Extraordinary Mountain Clouds Part 4: A Sea of Clouds

A sea of clouds is perhaps the most stunning cloud formation you can find in the Alps. It’s a very characteristic phenomenon in mountainous areas and it creates a surreal panorama with the valley hiding underneath a thick blanket of clouds from which peaks arise like islands out of a sea. In this series “Extraordinary Clouds”, I’ve already explained why you often see incredible cloud formations in the mountains. We also looked closely at the breathtaking lenticular cloud above Mont Blanc and the stunning banner cloud accompanying the Matterhorn. In this last blog of the series we’ll be exploring the jaw-dropping sea of clouds.

Warm air is trapped

A sea-like cloud arises in a warm layer of air that is trapped between the cold air in the valley and the cold air in the upper layer of the sky. This is called an inversion and in such a situation, the air temperature in the mountains is higher than in the valley. Normally, this is of course the other way around.

The top station of the Aiguille du Midi above an endles sea of clouds.

How an inversion is formed

So how does this warm layer of air get trapped? At the end of the afternoon when the sun disappears behind the mountains, cold air descends the mountain slopes and accumulates in the valley. The warmer air in the valley will try to escape (warm air wants to rise, while the “heavier” cold air tends to drop). However, due to the colder air layer higher up in the atmosphere, the hot air gets trapped. The warmer air can no longer escape from the valley and the inversion will be stuck halfway up the mountain slopes. This is especially the case if there is now wind. During the summer a sea of clouds is mostly formed at night or early in the morning. However, in winter it can also appear during the day.


If you’re hiking the Tour du Mont Blanc or the Chamonix to Zermatt Haute Route, I hope that one morning you’ll be high up in the mountains gazing over an endless sea of clouds. It’s such a spectacular panorama that you’ll remember it for the rest of your life! And if you’re lucky enough to witness this stunning cloud phenomenon, please take out your sunscreen and hat, because you can assume that it will be a bright and sunny day!

Simone van Velzen

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