Cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of the Water Vapour in free air at considerable elevations.
As the clouds are formed at some height over the surface of the earth, they take various shapes. According to their height, expanse, density and transparency or opaqueness clouds are grouped under four types :
(I) CIRRUS; (II) CUMULUS; (III) STRATUS; (IV) NIMBUS
CLOUDS AND ITS TYPES
a. HIGH CLOUDS: Mainly cirrus (Ci)which are feathery form at 6 km above the ground.
- CIRRUS (CI) – This looks fibrous and appears as wisps cotton in the blue sky. It indicates fair weather and gives brilliant sun set.
- CIRRO CUMULUS (Cc) – This appears as white globular masses, forming a mackerel sky.
- CIRROSTRATUS (Cs) – This resembles a thin white sheet. The sky looks milky and the sun and moon shines through this clouds and form a ‘halo’.
b. MIDDLE CLOUDS: Mainly Alto (Alt)clouds at 2 km to 6 km above the ground.
- Altocumulus (Alt-Cu): These are woolly, bumpy clouds arranged in layers appearing like waves in the blue sky. They indicate fine weather.
- Altostratus (Alt-St): These are denser and have watery look.
c. LOW CLOUDS:
Mainly Stratus or sheet clouds below 2 km height.
Stratocumulus (St-Cu): This is rough and bumpy clouds with wavy structure.
- Stratus (St): This is very low cloud, uniformly grey and thick, appears like highland fog. It brings dull weather and light drizzle. It reduces the visibility and is a hindrance to air transportation.
- viii. Nimbostratus (Ni-St): This is dark dull cloud, clearly layered, as it brings rain, snow and sleet and it is called as rainy cloud.
d. Clouds with vertical extent: Theseare mainly cumulus clouds whose heights extend from 2 km to 10 km approximately.
- Cumulus (Cu): This is vertical cloud with rounded top and horizontal base, associated with convectional process in the tropical region. It also called as ‘fair weather cloud’.
- Cumulonimbus (Cu-Ni): This is over grown cumulus cloud with great vertical extent, with black and white globular mass. The cauliflower top spreads like an anvil. This is formed due to heavy convection in the tropical regions. It is accompanied by lightning, thunder and heavy rainfall.
Precipitation The process of continuous condensation in free air helps the condensed particles to grow in size.
When the resistance of the air fails to hold them against the force of gravity, they fall on to the earth’s surface.
So after the condensation of water vapour, the release of moisture is known as precipitation. This may take place in liquid or solid form.
The precipitation in the form of water is called rainfall, when the temperature is lower than the 00 C, precipitation takes place in the form of fine flakes of snow and is called snowfall.
Sleet is frozen raindrops and refrozen melted snow-water. When a layer of air with the temperature above freezing point overlies a subfreezing layer near the ground, precipitation takes place in the form of sleet.
TYPES OF RAINFALL
On the basis of origin, rainfall may be classified into three main types – the convectional, orographic or relief and the cyclonic or frontal.
The, air on being heated, becomes light and rises up in convection currents. As it rises, it expands and loses heat and consequently, condensation takes place and cumulous clouds are formed. With thunder and lightening, heavy rainfall takes place but this does not last .
When the saturated air mass comes across a mountain, it is forced to ascend and as it rises, it expands; the temperature falls, and the moisture is condensed.
The chief characteristic of this sort of rain is that the windward slopes receive greater rainfall.
After giving rain on the windward side, when these winds reach the other slope, they descend, and their temperature rises.
Then their capacity to take in moisture increases and hence, these leeward slopes remain rainless and dry. The area situated on the leeward side, which gets less rainfall is known as the rain-shadow area. It is also known as the RELIEF RAIN.
- Cyclonic activity causes cyclonic rain and it occurs along the fronts of the cyclone.
- When two masses of air of unlike density, temperature, and humidity meet then it is formed.
- The layer that separates them is known AS FRONT.
- Warm front and the cold front are the two parts of the front.
- At the warm front, the warm lighter wind increases slightly over the heavier cold air.
- As the warm air rises, it cools, and the moisture present in it condenses to form clouds
- This rain falls gradually for a few hours to a few days.