Air Pressure Class 7 Geography Notes Maharashtra State Board
During the activity, you saw that the balance was tipped on the side of the full-blown balloon. This shows that the air has weight. Anything that has weight, exerts pressure on the thing that lies below it. Thus, the air in the atmosphere exerts pressure on the surface of the earth. Due to this air pressure, various phenomena like storms, precipitation, etc. occur in the atmosphere. There are variations in air pressure.
- Air pressure is not uniform in all places on the earth’s surface.
- Air pressure keeps on changing from time to time.
- The altitude of a region, the temperature of the air, and the amount of water vapor in the air are some factors influencing air pressure.
Altitude of the Region and Air Pressure:
The proportion of dust in the air, water vapor, 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.
Air Temperature and Air Pressure:
Take a flying lantern. Tie an approximately 5m long thread to the flying lantern so that you can bring the lantern down whenever required. After carefully reading the instructions given on the package of the lantern, open it and light the candle placed in it. Observe what happens. After some time, bring the lantern down with the help of the thread and put off the candle.
The air in the flying lantern gets heated once the candle is lit. The hot air expands, becomes lighter, and starts moving up. Therefore, the lantern is also lifted towards the sky. In nature too, a similar phenomenon occurs. 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.
Temperature zones and pressure belts are interrelated to each other but the latitudinal extent of the temperature zones is much larger while pressure belts are narrower. For example, the Temperate zone extends from 23° 30′ to 66° 30′. Compared to this, the air pressure belt has a limited extent. It is generally up to 10° parallel. 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. We have studied this in the previous class.
Equatorial Low-Pressure Belt:
The sun’s rays can be perpendicular between the Tropic of Cancer and the Tropic of Capricorn. So the temperature is higher in this region. Hence air in this region gets heated, expands, becomes lighter, and moves toward the sky. As this process operates continuously a low-pressure belt gets formed in the central part of this region between the parallels 5° north and 5° south.
Mid-Latitudinal High-Pressure Belts:
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. Due to low temperatures at higher altitudes, the air cools down and becomes heavier. This heavier air descends in both hemispheres in the region between 25° to 35° parallels. This leads to the formation of high-pressure belts in these parallels of latitudes in both hemispheres. This air is dry, hence the region does not get rainfall. Consequently, most of the hot deserts on the earth are found in these regions.
Sub-Polar Low Pressure Belts:
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. This condition is observed in the area between 55° and 65° parallels in both hemispheres.
Polar High-Pressure Belts:
In both polar regions, the temperature is below 0°C throughout the year. Hence, here the air is cold. As a result, high-pressure belts get formed. These are called polar high-pressure belts. They generally occupy the area between 80° and 90° parallels in both hemispheres. The duration and intensity of sunrays vary during particular periods of the year in both 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. This is called the oscillation of pressure belts.
The major difference between the temperature zones and pressure belts is that the temperature zones are continuous and are spread from the equator to the poles from Torrid to Frigid. Pressure belts are not continuous and areas of high and low pressure are found in different regions from the equator to the poles.
Air pressure has the following effects.
- Origin of winds.
- Generation of storms.
- Conventional type of rain.
- Air pressure affects breathing activity too.
The line that joins the places of equal pressure on the map is called an isobar.
Air pressure is measured in units of millibars. For this, an instrument called a barometer is used. The air pressure at the earth’s surface is measured with this instrument.
All things in and on the earth stay earthbound because of the earth’s gravity. This includes air which is in the gaseous form. Due to the earth’s gravity, air is pulled to the earth’s surface. That is why, air pressure is maximum at sea level. Note that air exerts pressure on everything and everyone including us. It is believed that the weight of the air column on any one person’s head amounts to 1000 kg.
Detailed Maharashtra State Board Class 7 Geography Notes Air Pressure is particularly useful for answering essay questions.