Notes Class 7 Chapter 4 Geography Maharashtra Board

Notes For All Chapters – Geography Class 7

Air Pressure

1. Introduction to Air Pressure

• The atmosphere consists of various gases, dust particles, and water vapor, all of which exert pressure on the Earth’s surface.
• This air pressure influences weather patterns, including storms, winds, and precipitation.
• Air pressure is not uniform across different regions and changes over time.

2. Factors Affecting Air Pressure

Air pressure varies due to the following factors:

(a) Altitude and Air Pressure

• Air pressure decreases with increasing altitude.
• The lower atmosphere contains more dust, water vapor, and heavier gases, making the air denser.
• As altitude increases, the air becomes thinner and exerts less pressure.
• Example: At sea level, air pressure is maximum, but at high mountains, it is much lower.

(b) Temperature and Air Pressure

• Higher temperature causes air to expand, become lighter, and rise, creating low pressure.
• Lower temperature makes air denser and heavier, leading it to descend, creating high pressure.
• Example: Hot air balloons rise because warm air is lighter.

(c) Water Vapor and Air Pressure

• Humid air is lighter than dry air, so areas with high humidity often have lower pressure.
• Dry air is denser and heavier, leading to high pressure.

3. Horizontal Distribution of Air Pressure

The uneven heating of Earth creates distinct pressure belts, arranged parallel to the Equator:

(a) Equatorial Low-Pressure Belt (5°N – 5°S)

• Sunrays fall vertically, causing high temperatures.
• Warm air expands, rises, and creates a low-pressure zone.
• This belt experiences high rainfall due to convectional currents.

(b) Subtropical High-Pressure Belts (25° – 35° N & S)

• Warm air from the equator rises and moves toward the poles.
• At higher altitudes, it cools and descends, forming high pressure.
• These regions are dry and contain hot deserts like the Sahara, Thar, and Atacama.

(c) Subpolar Low-Pressure Belt (55° – 65° N & S)

• The Earth’s rotation pushes air outward, creating a low-pressure belt.
• This zone experiences stormy weather due to the meeting of warm and cold winds.

(d) Polar High-Pressure Belts (80° – 90° N & S)

• The cold climate makes air dense and heavy.
• Cold air descends, creating high-pressure zones.
• These belts have extremely cold and dry conditions.

4. Oscillation of Pressure Belts

• Due to the Earth’s revolution, the intensity of sunlight varies throughout the year.
• The pressure belts shift 5° to 7° north in Uttarayan (Northern Hemisphere summer) and south in Dakshinayan (Southern Hemisphere summer).
• This movement is called the oscillation of pressure belts.

5. Effects of Air Pressure

• Origin of winds: Winds blow from high-pressure to low-pressure areas.
• Formation of storms: Variations in air pressure lead to cyclones and hurricanes.
• Convectional rainfall: Rising hot air cools and condenses, forming rain.
• Breathing activity: Lower air pressure at high altitudes affects oxygen levels.

6. Isobars

• Isobars are lines on a map that connect places with equal air pressure.
• These are used in weather forecasting to show high and low-pressure areas.

7. Measurement of Air Pressure

• Unit: Air pressure is measured in millibars (mb).
• Instrument: A barometer is used to measure air pressure.
• Standard air pressure at sea level is 1013.25 mb.

8. Summary of Key Pressure Belts