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Home Temporali nowcasting Tipi di nuvole I cumulonembi Supercelle e multicelle

Convective cells and streets

If the convection is not very strong, as is often the case on the sea where the surface temperature does not become so high as on land and changes more slowly, and at the same time flattens on a stable layer, the cells form stratocumulo fairly uniform in size over a wide area. The passage of time the cells often grow in width, so that starting with a width of 1 or 2 km when the regularity is initially appeared may widen up to 100 km without an appreciable increase in depth of the layer that contains them. These large cells are characteristic of light winds. When there is moderate or strong winds the only cells that last without change unless they are precipitants are the streets. Are aligned along the direction of the cut of the wind (wind shear) which is the same wind near the surface. The streets give a very clear indication of wind direction in satellite images. The width of the streets grows downwind but does not reach the size of the large cells. The growth in width is associated with increased surface temperature as when the air moves from the sea surface or ice cold to the hot surface, or from the sea to during the day. The cells have many different forms. Sometimes they are closed and pressed next to each other. These updrafts are stronger in the middle, where the top of the cloud is higher, and motion of descent around the edges. Clouds of this shape indicate that the stable layer on top is very developed and abruptly stops the upward growth of thermals. It also indicates that there is little or no subsidence in the cloud layer and no rainfall, otherwise appear larger spaces between the clouds as a result of heating or loss of liquid water from the cloud. The edges of very large cells are often indications of cyclonic motion where the air at the top of the region of descent converges in downdrafts. In these cases the loss of heat by radiation from the top of the layer is important to the same extent of the gain from the sea warmer, and we call this phenomenon convection descending cold for contrast with the ascending convection from a hot surface. In many cases, such as the sea-fog, where the air moves towards the sea colder and cools rather than heat, radiation continuously cools the layer at the top and the sea, paradoxically, heats the air so as to decrease the cooling rate. Open cells appear especially when the clouds produce rain. As soon as the rain falls in the open air cools evaporandovi inside. This produces a descending current (downdraft), which increased the weight of the rain which drags the air when it reaches its terminal velocity of fall. The downdraft opens on the surface and covers it with a layer of cooled air to be heated by new thermal before new piles may appear in it. In this way a large part of each cell is free from clouds.

 

Home Temporali nowcasting Tipi di nuvole I cumulonembi Supercelle e multicelle