Under certain weather conditions, fog can occur spectacularly. A literal wall of fog, in the shape of a giant wave, can spread over the beach in the space of a few minutes and cause a panic reaction in those who see it!

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When this incident is seen from a distance, it gives the impression of seeing a tsunami in the sky. This is why this weather phenomenon is known as a “fog tsunami”. This wave, not of water, but of water vapor can spread to a distance of 40 kilometers and to a height of several hundred meters. Typically, this phenomenon occurs when the sky was clear just before, so the fog tsunami appears to come out of nowhere.

It forms over a large body of water, a sea or a large lake, due to the condensation of water vapour: it is actually a convection fog associated with the presence of warm air masses at altitude and cold air masses below (a cold sea).

So this phenomenon often occurs in late spring or early summer, when the water has not yet had time to warm up, but beautiful warm days are already possible. The water then cools the soft air moving above it, causing condensation.

Thus a wall of fog is formed, and then it is pushed by the rotating winds: the wall rolls away and then the characteristic shape of the tsunami appears.

Fog tsunami over Lake Michigan in the United States. © Azmir Ben, Andrew Ballard

Fog tsunami is sometimes confused with thundercloud

A fog tsunami looks so frightening that it is sometimes confused with another completely different weather phenomenon: arcs.

The roller cloud, or arcus, appears at the front of a thunderstorm cloud and its appearance can actually be similar to that of a fog tsunami. Arcus on the other hand is thicker, therefore darker, while Fog Tsunami is generally very white.

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