Clouds are visible bodies of very fine water droplets or ice particles suspended in the atmosphere. The origin of term “cloud” can be found in the old english clod or clud, meaning a hill or a mass of rock. Around the beginning of the 13th century the term cloud was then intended as a metaphor to include rain clouds as masses of evaporated water in the sky because of the similarity in appearance between a mass of rock and a cumulus heap cloud.
Clouds can be classified in different ways, depending on their shapes, development and altitude. In the framework of the Earth Observation the remote sensing has a key role for the identification, study and evolution of the cloud multitude covering our Planet. Some of them are stunning if captured from the outer space: what about a swim in the clouds with the eyes of Sentinels Satellites?
Lee waves were discovered in 1933 by two German pilots and consist in periodic change of atmospheric pressure and temperature in the air caused by vertical displacement, e.g. when the wind blows over a mountain, an escarpment or plateau. They always occur in groups on the lee side (the side sheltered or away from the wind) of the terrain that triggers them, causing periodic changes in speed and direction of the air. At the beginning of April Sentinel-2A Copernicus satellite pictured a terrific image of lee waves covering the nothern part of the Scotland
For centuries those waves were shrouded in mystery as people stared toward the sky at those strange clouds that remained stationary above the peaks unlike other clouds that made smooth tracks across the sky. Pilots were the first to fully explore this phenomenon in a scientific way (and so the discovery by the two Germans). The most common form is the mountain wave but this phenomenon can occur also in the deep ocean: Sentinel-3A shows us lee clouds triggered in the Atlantic Ocean, with a particular “delta-wing” shape.
Von Karman vortex
The flow of atmospheric air over obstacles such as islands or isolated mountains sometimes can give birth to particular clouds named von Kármán vortex streets. When a cloud layer is present at the relevant altitude, the streets become visible. Those clouds appear like a set of organized turbolence and swirls and can occur usually over the sea where the temperature of the air gradually rises. Thanks to their ground conformation and geology there are several places in our planet where this vortex activity can be frequently observed . That’ s the case of the Bering Strait, between Russia and Alaska: Sentinel-3A satellites captured some of the most stunning pictures in the last months.
In the Bering Strait there are indeed several volcanic Islands, specifically in the Alaska peninsula, triggering this air flow as winds rush past the tall peaks on the islands. As a matter of facts the volcanic peaks stand as a barrier for the wind flow: in those cases a turbulence street can form and the further away the barrier the turbulences spread, the bigger they appear.
Sentinel-3A and its constant watch on the Earth give us a clear picture of von karman streets also over Canary Islands (on the left of the image below) and Madeira (on the right). Those islands, just like the volcanic islands in the Bering Strait, are subjected to those kinds of phenomenon, where whirls and turbolences however seem to have a different shape
The Hungarian engineer Theodore von Kármán gives the name to this phenomenon. He’s considered a pioneer in the modern fluid dynamics as the turbolent flow has a key role in the airplane and automotive aerodynamics, regulating lift and drag. In the following air flow simulation over a Ferrari the turbolent flow triggered just after the back of the car is evident, where the spoiler can be seen as an obstacle to the air flow just like the volcanic islands are to the wind direction.
Stratocumulus clouds are grouped in different sub-types. Stratocumulus stratiformis are the most common: they are rolls or large rounded masses arranged in an extended sheet or layer. The elements are more or less flattened. Sometimes Stratocumulus stratiformis occurs in the form of a large single roll (roll cloud). The image below has been captured by Sentinel-3A over the Atlantic Ocean, where the clouds (with a structure of tightly packed “closed cells”) seem to envelop a theoretical centre and they are growing in dimension as they drift away from it.
The last cloud type we encountered during our journey are cirrus, characterized by their transparency. There are different kind of Cirrostratus, by definition transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance, totally or partly covering the sky, and generally producing halo phenomena. The optical thickness in the visible is such that surface properties and spatial characteristics are still “visible” in the satellite images. Here below a picture sensed by Sentinel-3A OLCI, in the Pacific Ocean south of the Bering Strait, where those type of clouds are clearly visible in the middle.
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