Under specific weather conditions, fog can occur dramatically. A genuine wall of fog, resembling a giant wave, can roll onto a beach within minutes, causing a panic reaction among those who witness it!
When this phenomenon is observed from a distance, it gives the impression of seeing a tsunami in the sky. This meteorological phenomenon is known as a “fog tsunami” for this reason. This wave, not of water but of water vapor, can spread over a distance of 40 kilometers and stretch several hundred meters in height. Generally, this phenomenon occurs when the sky was clear just before, so the fog tsunami seems to emerge out of nowhere.
It forms above a large body of water, such as the sea or a large lake, due to the condensation of water vapor; it is, in fact, an advection fog related to the presence of warm air above and cold air below (a colder sea).
The phenomenon most often occurs at the end of spring or at the very beginning of summer, when the water has not yet had time to warm up, but warm days are already possible. The water then cools the milder air circulating above, leading to condensation.
A fog wall is thus formed, and it is subsequently pushed by the breezes in circulation; the wall curls, and that’s when the characteristic shape of a tsunami appears.
The Fog Tsunami Is Sometimes Mistaken for a Storm Cloud
The fog tsunami looks so threatening that it is sometimes confused with another, completely different weather phenomenon: arcus cloud.
The roll cloud, or arcus, appears at the front of a thundercloud, and its appearance can indeed be similar to that of a fog tsunami. The arcus, however, is much thicker and therefore darker, whereas the fog tsunami is generally very white.
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