Page 88 - Journeys Through Society-7

Strong winds do not blow continuously with the same
degree of intensity and, after a while, they become
gentle before regaining force once again. Sometimes
when the wind blows, it forms whirls of dust or eddies.
It especially takes place in areas where there are tall
buildings. When the eddies lie in the direction of the
winds, the winds become strong.
Winds may be broadly classified into the following
four categories:
Permanent Winds: These winds blow the all year
round in a fixed direction. Trade winds that come from
the east, blow from sub-tropical high pressure zones Aerial view of a cyclone
to equatorial low pressure zones. Westerly winds that
occur. In the Eastern Pacific they are called hurricanes,
come from the west blow from sub-tropical high
those in Southeast Asia are called typhoons, and those
pressure zones to sub-polar low pressure zones. Polar
in the Indian Ocean and Western Pacific regions are
winds coming from the east blow from polar high
called cyclones.
pressure zones to sub-polar low pressure zones.
Temperature
Temperature is measured by a thermometer. The
simplest thermometer is the mercury-in-glass or rod
thermometer. When it heats up, the mercury in the
bulb at the bottom of the thermometer expands and
rises in a thin vacuum glass, within the glass rod. When
it cools, the mercury contracts and drops down in the
tube, thereby, depicting both maximum and minimum
temperatures of the day. The recording thermometer
automatically records all temperature changes.

The air temperature of a particular place denotes the
degree of hotness and coldness of air at that place. It is
measured in Celsius or Fahrenheit. The surface of the
Planetary winds of the earth Earth is heated by the sun's rays, and the air above the
Earth surface is heated by the heat reflected from the
Seasonal Winds: Unlike permanent winds, they blow
warm surface.
only during a particular season and are, therefore,
In summers, the Sun is high in the sky. Therefore, the
called periodic winds. The monsoon winds are a good
amount of heat that reaches the Earth's surface is
example of such winds.
higher. In winters, the Sun is low in the sky and the rays
Local Winds: These winds are confined to a small area
slant towards the surface of the Earth, due to which
and have certain characteristic features of their own.
there is less heat. For these reasons, summer is always
An example of local wind is the loo which is a hot, dry
warmer than winter. Similarly, the Sun reaches its
and dusty wind that blows during the hot, dry season
highest position or zenith in the sky at midday. As
in the Northern Plains of India.
more of the Sun's heat reaches the Earth's surface at
Cyclonic Winds: These winds occur when cold air midday than at any other time, the hottest period of
meets warm air. Their occurrence varies with time and
any day is right after noon. The temperatures are
place. Cyclones create chaos and devastation moving
lowest just before dawn. This is because the surface of
at the rate of up to 74 miles per hour. The name of a
the Earth cools rapidly after the Sun has set.
tropical cyclone depends on the region in which they
Temperature falls steadily between sunset and dawn.
88
Social Studies-7