What is Cloud?
Cloud has been defined as an aggregation of innumerable tiny water droplets, ice particles or mixture of both in the air usually above the ground level.
• Clouds are formed due to condensation of water vapour around hygroscopic nuclei caused by adiabatic cooling.
• All clouds do not yield precipitation.
• In the absence of clouds, days would have been much warmer and nights much colder.
CLASSIFICATION OF CLOUD
In 1803 Luke Howard published his paper “on modification of clouds” in which a system of cloud classification was developed.
• As a result of the efforts made by the first International Meteorological Congress in 1874, the International Clouds Atlas could be prepared in 1896.
• The World Meteorological Organisation describe the 10 main cloud families.
A. High clouds(height 6-20km)
Cirrus
• These clouds have fibrous (hair like) appearance.
• All the cirrus or cirro-type clouds are composed of ice crystals.
• The sun or moon shining through these clouds produces a halo.
• These cloud do not give precipitation. In fact they are indicative of dry weather.
Cirro-cumulus
• Thin, white patch, sheet or layer of cloud without shading, composed of very small elements in the form of grains, ripple etc.
• They appear as ripples similar to sand ripples in the desert.
• They are not common type of clouds.
• They are often connected with cirrus or cirro-stratus.
• When they are arranged uniformally, they form a ‘Mackerel sky’.
Cirro-stratus
• Transparent, whitish cloud vell of fibrous or smooth appearance, totally or partly covering the sky and generally producing halo phenomena.
• It gives the sky a milky appearance.
• It produces halos around the sun or moon.
• It is formed of icy crystals.
B. Middle clouds(height 2.5-6 km)
Alto-cumulus
• White or grey or both white and grey, patch, sheet or layer of cloud are composed of laminae.
• They do not form halos around the sun or moon.
• High globular altocumulus groups are sometimes referred to as ‘sheep clouds’ or ‘woolpack clouds’.
Alto-stratus
• Greyish or blush cloud sheet or layer of airlated, fibrous or uniform appearance.
• Alto stratus does not show halo phenomena.
• This cloud consists of water droplets.
• Precipitation may fall either as fine drizzles or snow.
Nimbo-stratus
• Grey cloud layer, often dark, the appearance of which is rendered diffuse by more or less continuously failing rain or snow.
• It is a rain or sheet cloud.
• It is never accompanied by lightning & thunder.
• Streaks of rain or snow failing from these clouds but not reaching the ground are called virga.
C. Low cloud (height 0-2.5 km)
Strato-cumulus
• Grey or whitish or both grey and whitish, patch, sheet or layers of cloud which almost always has dark parts, composed of rounded masses and rolls.
• It is associated with clear or fair weather but occasional rain or snow may occur.
Stratus
• Generally grey cloud layer with a fairly uniform base, which may give drizzle, ice prisms or snow grains.
• It does not produce halo phenomena except possibly at very low temperatures.
• When it is associated with rain or snow it is called nimbo-stratus cloud.
Cumulus
• Detached clouds generally dense and with sharp outlines, developing vertically in the form of rising mousds. dome or towers, of which the bulging upper parts often resemble a cauliflower.
• Cumulus clouds represent the tops of strong convective currents.
• Irregular patches of cumulus are called fracto cumulus.
• Cumulus is generally found in the day time over land area. It dissipates at nigjht.
Cumulo-nimbus
• Heavy and dense cloud, with a considerable vertical extent, in the form of a mountain or huge towers.
• It is a towering cloud sometimes spreading out on top to form an anvil head.
• This is associated with heavy rainfall, thunder lightning, hail and tornadoes.
This article is shared by Priyanka Duta. Priyanka is a Guest Lecturer of Geography at New Alipore College, Kolkata.
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