Atmospheric Pressure: Distribution and Pressure Belts
Understanding atmospheric pressure is fundamental to grasping global weather patterns and climate. This section delves into how pressure is distributed across the Earth's surface and the formation of distinct pressure belts.
What is Atmospheric Pressure?
Atmospheric pressure is the weight of the air column pressing down on a unit area.
Imagine a tall column of air extending from the Earth's surface to the edge of space. The total weight of this air column, distributed over the area it covers, is what we call atmospheric pressure. It's typically measured in millibars (mb) or hectopascals (hPa).
Atmospheric pressure is defined as the force exerted by the weight of the atmosphere per unit area. This force is a result of gravity pulling the air molecules towards the Earth's center. While air is light, the sheer volume of it in the atmosphere creates a measurable pressure. At sea level, the average atmospheric pressure is approximately 1013.25 millibars (mb) or 1013.25 hectopascals (hPa), which is equivalent to about 14.7 pounds per square inch.
Gravity, which pulls air molecules towards the Earth's center.
Factors Affecting Atmospheric Pressure
Several factors influence the distribution of atmospheric pressure across the globe. The most significant ones are temperature and altitude.
| Factor | Effect on Pressure | Explanation |
|---|---|---|
| Temperature | Inverse Relationship | Warm air is less dense and rises, creating lower pressure at the surface. Cold air is denser and sinks, creating higher pressure at the surface. |
| Altitude | Inverse Relationship | As altitude increases, the density of air decreases, meaning there are fewer air molecules above. Therefore, atmospheric pressure decreases with increasing altitude. |
Think of pressure like a stack of books. The more books you have (denser air, lower altitude), the more pressure at the bottom. Fewer books (less dense air, higher altitude) mean less pressure.
Global Pressure Belts
The differential heating of the Earth's surface by the sun leads to the creation of distinct zones of high and low pressure, known as global pressure belts. These belts are not static and shift seasonally.
The Earth's surface is divided into several major pressure belts due to the uneven distribution of solar radiation and the resulting temperature differences. These belts are characterized by rising or sinking air masses, which dictate whether they are high or low-pressure zones. The primary belts include the Equatorial Low, Subtropical Highs, Subpolar Lows, and Polar Highs. The movement of air between these belts drives global wind systems.
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1. Equatorial Low-Pressure Belt (Doldrums)
Located around the equator (0° to 10° N and S latitude), this belt is characterized by intense solar heating. Warm, moist air rises here, leading to convectional rainfall and generally calm winds. This zone is also known as the 'Doldrums' due to the frequent lack of significant wind.
2. Subtropical High-Pressure Belts
Found around 30° to 35° N and S latitude, these belts are formed by the descending air from the upper atmosphere (Hadley Cell). As air sinks, it warms and dries, leading to arid conditions and the formation of major deserts. Winds are generally light and variable in these regions.
3. Subpolar Low-Pressure Belts (Subarctic and Subantarctic Lows)
Located around 60° to 65° N and S latitude, these belts are characterized by rising air. As the cold, dense air from the polar regions meets the warmer air from the tropics, it is forced upwards. These regions experience stormy weather and are associated with the Westerlies.
4. Polar High-Pressure Belts
Found at the North and South Poles (90° N and S latitude), these are zones of permanent high pressure. The extremely cold temperatures cause the air to be very dense and sink. These regions are characterized by very cold, dry, and stable conditions.
The Subtropical High-Pressure Belts.
Seasonal Shift of Pressure Belts
Due to the Earth's axial tilt and its revolution around the sun, the direct rays of the sun shift between the Tropic of Cancer and the Tropic of Capricorn. This causes the global pressure belts to migrate northwards in July and southwards in January. This seasonal shift significantly influences monsoon patterns and regional climates.
The seasonal migration of pressure belts is a key driver of the Indian Monsoon system, demonstrating the practical impact of these global patterns.
Learning Resources
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