Orographic clouds

Orographic clouds are clouds that form in a stream of air that passes through a hill, an isolated mountain or a mountain range. They may be located below, on the level or above the highest part of the obstacle. They most often belong to the genera Altocumulus, Stratocumulus and Cumulus.

The physical constitution of orographic clouds is, as a whole, similar to that of the clouds belonging to the genre in which they are classified. Orographic clouds, being associated with terrestrial relief, generally have zero or very slow joint movement, although the wind at cloud level is very strong.

In certain cases, wind speed may be highlighted by the movement of certain appreciable details, such as that of isolated elements that are dragged from one end of the cloud to the other.

Orographic clouds can have many different aspects. In the case of an isolated mountain, orographic clouds can often take the form of a necklace around the mountain, or a hood covering its summit, both forms arranged almost symmetrically. Clouds of this type are lenticularis, and usually do not produce precipitation and when they do, they are always very weak.

Hills or high mountains can produce, on the windward slope (windward, with respect to the wind direction), clouds of large horizontal extent that give rise to precipitation. These clouds surround the crest of the mountain and dissolve immediately behind it. When they are observed from a place on the slope under the wind (leeward behind with respect to the wind direction), these clouds take the form of a broad wall.

At a time of strong wind, orographic clouds may form in the vicinity of the ridges or ridges, which have, like the leeward relief, the appearance of a flag fluttering in the wind (the mountain then appears to "smoke").

It can often happen from an orographic cloud - or a union of several of these clouds - usually shaped like lenticularis, appearing over the hill or the mountain, and sometimes slightly up or downwind. Some mountain ranges or even some relatively low ridges over low-lying regions can cause the formation of standing waves in the airstream that flows through them.

When the air is sufficiently humid, orographic clouds, called "wave clouds", may appear on the crest of these standing waves; then you can see a cloud over the mountain ridge or slightly ahead and at the same time one or several different ones a little behind. In the latter case, the clouds appear at regular intervals scattered over several kilometers.

Wave clouds can also appear simultaneously on several levels. The leeward waves are often accompanied in the lower layers of the atmosphere by large, horizontally stationary swirls with a roll-like cloud ("leeward whirl cloud") appearing on top. Due to this turbulence, orographic clouds are the most dangerous for helicopters.

The orographic thunderstorms, very common in the Serra de Ubatuba, in São Paulo, are a strong sign of how the microclimate works around the mountains. There there is an intense sea breeze rising over 1000 meters from the Serra do Mar, and a large amount of humidity and heat, all providing an upstream and causing instability only in that place. Orographic Cumulonimbus are not long lasting, but can cause as much damage as others of diverse origins.

In São Paulo, the Mantiqueira, Mar and Canastra Mountains, mainly in the faces facing the valleys and the ocean, are the main orographic cloud forming regions. The Meteorological Surveillance Centers (CMV) inform if the Brazilian mountains are obscured by orographic clouds, and pilots who fly over such places should previously check such information to avoid possible accidents.