The general circulation of the atmosphere refers to the global-scale patterns of atmospheric motion driven by the unequal heating of the Earth's surface. These patterns give rise to the planetary winds, which are the dominant winds found at different latitudes.
The general circulation consists of six major wind belts that encircle the Earth:
1. Equatorial Low-Pressure Belt (Doldrums): Near the equator, intense solar radiation heats the air, causing it to rise and create a region of low pressure. This results in the formation of the Inter-Tropical Convergence Zone (ITCZ) or the Doldrums. The air rises and expands, leading to the development of cumulonimbus clouds and frequent thunderstorms. The air then flows poleward in the upper atmosphere.
2. Trade Winds: The air that rises in the equatorial region diverges in the upper atmosphere and flows towards the poles. As the air moves away from the equator, it gradually descends around 30 degrees latitude in both hemispheres. Due to the Coriolis effect caused by the Earth's rotation, the air is deflected to the west, resulting in the formation of the trade winds. The trade winds blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. They are relatively steady and persistent, with speeds averaging between 10 and 20 knots.
3. Subtropical High-Pressure Belt: The descending air around 30 degrees latitude creates a region of high pressure known as the subtropical high-pressure belt. In the Northern Hemisphere, this is often referred to as the Bermuda-Azores High, while in the Southern Hemisphere, it is known as the South Atlantic or South Pacific High. The subtropical high-pressure belt is generally associated with stable weather conditions, clear skies, and relatively light winds.
4. Westerlies: Poleward of the subtropical high-pressure belts, around 30 to 60 degrees latitude, the air from the subtropics meets the polar air masses. The interaction between these air masses creates the prevailing westerly winds, also known as the mid-latitude westerlies. In the Northern Hemisphere, these winds blow from the southwest, while in the Southern Hemisphere, they blow from the northwest. The westerlies are strong, particularly in the Southern Hemisphere, and are responsible for the movement of weather systems, including mid-latitude cyclones.
5. Subpolar Low-Pressure Belt: Around 60 degrees latitude in both hemispheres, the westerlies encounter the cold polar air masses. The warm air from the mid-latitudes rises over the colder air, creating a zone of low pressure known as the subpolar low-pressure belt. This region is characterized by stormy weather and is a frequent site of the development of mid-latitude cyclones.
6. Polar Easterlies: Poleward of the subpolar low-pressure belts, near the poles, the cold air descends and flows towards the lower latitudes. These winds are known as the polar easterlies. In the Northern Hemisphere, they blow from the northeast, while in the Southern Hemisphere, they blow from the southeast. The polar easterlies are weak and variable, influenced by the proximity of the polar ice caps.
It's important to note that the general circulation of the atmosphere is not perfectly uniform or symmetrical due to various factors, such as land-sea distribution, mountain ranges, and ocean currents. Additionally, the positions of the wind belts can shift slightly throughout the year in response to seasonal changes in solar radiation.
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