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Translations of Encyclopedia about Geology


Cloud Types

Although the clouds in the sky seem to be very different, it is possible to classify them by common characteristics into several categories. In 1803, amateur meteorologist Luke Howard (1772-1864) put together a table for distinguishing types of clouds and gave them Latin names. The classification, which is based on the shape and altitude of the clouds, gives us the following groups: high clouds, middle clouds, and low clouds.

High clouds are grouped together under a general name of "cirrus." This group includes cirrus, cirrocumulus, and cirrostratus. In temperate geographical latitudes, these clouds move in altitudes of

between 5 and 13 kilometres, in the tropics, the altitude is between 6 and 18 kilometres, while in the polar regions it is between 3 and 8 kilometres.

Cirrus clouds, also called "white clouds," are formed by ice crystals. They occur either individually or in groups, have a fibrous appearance, silky glow in the sunlight, and are surrounded by white reflections. Depending on the wind speed, they may travel in these altitudes at 150 to 300 km/h. The wind scatters them far away from each other. The sunlight penetrates very well these thin layers of ice particles. At dusk, cirrus clouds light up with intense colours.

Cirrocumulus clouds, also called high (small) fleecy clouds, occur relatively infrequently. They are formed by ice crystals. Of ribbed appearance, they join in groups, forming cloud fields.

Cirrostratus clouds are also described as high cloudy veil, or high cloudiness. This type of clouds consists mainly of ice crystals and forms thin, expansive cloudy veils, reducing the intensity of the sunlight. If these clouds cover the sun or the moon, they create a light circle around these heavenly bodies. This phenomenon is called "halo."

A preposition "alto" is given to the name of the middle high clouds. They include altocumulus and altostratus. These clouds move in temperate geographical latitudes in an altitude of 2 to 6 kilometres, in the tropics at 2 to 8 kilometres, and in the polar regions at 2 to 4 kilometres.

Altocumulus clouds, the so-called large fleecy clouds, are similar in their appearance to the cirrocumulus clouds. They occur at an altitude of 3 to 4 kilometres. They appear in the shape of nuggets, waves, or contiguous fields. Sometimes they appear layered or in a belt formation. They are formed by water droplets. When these clouds cover the sun, they create wreaths of light, but these are smaller than the "halo" phenomenon in the case of the cirrostratus clouds. Individual cloud nuggets often separate and rise. This strange shape is called altocumulus castellanus. They are an almost certain indication of a coming storm.

Altostratus clouds are middle high grey layers in the altitude of approximately 3 to 4 kilometres. They often originate from the cirrostratus clouds and create felt-like grey layers covering large areas of the sky.

They gradually cover the sun, so that it appears as if made of milky glass. These large clouds are made up of water droplets and ice crystals. They signal the arrival of rain. If, due to high rate of evaporation, the rain does not reach the ground, they form fringes on the lower edges.

Low clouds occur in the altitude of up to 2 kilometres. They include stratocumulus and stratus clouds. Stratus clouds are also called cloud layers. They extend from the vicinity of the ground to about 2000 metres above ground. Because of that, they are sometimes called high fog. Their base structure is not well organized and they give the impression of grey foggy mist. They often have "rags" (stratus fractus) hanging down. Stratus clouds consist of droplets, and at lower temperatures there will also be ice crystals. These clouds form when the ground warms up. At times, when they are very dense, they may produce drizzle, or sometimes even tiny particles of snow, but not large snowflakes. Stratus clouds may envelop mountain ranges and towers in a fog.

Stratocumulus clouds are the fair-weather clouds. They consist of water droplets, and sometimes in winter also of ice crystals. They re situated in the low cloud layer. Grey or whitish clouds, they occur in the shape of piled-up packages or nuggets, arranged above each other or over each other. There are no firm shapes or boundaries. These clouds often occur in evening hours and in winter.

Clouds that develop vertically extend through several altitude groups. Since their base is in the low cloud layer, they are included in those groups. They include cumulus, cumulonimbus, and nimbostratus clouds.

Cumulus cloud, also called dome-shaped cloud, is generally seen as an isolated, sharp-edged white dome. The upper part glows with white light, the lower portion is often dark. These clouds form when the ground is heated, most often in the afternoon and in summertime.

Cumulus clouds partially reach the upper cloud layers, where their tops may freeze. In general, no precipitation is expected from these clouds, however, if they continue to develop into the cumulonimbus type, there may be heavy downpours.

Cumulonimbus clouds are also called storm or rain clouds. They may reach up to 18 kilometres into the atmosphere (in the tropics) and run through several cloud layers, which means that they encompass almost the entire depth of the troposphere. Their characteristic is the widening of the upper part into a vaulted shield, also called storm shield. The shield is generally frozen and shows anvil-like fibrous structure, similar to the one seen in cirrus clouds. Generally, the cumulonimbus clouds appear alone, like a tower, or are in a mountain-range formation. When observed from the side, the view is impressive. However, standing underneath these clouds, you see only dark mass, which does not show any of its luminosity. If there are gleaming yellow patches in the cloud, it means hail will fall on the place below this cloud. Cumulonimbus clouds form during hot and humid summer days. They produce heavy precipitation, both rain and hail, which may be accompanied by high winds and thunderstorms.

Nimbostratus clouds are classified as layered, precipitation-laden clouds, which completely cover the sun. They are thick, grey layers of clouds, with "rags" hanging on the lower part, and they extend to an altitude of 5 kilometres. They form through gradual rise of extensive layers of air, bringing bad weather with continuing precipitation, either in the form of rain or snow.

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