Maharashtra Board Class 7 Geography Chapter 4 Air Pressure PDF Download

Air Pressure

Let's Recall

Have you tried the activity that shows 'Air has weight' given on page 16 of your Science textbook?

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.

(Instructions for teachers/parents: This activity is to be performed under your supervision and presence with utmost care. The teacher should arrange a discussion after completing the activity given above. Use following questions to initiate the discussion.)

Geographical Explanation

During the activity, you saw that the balance 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 on all places on the earth's surface.

Air pressure keeps on changing from time to time.

The altitude of a region, temperature of the air, and amount of water vapour in the air are some factors influencing air pressure.

Altitude of the Region and Air Pressure

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.

Teacher's Note

Air pressure is heavy on us! When you go up a mountain in India, you feel tired because there is less air. The higher you go, the thinner the air becomes.

Exam Trick

Remember: Higher the place, lower the pressure. Just like in Mumbai the air is heavy, but in Shimla mountains the air is thin and light.

Points to Remember

Air has weight and pushes down on everything.
Air pressure is stronger at sea level and weaker on mountains.
Temperature, altitude, and water vapour change air pressure.
Not all places on Earth have the same air pressure.

Air Temperature and Air Pressure

Try this.

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.

Think About It

What will be the effect on air pressure if the temperature drops? Why?

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 up 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.

Can You Tell

Did the flying lantern start ascending immediately after the candle was lit?

What would have happened to the flying lantern had the candle got extinguished after the lantern had gone up in the air?

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. The Temperate zone extends from 23° 30' to 66° 30'. Compared to this, the air pressure belt has 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.

Observe the figures and answer the following questions:

Which pressure belt is mainly found in the Tropics?

With which pressure belt are the polar winds associated? In which temperature zone are they observed?

What could be the reason behind a low pressure belt in the Tropics?

With which pressure belts are the winds in the Temperate zone associated?

Write the latitudinal extent of the low pressure belts.

Teacher's Note

Temperature and pressure are like friends. When it is hot, the air goes up and pressure becomes low. When it is cold, air comes down and pressure becomes high.

Exam Trick

Remember: Hot = Low Pressure, Cold = High Pressure. Think of a hot day - everyone feels light and wants to go up to the hills!

Points to Remember

Hot air goes up and makes low pressure.
Cold air comes down and makes high pressure.
Temperature and pressure always work together.
Air pressure changes with heat.

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 sunrays 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 towards 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 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. This leads to the formation of high pressures belts in these parallels of latitudes in both the 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 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 the hemispheres.

Polar High Pressure Belts

In both the 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 the hemispheres.

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° 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.

Teacher's Note

There are different pressure belts around the Earth like rings. The equator has low pressure, but at 30° latitude we have high pressure where deserts are found.

Exam Trick

Remember: Equator = Low Pressure, 30° = High Pressure with Deserts. Think of the Sahara and Thar deserts - they are at 30° latitude with high pressure and no rain!

Points to Remember

Equatorial belt has low pressure between 5°N and 5°S.
High pressure belts are at 25° to 35° where deserts form.
Polar regions have high pressure because they are very cold.
Pressure belts move 5° to 7° with seasons.

Always remember - Effects:

Air pressure has the following effects.

Origin of winds.

Generation of storms.

Convectional type of rain.

Air pressure affects the breathing activity too.

Isobars

The line that joins the places of equal pressure on the map is called an isobar.

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