Cloud Spotting for Beginners Part 4: Cumulus

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

Next up is the Cumulus cloud!

Close your eyes and think of a cloud. What image comes to mind? I bet it’s a Cumulus cloud. This feels like the generic cloud, the one that stands for cloud-kind in general.

It is the ‘fair-weather cloud’, and the most light-hearted of all the cloud forms. The reason Cumulus is a fair-weather cloud is because it often forms as a direct consequence of the sunshine warming the ground.

How are Cumulus clouds formed?

This warmth can cause the air just above the ground to expand slightly and float upwards in invisible currents known as thermals. Whenever air rises, it expands, and when air expands, it cools. The cooling of the rising column of air can cause some of the moisture it carries to change form.

The water vapour – the invisible gaseous form of water in which it is bouncing around as individual molecules – can condense into droplets of liquid. These countless, minuscule droplets scatter the sunlight as they are borne on the rising thermals, to appear as the distinctive, brilliant- white mounds of Cumulus clouds.

Named from the Latin for a stack or heap, Cumulus are low, solid-looking clumps with flattish bases typically below 2,000 m (6,500 ft) and heaped, cauliflower tops. They are a common cloud that forms right around the world except in polar regions, where the sun never rises high enough in the sky to produce thermals of any consequence.

Cumulus look like they would be the most comfortable clouds to sit on and let your imagination kick back.

This fact was not missed by the Italian painters of the Baroque period who rarely depicted a saint or an angel without a comfy Cumulus to perch upon. Cumulus may be common, but how many others can claim to be the sofas of the saints?

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.