4. Air Pressure
Complete textbook exercise solutions and geographical reasons. Maharashtra State Board (Balbharati).
ShareAdvertisement
Exercises
(1) Air pressure decreases with increasing altitude.
The proportion of dust in the air, water vapour, heavy gases, etc., is higher in the air closer to the surface of the earth. This proportion decreases with increasing altitude. As one moves higher and higher from the surface of the earth, the air becomes thinner and thinner. As a result, the air pressure decreases with increasing altitude.
(2) Pressure belts oscillate.
The duration and the intensity of sunrays vary during particular periods of the year in both the hemispheres. As a result, the locations of the temperature zones and the pressure belts dependent on the sun's heat also vary. This change is of the order of 5° to 7° towards the north in Uttarayan, and 5° to 7° south in Dakshinayan. Because of this shifting, it is said that the pressure belts oscillate.
(1) What effect does temperature have on air pressure?
Temperature and air pressure are closely related. Wherever the temperature is high, the air pressure is low. As the temperature rises, the air gets heated, expands, and becomes lighter. This lighter air in the vicinity of the earth's surface starts moving up towards the sky. As a result, the air pressure in such areas decreases.
(2) Why is the subpolar low pressure belt formed?
Due to the earth's curvature, the area between two parallels gets reduced as we move towards the poles. This results in lesser friction of the air with the earth's surface. Air in this region is thrown out because of this reduced friction and also because of the earth's rotational motion. This leads to the development of a low pressure belt in the area between 55° and 65° parallels in both the hemispheres.
Advertisement
(1) Mid-latitudinal high pressure belts
The heated air from the equatorial region becomes lighter, starts ascending and after reaching higher altitudes, moves towards the polar region (towards the North and the South Pole). Due to low temperatures at the higher altitudes, the air cools down and becomes heavier. This heavier air descends down in both the hemispheres in the region between 25° to 35° parallels, leading to the formation of high pressure belts. This descending air is dry, hence the region does not get rainfall, and consequently, most of the hot deserts on the earth are found in these regions.
(2) Horizontal distribution of air pressure
The heat received from the sun is uneven in different regions, leading to an uneven distribution of temperature from the equator to the poles. This uneven distribution of temperature influences the distribution of air pressure. As a result, it leads to the formation of distinct low and high pressure belts horizontally across the earth's surface. These include the Equatorial low pressure belt, Mid-latitudinal high pressure belts, Sub-polar low pressure belts, and Polar high pressure belts.
- (1) At higher altitudes air becomes thinner.
(Options: thicker, thinner, hotter, more humid) - (2) Air pressure is expressed in millibars.
(Options: millibars, millimetres, millilitres, milligrams) - (3) On the earth, air pressure is uneven.
(Options: uniform, uneven, high, low) - (4) The Equatorial low pressure belt spreads between 5° North and 5° South parallel.
(Options: Equatorial low, Polar high, Subpolar low, Mid-latitudinal high)
The heated air from the equatorial region becomes lighter, ascends, and after reaching higher altitudes, moves towards the polar regions. Due to low temperatures at higher altitudes, this air cools down and becomes heavier. This heavier air descends in both hemispheres in the region between 25° to 35° parallels (which includes the 30° parallel), forming the mid-latitudinal high pressure belts.
This region has hot deserts because the descending air is dry. Since the air lacks moisture, the region does not get rainfall, resulting in the formation of most of the hot deserts on the earth in these specific latitudes.
Students should refer to Figure 4.2 b: Pressure belts and planetary winds on page 17 of the textbook to draw the diagram. Ensure you correctly label the Equatorial low pressure belt (5° N to 5° S), Mid-latitudinal high pressure belts (25° to 35°), Subpolar low pressure belts (55° to 65°), and Polar high pressure belts (80° to 90°) in both hemispheres.