SUB-THEMES

Earth & Atmosphere

Clouds are formed when water vapour from the earth raises in the atmosphere and during rise, due to drop in air pressure and temperature it gets condensed on very small particles in the air (aerosols), eventually the condensed vapour gets denser and according to its weight, gets distributed in different heights of atmosphere in the form of clouds. Millions of very small water droplets collect to form cloud. The distribution of clouds in the atmosphere is also influenced by the rotational movement of the Earth as well as other atmospheric phenomenon like temperature, pressure and atmospheric circulations. Clouds formed in a part of the Earth may shift in the atmosphere in different directions, gets condensed and fall as rain in other far off parts of the Earth. For example, during monsoon season in India, clouds that are formed mostly above the oceans travel in the atmosphere and enter above the Indian subcontinent, circulate differently with time and fall in different forms of precipitation.     
Cloud formation is an interesting natural phenomenon that people are generally not aware of. By keenly watching the clouds and their types we learn to understand their types and distribution in the atmosphere. By practicing we can also anticipate weather conditions during different times/seasons according to the cloud formations. When the water droplets are not collecting and remain in scattered form the amount of sunlight reflected through them is more, and from the Earth these clouds look light coloured/white (non-rainy clouds) whereas when water droplets collect together into large drops, the amount of sunlight reflected from them is less and they look dark (rainy clouds).

Basic cloud conditions that are referred in the classification are –

1) Strato – layer                2) Cumulo – heap, cotton ball          3) Cirro – high, curl of hair 4) Alto – mid                    5) Nimbo – precipitation, mist, vapour or fog

The knowledge about clouds may serve very useful in broadcasting information and alerts to farmers and common masses for possible weather conditions like rain, storm, thunder etc especially during monsoon season.
Clouds are classified according to their vertical distribution in the atmosphere from the ground as low level clouds, middle level clouds, high level clouds and clouds with vertical development. The vertical categories are further classified according to their composition and formations.

A.    Low Level Clouds:
Low level clouds form in stratus (develop horizontally) pattern in lower atmosphere up to 2 km from the Earth’s surface. Their distribution heights are same in polar, temperate and tropical zones. These are classified further in three types -

i)       Stratus Clouds:  These Clouds are grey and lie very close to the Earth’s surface. They usually look like a sheet layer but sometimes are also found in patches. They rarely produce precipitation.  

ii)     Stratocumulus Clouds: These clouds are of grey or whitish colour. The bases of clouds tend to be more round than flat. They can be formed from old stratus clouds or from cumulus clouds that are spreading out. They appear like cloud clumps with thin and thick areas.

iii)   Nimbostratus Clouds: These clouds are named from the Latin word ‘nimbus’ that means rain cloud. These are dense, dark, grey, featureless layers of cloud which are thick enough to block the Sun and produce precipitation in the form of persistent rain and perhaps snow as well.  

B.    Middle Level Clouds
Middle level clouds form in the atmosphere from 2 to 8 km altitude above the Earth’s surface. Their heights in different latitudinal zones are different. In polar zones these are formed from 2 to 4 km altitude, in temperate zone these are formed from 2 to 7 km altitude and in tropical zones these are formed from 2 to 8 km altitude from the Earth’s surface. These are classified further in two types -  

i)       Altostratus Clouds: These clouds appear like flat and uniform type texture having bluish or grayish veil that totally or partially covers the sky. The light of the sun can be seen through them but there is no halo effect. They themselves do not produce significant precipitation unless they lower into stratus and become thicker.

ii)     Altocumulus Clouds: These clouds exhibit ‘cumulo’ type characteristic and align like rows of clouds, like waves of the sea with white and grey coloring and shadows. These clouds contain mostly water droplets and perhaps some ice crystals too.

C.    High Level Clouds
High level clouds form in the atmosphere from 3 to 18 km altitude above the Earth’s surface. Their heights in different latitudinal zones are different. In polar zones these are formed from 3 to 8 km altitude, in temperate zone these are formed from 5 to 14 km altitude whereas in tropical zones these are formed from 6 to 18 km altitude from the Earth’s surface. These are classified further in three types - 

i)       Cirrus Clouds: These clouds look like white delicate feathers. They are generally wispy forms and are composed entirely of ice crystals.

ii)     Cirrostratus Clouds: These clouds appear as a thin, almost transparent, whitish veil like layer composed of ice crystals. When moonlight or sunlight passes through their ice crystals the light is dispersed similar to when light passes through a prism, and that result into formation of a halo.

iii)    Cirrocumulus Clouds: These clouds are thin white layers with a texture giving them the look of patches of cotton or ripples without shadows. These contain primarily ice crystals and perhaps some very cold water droplets too.

D.   Clouds with Vertical Development

These types of clouds form in cumulus (develop vertically) pattern in the lower atmosphere near to the Earth’s surface and move upward through higher altitude up to high level. These are classified further in two types -

i)       Cumulus Clouds: These clouds have a flat base and a dense, mound-shaped top that resembles a large cauliflower and grows vertically. Where the sun hits these clouds, they are a brilliant white. The base tends to be a darker grey. They generally do not produce precipitation.

ii)     Cumulonimbus Clouds: These are large, heavy and dense clouds. They have a generally flat, dark surface with very tall and large tops like the shape of massive mountain or block. These clouds are often characterized with lightning and thunder due to collision between charged water droplets, and sometimes hail. They may also produce tornadoes.