Cloud Spotting for Beginners Part 5: Fluctus

Welcome to part five of our cloud spotting for beginners series! A series where we guide you through the most common clouds in the sky.

Next up we have Fluctus clouds!

The Fluctus cloud looks like a succession of enormous waves breaking onto an invisible shore. It is known by some as the Kelvin- Helmholtz wave cloud, after two 19th-century pioneers in the study of turbulence, physicists Lord Kelvin (William Thomson) and Hermann von Helmholtz. 

Rare and fleeting, Fluctus is not one of the ten main cloud types but classed as a supplementary feature that can appear on a part of one of the main clouds. 

As such, it can form at all heights in the troposphere – the lower 17 km (11 miles) or so of our atmosphere (in mid- latitude regions) where weather happens – and it can be thought of as a very specific example of the wavy-looking cloud variety called undulatus. 

How are Fluctus clouds formed?

Waves of rising and dipping air occur throughout our atmosphere, which can be thought of as an ocean – just one that consists of gases rather than liquid. 

When clouds appear in undulating forms they show these otherwise invisible waves of air. The distinctive breaking-wave tops to the undulations of fluctus are caused by wind shear, when there is a distinct change in windspeed with altitude. 

When cloud develops at the boundary between a region of colder air below and warmer air above, and when the upper layer is moving more rapidly than the lower one, undulations start to develop along the upper surface of the cloud. 

If the conditions are just right, the tops of the undulations curl over into a succession of vortices. Such curling Kelvin-Helmholtz waves are found not only in our clouds. They develop deep in the oceans between differing layers of seawater, as well as in the churning clouds on the planets Jupiter and Saturn. 

Since Fluctus clouds in our atmosphere hold their shape for no more than a few moments, they are relatively rare and a really great example is the ultimate prize for a cloud spotter.

Keep your eye out for the rest of our Cloud Spotting series!

This post is an excerpt from the RMetS book Weather A-Z. The original author is Gavin Petor-Pinney, Founder of the Cloud Appreciation Society, who wrote a portion of the book on the clouds that capture his imagination. Photo © Frank Le Blancq.