Page 118 - SST Class 07

The atmospheric pressure is affected by altitude pressure belts are called trade winds. These winds
(height). It decreases with altitude. As we go blow constantly in the same direction. They are
higher, the layers of atmosphere get thinner. This also called the easterlies as they blow from the
is the reason why the level of oxygen is low on east.
high mountains. Anti-Trade Winds : The westerlies blow in the
You must have seen the water vapours coming out temperate zone from the subtropical high pressure
from the spout of the kettle. It always goes areas to the sub-polar low pressure areas. They
upwards. Because water vapour is lighter than air. generally originate from the western direction and
Moist air exerts less pressure than dry air and it is are called westerlies.
lighter than the dry air. The excessive evaporation Polar Winds : The polar winds blow from the
adds more water vapour in the air in low latitudes polar high pressure belt to the sub-polar low
in the torrid zone. It makes the atmospheric pressure belt. These winds are very cold and dry.
pressure low in such areas. They blow in both Northern and Southern
Hemispheres.
Activity
Collect the weather report of your town or city from Activity
the newspaper for a month. Analyse the change
and draw up a weather report. On an outline map of the world, mark the planetary
winds and label each wind type. And also label the
Wind pressure belts of the world.

The movement of air from high pressure area to
low pressure area is called Periodic Winds
wind . It is the horizontal These are the winds which change their direction
movement of air. The vertical in different seasons during the year. They blow in
movement of air is called air a definite direction during a particular period of
current . Wind is named the day or year. These winds are caused by the
according to the direction from unequal heating and cooling of the earth’s surface
which it blows. If the wind Anemometer and the localised difference in the atmospheric
blows from the western pressure.
direction, it is called west wind .
Monsoon Winds : Monsoons in India are a good
Wind direction is measured by example of periodic or seasonal winds. Since the
an instrument known as the monsoons blow from the sea, they have a lot of
wind vane and its speed is
water content due to which they bring widespread
measured by an anemometer . rains to many parts of Asia. The monsoons which
We can divide wind into three Wind Vane bring rains to the whole of India are called
categories :
summer or south-west monsoons .
Planetary Winds
Land and Sea Breeze : The land and sea breeze
The winds which blow in fixed direction blow along the coastal regions for a few hours
throughout the year are called prevailing winds or only. During the day, the land usually gets hot
permanent winds or planetary winds . These winds while the sea remains cool and pressure difference
originate due to the presence of permanent high is created. The wind that blows from sea towards
pressure and low pressure belts on the surface of the land brings the temperature down in the
the earth. The main planetary winds are the trade coastal regions during the day. It is called sea
winds, westerlies winds and polar winds . breeze. At night, the land cools down much faster
than the sea. The wind blowing from the sea is
Trade Winds : Winds which blow from
subtropical high pressure belts to equatorial low warmer and lighter. So the pressure over the sea is
Contemporary Social Science-7
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