Earlier Chapter 9 described the uneven distribution of temperature over the surface of the earth. Air expands when heated and gets compressed when cooled. This results in variations in the atmospheric p essure. The result is that it causes the movement of air from high pressure to low pressure, setting the air in motion. You already know that air in horizontal motion is wind. Atmospheric pressure also determines when the air will rise or sink. The wind redistributes the heat and moisture across the planet, thereby, maintaining a constant temperature for the planet as a whole. The vertical rising of moist air cools it down to form the clouds and bring precipitation. This chapter has been devoted to explain the causes of pressure differences, the forces that control the atmospheric circulation, the turbulent pattern of wind, the formation of air masses, the disturbed weather when air masses interact with each other and the phenomenon of violenttropical storms.
ATMOSPHERIC PRESSURE
Do you realise that our body is subjected to a lot of air pressure. As one moves up the air gets varified and one feels breathless. The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure. The atmospheric pressure is expressed in units of mb and Pascals. The widely used unit is kilo Pascal written as hPa. At sea level the average atmospheric pressure
is 1,013.2 mb or 1,013.2 hPa. Due to gravity the air at the surface is denser and hence has higher pressure. Air pressure is measured with the help of a mercury barometer or the aneroid barometer. Consult your book, Practical Work in Geography — Part I (NCERT, 2006) and learn about these instruments. The pressure decreases with height. At any elevation it varies from place to place and its variation is the primary cause of air motion, i.e. wind which moves from high pressure areas to low pressure areas. Vertical Variation of Pressure In the lower atmosphere the pressure decreases rapidly with height. The decrease amounts to about 1 mb for each 10 m increase in elevation. It does not always decrease at the same rate. Table 10.1 gives the average ressure and temperature at selected levels of elevation for a standard atmosphere. The vertical pressure gradient force is much larger than that of the horizontal pressure gradient. But, it is generally balanced by a nearly equal but opposite gravitational force. Hence, we do not experience strong upward winds.
Horizontal Distribution of Pressure
mall differences in pressure are highly significant in terms of the wind direction and velocity. Horizontal distribution of pressure is studied by drawing isobars at constant levels. Isobars are lines connecting places having equal pressure. In order to eliminate the effect of altitude on pressure, it is measured at any station after being reduced to sea level for purposes of comparison. The sea level pressure distribution is shown on weather maps. Figure 10.1 shows the patterns of isobars corresponding to pressure systems. Lowpressure system is enclosed by one or more isobars with the lowest pressure in the centre. High-pressure system is also enclosed by one or more isobars with the highest pressure in the centre.
World Distribution of Sea Level Pressure
The world distribution of sea level pressure in January and July has been shown in Figures 10.2 and 10.3. Near the equator the sea level pressure is low and the area is known as equatorial low. Along 30° N and 30o S are
found the high-pressure areas known as the subtropical highs. Further pole wards along 60o N and 60o S, the low-pressure belts are termed as the sub polar lows. Near the poles the pressure is high and it is known as the polar high. These pressure belts are not permanent in nature. They oscillate with the apparent movement of the sun. In the northern hemisphere in winter they move southwards and in the summer northwards.Forces
Affecting the Velocity and Direction of Wind
You already know that the air is set in motion due to the differences in atmospheric pressure. The air in motion is called wind. The wind blows from high pressure to low pressure. The wind at the surface experiences friction. In addition, rotation of the earth also affects the wind movement. The force exerted by therotation of the earth is known as the Coriolis force. Thus, the horizontal winds near the earth surface respond to the combined effect of three forces – the pressure gradient force, the frictional force and the Coriolis force. In addition, the gravitational force acts downward.
1. Multiple choice questions.
(i) If the surface air pressure is 1,000 mb, the air pressure at 1 km above the surface will be:
(a) 700 mb (c) 900 mb
(b) 1,100 mb (d) 1,300 mb
(ii) The Inter Tropical Convergence Zone normally occurs:
(a) near the Equator (b) near the Tropic of Cancer
(c) near the Tropic of Capricorn (d) near the Arctic Circle
(iii) The direction of wind around a low pressure in northern hemisphere is:
(a) clockwise (c) anti-clock wise
(b) perpendicular to isobars (d) parallel to isobars
(iv) Which one of the following is the source region for the formation of air masses?
(a) the Equatorial forest (c) the Siberian Plain
(b) the Himalayas (d) the Deccan Plateau
2. Answer the following questions in about 30 words.
(i) What is the unit used in measuring pressure? Why is the pressure measured at station level reduced to the sea level in preparation of weather maps?
(ii) While the pressure gradient force is from north to south, i.e. from the subtropical high pressure to the equator in the northern hemisphere, why are the winds north easterlies in the tropics.
(iii) What are the geotrophic winds?
(iv) Explain the land and sea breezes.
3. Answer the following questions in about 150 words.
(i) Discuss the factors affecting the speed and direction of wind.
(ii) Draw a simplified diagram to show the general circulation of the atmosphere over the globe. What are the possible reasons for the formation of subtropical high pressure over 30o N and S latitudes?
(iii) Why does tropical cyclone originate over the seas? In which part of the tropical cyclone do torrential rains and high velocity winds blow and why?
Please refer to attached file for NCERT Class 11 Geography Atmosphoric Circulation And Weather System
