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

Introduction :

• The air in the atmosphere puts pressure on the earth's surface. As a result of the air pressure, numerous phenomena such as storms, precipitation, and so on occur in the atmosphere.
• There are changes in air pressure. The air pressure on the earth's surface is not uniform. It fluctuates from time to time. Air pressure is influenced by elements such as the height of the region, the temperature of the air, and the amount of water vapour in the air.

Altitude of the region and air pressure :

• The quantity of dust in the air, water vapour, heavy gases, and so on is higher towards the earth's surface. This percentage reduces as altitude increases.
• 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 the increasing altitude.

Temperature and air pressure :

• 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.

Temperature zones and pressure belts :

• Temperature zones and pressure belts are interrelated to each other.

• The latitudinal extent of the temperature zones is much larger while that of pressure belts is narrower.
• The uneven distribution of temperature influences the distribution of air pressure too. This leads to the formation of low and high pressure belts horizontally between the equator and the poles.

Pressure belts on the earth’s surface :

• The heat received from the sun is uneven in different regions. Hence, the distribution of the temperature is uneven from the equator to the poles.
• As a result, the temperature zones are created. Temperature zones then create pressure belts.

Equatorial low pressure belt:

• Equatorial low pressure belt is formed between 5°N and 5°S parallels.
• Sunrays are perpendicular between the Tropics of Cancer and Capricorn. As a result, the temperature is higher in this area.
• As a result, the air in this region heats up, expands, becomes lighter, and travels upward.
• As this process continues, a low pressure belt forms in the centre of this region between parallels 5° north and 5° south.

Mid-latitudinal high pressure belts :

Mid-latitudinal high pressure belts are formed between 25° and 35° parallels in both the hemisphere.

• The warm air from the equatorial zone gets lighter, ascends, and flows towards the polar region, i.e. the North and South poles, after reaching higher latitudes.
• Because of the lower temperatures at higher altitudes, the air cools and gets heavier.
• As a result, heavier air descends in both hemispheres between the 25° and 35° parallels.
• This causes high pressure belts to emerge in both hemispheres.
• These high pressure belts are referred to as mid-latitudinal high pressure belts.
• The air in the mid-latitudinal high pressure belts is dry, resulting in little rainfall. As a result, the majority of the world's hot deserts may be found in these areas.

Sub polar low pressure belts :

Sub polar low pressure belts are formed between 55° and 65° parallels in both the hemisphere.

• 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.

Polar high pressure belts :

These pressure belts are found in both the hemispheres in the region between 80° and 90° parallels.

• In both the polar regions, the temperature is below 0° C throughout the year.
• Hence, the air is found to be cold in these regions.
• As its effect, high pressure belts get formed in polar regions.
• These pressure belts are called polar high pressure belts.

 Oscillation of pressure belts :

• The duration and the intensity of sunrays varies 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 north in Uttarayan, and 5° to 7° towards south in Dakshinayan. This is called the oscillation of pressure belts.

Difference between the temperature zones and pressure belts :

• The temperature zones are continuous. On the other hand, pressure belts are not continuous.
• The temperature zones are spread from the equator to the poles from Torrid to Frigid. On the other hand, areas of high and low pressure are found in different regions from the equator to the poles.
• The latitudinal extents of the temperature zones are much larger. Temperate zones are found between 23° 30’ and 66° 30' parallels in both the hemispheres. On the other hand, the latitudinal extents of pressure belts are narrower. It is generally only up to 10° parallel.

Effects of air pressure : Effects of air pressure are as follows :

• Air pressure generates winds.
• It also generates storms.
• It leads to convectional type of rain.

Isobars :

• The line that joins the places of equal pressure on the map is called an isobar.
• Isobars in the northern hemisphere run zigzag and they are not parallel to the latitudinal lines.
• Isobars in the southern hemisphere run almost straight and they are fairly parallel to the latitudinal lines.