Solutions-Class-7-Geography-Chapter-4-Air Pressure-Maharashtra Board

• 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.

• The intensity and the duration of sunrays varies during particular periods of the year in both the hemispheres.
• Due to this, 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 north in Uttarayan, and 5° to 7° towards south in Dakshinayan. In this way, pressure belts oscillate.

• Temperature and air pressure are closely related.
• Whenever 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 upwards the sky. As a result, the air pressure in such areas decreases.

• 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. As its effect, the sub-polar low pressure belt formed.

• The heated air from equatorial region becomes lighter, starts ascending and after reaching higher latitudes, moves towards the polar region i.e. towards the North and South
• The air cools down and becomes heavier because of low temperatures at the higher altitude,
• Thus heavier air descends down in both the hemispheres in the region between 25° and 35° parallels.
• This leads to the formation of high pressure belts in both the hemispheres.
• These high pressure belt are called mid-latitudinal high pressure belts.
• The air in the mid-latitudinal high pressure belts is found to be dry leading to scarce Consequently, most of the hot deserts on the earth are found in these regions.

• Air pressure is not horizontally uniform on all places on the earth's surface.
• The air pressure is found to be high in some regions and low in some other regions on the earth's surface.
• Such distribution of air pressure is known as horizontal distribution of air pressure.
• Polar high pressure belts.

(1) At higher altitudes air becomes thinner

(2) Air pressure is expressed in millibars.

(3) On the earth, air pressure is uneven

(4) The Equatorial low pressure belt spreads between 5° North and 5°South parallel.

Formation of a high pressure belt near 30° parallel :

(1) The heated air becomes lighter, starts ascending and after reaching higher altitudes, moves towards the polar region, i.e., towards the North and the South Pole.

(2) Due to low temperatures at the higher altitude, the air cools down and becomes heavier.

(3) Thus heavier air descends down in both the hemispheres in the region between 25° and 35° parallels.

(4) This leads to the formation of high pressure belts in both the hemispheres. In this way, high pressure belt gets formed near 30° parallel. ,

Reasons for the region near 30° parallel having hot deserts :

(1) The air in the mid-latitudinal high pressure belts (between 25° and 35° parallels in both the hemispheres) is found to be dry.

(2) The amount of water vapour is found to be very low in dry air. Hence, this region gets extremely scares or no rainfall.

(3) Consequently, this region have hot deserts.