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Pressure Belts of the Earth – UPSC Geography Notes
Overview: The distribution of air pressure over the Earth’s surface is not uniform. It forms distinct zones called pressure belts. These belts play a crucial role in controlling global wind systems, weather, and climate. Understanding the pattern of pressure belts is vital for UPSC Geography and Climatology.
Table of Contents
- 1. Introduction
- 2. What is Air Pressure?
- 3. Major Pressure Belts of the Earth
- 4. Characteristics of Each Pressure Belt
- 5. Shifting of Pressure Belts
- 6. Effects on Global Climate and Winds
- 7. UPSC Quick Revision Notes
- 8. Previous Year Question
1. Introduction
The unequal heating of the Earth’s surface by the Sun causes differences in temperature and pressure. Warm air rises, creating low pressure, while cold air descends, creating high pressure. These alternating high and low pressure zones are arranged almost parallel to the equator, forming pressure belts.
2. What is Air Pressure?
Air pressure is the weight or force exerted by the air on the Earth’s surface. It is measured using a barometer and expressed in millibars (mb).
Standard sea level pressure = 1013.25 mb.
Air pressure decreases with altitude and changes with temperature and humidity.
3. Major Pressure Belts of the Earth
The Earth has seven pressure belts — alternating high and low pressure zones distributed symmetrically on both sides of the equator.
| Latitude (Approx.) | Pressure Belt | Type |
|---|---|---|
| 0° | Equatorial Low Pressure Belt | Low Pressure |
| 30° N & 30° S | Subtropical High Pressure Belts | High Pressure |
| 60° N & 60° S | Subpolar Low Pressure Belts | Low Pressure |
| 90° N & 90° S | Polar High Pressure Belts | High Pressure |
4. Characteristics of Each Pressure Belt
(a) Equatorial Low Pressure Belt (Doldrums)
- Located around the equator (0°–5° N/S).
- Region of intense heating – warm, moist air rises due to convection.
- Characterized by high temperature, high humidity, and calm winds.
- Associated with heavy rainfall and thunderstorms.
(b) Subtropical High Pressure Belts (Horse Latitudes)
- Located around 30° N and 30° S.
- Descending air from the equator creates dry and stable conditions.
- Regions of calm winds and clear skies.
- Major deserts of the world (Sahara, Kalahari, Australian) are found here.
(c) Subpolar Low Pressure Belts
- Located around 60° N and 60° S.
- Warm westerlies meet cold polar easterlies, causing rising air and cyclonic activity.
- These belts are zones of frequent storms and low pressure.
(d) Polar High Pressure Belts
- Located near the poles (90° N & S).
- Extremely cold air sinks, creating high pressure.
- Air is dense and dry; winds move outward as polar easterlies.
5. Shifting of Pressure Belts
The pressure belts are not fixed. They shift north and south with the apparent movement of the Sun during the year.
- In summer (Northern Hemisphere) – belts shift northward.
- In winter (Northern Hemisphere) – belts shift southward.
- This shifting causes the seasonal migration of winds and rainfall zones.
Example: The monsoon winds in India are caused by the shifting of pressure and wind belts.
6. Effects on Global Climate and Winds
- Pressure belts determine the origin and direction of planetary winds such as trade winds, westerlies, and polar easterlies.
- They control rainfall patterns and climatic zones across the Earth.
- Influence the formation of deserts, cyclones, and monsoon systems.
7. UPSC Quick Revision Notes
Equatorial – rising air, low pressure, heavy rain.
Subtropical – sinking air, high pressure, deserts.
Subpolar – cyclonic storms.
Polar – cold, high pressure.
Belts shift north and south with the Sun’s movement.
Horse Latitudes → calm, dry regions near 30°.
Doldrums → calm, rainy region near equator.
8. Previous Year Question
Q. Which of the following regions is known as the “Doldrums”?
(a) Around 30° N and S
(b) Around the equator
(c) Around 60° N and S
(d) Around the poles
Answer: (b) Around the equator
Related Keywords
Pressure Belts of the Earth, Equatorial Low Pressure, Subtropical High Pressure, Horse Latitudes, Doldrums, Global Wind System, UPSC Geography Notes, Climatology UPSC, Physical Geography NCERT Notes.
Sources: NCERT Class XI – Fundamentals of Physical Geography, Goh Cheng Leong (Certificate Physical and Human Geography), IMD & NOAA Climate Data (simplified for UPSC learners).
Next Topic → Planetary Winds and Air Circulation (UPSC Geography Notes)
