NCERT Book Class 11 Geography (Fundamental of Physical Geography) Chapter 9 Atmospheric Circulation and Weather Systems

CHAPTER

ATMOSPHERIC CIRCULATION
AND WEATHER SYSTEMS

E
arlier Chapter 8 described the uneven pressure is measured with the help of a
distribution of temperature over the mercury barometer or the aneroid barometer.
surface of the earth. Air expands when Consult your book, Practical Work in
heated and gets compressed when cooled. This Geography — Part I (NCERT, 2006) and learn
results in variations in the atmospheric about these instruments. The pressure
pressure. The result is that it causes the decreases with height. At any elevation it varies
movement of air from high pressure to low from place to place and its variation is the
pressure, setting the air in motion. You already primary cause of air motion, i.e. wind which
know that air in horizontal motion is wind. moves from high pressure areas to low
Atmospheric pressure also determines when pressure areas.
the air will rise or sink. The wind redistributes
the heat and moisture across the planet, Vertical Variation of Pressure
thereby, maintaining a constant temperature
In the lower atmosphere the pressure
for the planet as a whole. The vertical rising of
decreases rapidly with height. The decrease
moist air cools it down to form the clouds and
amounts to about 1 mb for each 10 m
bring precipitation. This chapter has been
increase in elevation. It does not always
devoted to explain the causes of pressure
decrease at the same rate. Table 9.1 gives the
differences, the forces that control the
average pressure and temperature at
atmospheric circulation, the turbulent pattern
selected levels of elevation for a standard
of wind, the formation of air masses, the
atmosphere.
disturbed weather when air masses interact
with each other and the phenomenon of violent Table 9.1 : Standard Pressure and Temperature at
tropical storms. Selected Levels
Level Pressure in mb Temperature °C
ATMOSPHERIC PRESSURE
Sea Level 1,013.25 15.2
Do you realise that our body is subjected to a
1 km 898.76 8.7
lot of air pressure. As one moves up the air
gets varified and one feels breathless. 5 km 540.48 –17. 3
The weight of a column of air contained in 10 km 265.00 – 49.7
a unit area from the mean sea level to the top
of the atmosphere is called the atmospheric The vertical pressure gradient force is much
pressure. The atmospheric pressure is larger than that of the horizontal pressure
expressed in units of milibar. At sea level the gradient. But, it is generally balanced by a
average atmospheric pressure is 1,013.2 nearly equal but opposite gravitational force.
milibar. Due to gravity the air at the surface is Hence, we do not experience strong upward
denser and hence has higher pressure. Air winds.