Cumulus congestus – When a Fair-weather Cloud Grows Up

When you picture a cloud, it’s probably a little Cumulus that comes to mind. Bright, solid-looking, with a bumpy top and shaded underside, this is the classic fair-weather cloud. The crisp outlines of Cumulus make it the cloud type that’s best for finding shapes in. And it’s the cloud that features in the Cloud Appreciation Society’s own logo. But not all Cumulus are such happy-go-lucky fair-weather formations.

The huge, towering form of Cumulus known as Cumulus congestus, like the one spotted here over Orkney, Scotland by Antonia Thomas (Member 63,492), can rain a fair shower on you. It is the largest, most developed form of Cumulus, having a height from cloud base to summit that’s much greater than that of its base above the ground. Though Antonia’s Cumulus congestus has yet to produce any showers, this is a formation known for its tendency to develop into a storm cloud.

Like any form of Cumulus, congestus develops when warm, moist air near the ground rises in a column called a thermal and cools enough as it does so for its moisture to condense into droplets. If the atmosphere is unstable, the heat released as these droplets form keeps the air rising more and the cloud building upwards. In this way, Cumulus congestus clouds can grow to great heights, their bases becoming darker and darker the taller they grow. This is because sunlight can no longer pass easily down through tall clouds. Grab your raincoat; this is a sign that showers may be on the way.

That’s because Cumulus congestus clouds often continue to build, developing into huge, towering Cumulonimbus storm clouds. But not always. Other times, a Cumulus congestus like Antonia’s simply rises, reshapes, and dissipates again, reminding us that even the boldest clouds are only ever passing through.

Cumulus congestus, gold in the evening light, spotted over Orkney, Scotland by Antonia Thomas (Member 63,492). View this in the photo gallery.