Page 141 - SST Class 07

water. Droplets joining together form bigger when warm air meets cold air. Stratiform
drops. Huge masses of water droplets are called rainfall usually occurs from nimbostratus
‘clouds’. cloud.

When these droplets become too heavy to float 3. Relief or Orographic Rainfall
in the air, they come down as ‘precipitation’, Orographic
Precipitation is of different types. That, which rainfall occurs
comes down to the earth in liquid form is called when a mass of
rain. The other forms are snow, sleet and hail. air is pushed up
Rainfall is the main source of groundwater which because of a
is very important for the vegetation-cover on the structure with
earth surface. It is also an important source of very high eleva-
fresh water. Very tiny drops of rain water are tion such as a
called drizzle. The instrument used to measure mountain. The
rainfall is called the rain gauge. lifted air cools down and precipitation
Rainfall itself is of three types : occurs. Rain shadow area is formed on the
Convectional, Cyclonic or Stratiform and Relief other side of the elevation.
or Orographic rainfall. Rainfall is extremely important for existence of
1. Convectional Rainfall life on the earth. It is not only a source of fresh
When a portion of water, it also effects the vegetation of any area.
the earth becomes AIR POLLUTION
heated more than Any change in the composition of air is termed
its surrounding, it as air pollution. It results from release of certain
creates significant gaseous, solid or liquid agents in the atmosphere.
amount of evapora- The obvious causes of air pollution are—growing
tion. Convection industrial activity, excessive burning of fossil
currents then push fuels, rising numbers of automobiles, fires and
up the moist air and burning of solid wastes. The chief air pollutants
cumulonimbus clouds are formed. These are – carbon dioxide, carbon monoxide, chloro-
clouds result in convectional rainfall. fluorocarbons, suspended particulate matter,
Convectional rainfall is in the form of sulphur dioxide, nitrogen oxide and lead.
showers with rapidly changing intensity. This Air pollution leads to serious health problems
type of rainfall happens for a short duration such as loss of biodiversity and damage to
because horizontal expanse of a cumulo- historical monuments. It is a serious environmen-
nimbus cloud is small. tal problem and must be tackled urgently.
2. Cyclonic or Greenhouse Gases and Ozone
Stratiform Rainfall
Gases like water vapor, carbon dioxide and
This type of rainfall is
Warm air methane are greenhouse gases which are present
formed by frontal Cold air in the small amounts. Like a thick blanket that
system surrounding covers the earth, they are especially good at
extratropical cyclones. trapping heat. And, because their concentrations
Cyclone is formed are increasing due to human activity, more heat

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