How clouds are formed ?

•  As sun rays heat heat the ground and the air immediately above it , the air gets warm and becomes light. This warm and lighter air is carried upward. 
• When air rises in the atmosphere it gets cooler and is under less pressure. When air cools, it is not able to hold all of the water vapor.
• The vapor becomes small water droplets or ice crystals (condenses) and a cloud is formed.

How condensation occurs ?

• Water vapor to condense into water droplets, require a particle to condense upon.
• These particles, such as dust and pollen, are called condensation nuclei. Eventually, enough water vapor condenses upon pieces of dust, pollen or other condensation nuclei to form a cloud.  

Let us study further in detail about clouds and cloud types.

Clouds are classified based on their shape and their height from the earth’s surface. There are typically 10 main clouds, which are further classified into 27 sub types. 

On the basis of height, clouds are divided into three groups

1. High Level Clouds (above 6 km)
2. Middle Level Clouds (2.5 to 6 km)
3. Low Level Clouds (below 2.5 km)

1.  High Level Clouds(above 6 km) are Cirrus, Cirrocumulus and Cirrostratus.

Characteristics –

• High Clouds are composed of ice crystals, because they are formed at such altitudes temperatures that are below freezing. 
• They do not produce rain and often indicate fair weather.

Cirrus	Cirrocumulus	Cirrostratus
Detached clouds in the form of white, delicate filaments, mostly white patches or narrow bands	Thin, white layers of clouds without shading. They are composed of very small elements in the form of ripples	Transparent, whitish veil clouds with a fibrous or smooth appearance
Made Up Of Ice Crystals	Ice Crystals	Ice Crystals
No Precipitation	No Precipitation	No Precipitation.

 Cirrostratus is a thin cloud layer that causes a halo to appear around the sun and moon.

2. Middle Level Clouds (2.5 to 6 km) are Altostratus, altocumulus and nimbostratus.

Altocumulus	Altostratus	Nimbostratus
White and/or gray sheet or layered clouds.	Gray or bluish cloud sheets or layers of striated or fibrous clouds that totally or partially covers the sky.	Dark gray cloud layer
They may be partly fibrous or diffuse.	Thinner Layer Allows Sun To Appear As Through Ground Glass	Diffused by falling rain or snow. Resulting from thickening of Altostratus
Precipitation: May Produce Light Showers	Precipitation: Rain Or Snow.	Precipitation: Heavier Intensity Rain Or Snow.

 When the edge or a thin semitransparent patch of altocumulus passes in front of the sun or moon a corona appears. This colored ring has red on the outside and blue inside and occurs within a few degrees of the sun or moon.

3. Low Level Clouds (below 2.5 km) are Stratocumulus, Stratus, Cumulus, Cumulonimbus

Stratocumulus	Stratus	Cumulus	Cumulonimbus
Low Level Layered Cloud, Gray or whitish patch, sheet, or layered clouds	Low Level Layer Grey, Uniform Base; If Ragged, Referred To As ‘Fractostratus’.	Low Level, Detached, generally dense clouds and with sharp outlines that resembles a cauliflower. Dark bases and bright tops	The upper portion is usually smoothed, fibrous or striated and nearly always flattened in the shape of an anvil.
Precipitation: Drizzle	Precipitation: Drizzle	Precipitation: clear skies. It appears in the morning, grows, and then dissolves again toward evening.	Precipitation: Showers Or Snow. (thunderstorm cloud)

Hail or tornado( rarely) comes from a cumulonimbus cloud.

Note : Clouds are given Latin names which describe their characteristics, e.g.

• cirrus is derived from ‘cirro’ which meanscurly or fibrous
• cumulus is derived from ‘cumulo’ which means puffy or heap.
• stratus from ‘strato’ which means a layer
• nimbus from ‘nibo’ meaning rain-bearing

Image Source : scied.ucar.edu