The air exerts pressure on the surface
of earth due to its weight, which is called atmosphere
pressure.
The instrument
used to measure the atmospheric pressure is
calledBarometer.
Isobar (خطوط مساوی البار): Isobars are the lines used to show atmospheric pressure.
As the temperature
is high there pressure of atmosphere is low.
Low pressure
belt is located between 100 North and 100
South of the equator.
Horizontal
movement of the air over the surface of earth is called wind.
There are three
types of wind.
a) Permanent winds (دائمی ہوائیں )
b) Seasonal Wind (موسمی
ہوائیں)
c) Local wind (مقامی
ہوائیں)
Permanent
winds / Planetary winds (دائمی
ہوائیں ) :These wind s
are blowing throughout the year in a particular direction from areas of high
pressure to low pressure.
Permanent winds
further divide in to three types.
Easterlies or
Trade Winds (مشرقی یا تجارتی ہوائیں):
In equilateral
area the air pressure is low because of high
temperature.
In the north
and South hemispheres the air pressure is high
by the cause of low temperature.
The winds blow
from east to west so they are called Eastern wind or Easterlies.
Due to high
pressure the Wind permanently blowing in one direction they help in sailing of
ships, therefore they are also called trade winds.
b) Westerlies
or West winds (مغربی ہوائیں):
The wind blow
from west to east so they are called Westerlies or west
winds.
c) Polar wind (قطبی
ہوائیں):
The wind
blowing from polar high pressure belts to sub polar low pressure belts are also
known as polar winds.
As these winds
blow from east to west therefore they are also called polar easterlies.
Seasonal winds (موسمی ہوائیں):Those wind which change their direction with season is called seasonal
wind.
The example
of Seasonal winds is monsoon winds.
In summer
Seasonal winds blow from ocean towards land and cause
sufficient rainfall, they are called summer monsoon.
In winter
Seasonal wind direction is from land to Ocean and are called winter
monsoon.
Local wind (مقامی ہوائیں):In different areas of the world at local level winds are blowing
due to difference in temperature and pressure is called local winds.
They are further divide in the following.
a) Sea
Breeze and Land Breeze (نسیم بری وبحری):During day time when land is hot and ocean
water is comparatively cold the pressure is low at
land and high at ocean so winds blow from ocean
toward land. They are called sea breeze (نسیم بری).
While during
night the land is comparatively cold having
high pressure and the ocean water is warm
having low pressure. So the winds blow from land toward
ocean. They are called land breeze (نسیم بحری).
b) Mountain
and Valley Breezes (نسیم کوہی اور نسیم وادی):The difference between the height of mountain and valley the temperature
of the mountain is low verses valley, so the hot
wind goes to upward from valley this is
called Valley breeze (نسیم وادی).
Similarly the
space of the wind in valley filled during night and cold
wind comes from the mountain is called mountain
Breeze (نسیم
کوہی).
MCQs
………………is the instrument used to measure the atmospheric pressure?
a) Wind vane
