An imposing Shelf Cloud (in Italian “Nube a shelf”) Oveled the sky above Washington DC yesterday and Maryland just before a summer storm, causing amazement and fright between the population. Despite its threatening appearance, this type of cloud it is not dangerousbut it is formed at the base of cumulonemb and storm clouds, therefore it indicates the arrival of even strong showers and gusts of wind: it is easier to spot in summerwhen intense thunderstorms are more frequent.
It is a accessory cloudlow and horizontal, which is formed in the lower edge of a temporal cloud in the direction of advancement, therefore “in front” to the rain. Its elongated shape and its “threatening” appearance can confuse it with a roll cloud, but these are two different clouds even if they are part of the same type, called Arcus because of their form – in fact – in Arco.
Everything arises from the delicate balance between Ascending hot currents And Descendant cold currents during a storm. The rain begins to descend when the ascending currents are no longer sufficiently supported to keep the droplets of water in suspension. Here these precipitate and the real storm begins. The rain, falling, drags cold air with it that can also come from very high altitudes (a “well placed” cumulonembo can also reach up to 10 km from the ground): a descending cold current is formed which is called in jargon in jargon Downraft.
If this current is quite intense, once you arrive on the ground it “drains” in all directions by creating what is called front of gustsor gust In English. A kind of Downburst In short, less intense. The part of the Gust Front that extends under the front of the storm cloud, in front of the rain, interacts with the air currents that the cumulonembo “aspires” near the ground. This air, in fact, being warmer and therefore less dense than the cold one that descends with the downndraft, spontaneously tends to go up because of the push of Archimedes, creating one Ascending hot air current which reaches up to a few hundred meters from the ground. At this height the humidity of the air can condense around the front base of the cumulonembo, going to create a sort of “shelf” (the Shelf Cloudin fact) which heralds the arrival of the gust front first and possibly of the rain then.
