summer storms.
Under a thunderstorm, fractus is often known as “scud”, which appear under the main cumulonimbus. They can often be mistaken for a developing tornado or funnel cloud — you can tell the difference by looking to see if there’s any rotation. If rotation is seen, then there’s a good chance of seeing a tornado, so take cover.
Anything else?
Big masses of many fractus clouds under a main cloud are called pannus — which often merge together to form a layer under the main cloud called an accessory cloud.
Fractus clouds are often found close to rainshafts, representing rising air with high humidity due to the evaporation of rain or water on the ground. They sometimes evaporate themselves before reaching the main cloud due to turbulent mixing with the dry air in between.
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