Equatorial Low-Pressure Belt (Doldrums):
This belt lies between 10°N and 10°S, where trade winds from the subtropical high-pressure belts converge. The region is known for calm winds, hence called the Doldrums. Its position shifts slightly due to the apparent movement of the sun. The equator receives maximum solar radiation, warming the air which rises, creating a low-pressure zone. Vertical air currents dominate here, with winds rising near the margins. The abundant moisture from surrounding oceans leads to humid conditions, but cyclones do not form at the equator due to the lack of Coriolis force.
Subtropical High-Pressure Belt (Horse Latitudes):
Found around 30°N and 30°S, these belts form as dry, cold air descending from the upper troposphere after losing moisture in the Intertropical Convergence Zone (ITCZ). This subsiding air creates high-pressure zones and calm, anticyclonic conditions. Deserts are common here due to warm, dry air. The belt is called Horse Latitude because historical sailing ships carrying horses struggled in its calm conditions, sometimes discarding livestock when supplies ran out.
Subpolar Low-Pressure Belt:
Located between 45°N/S and the Arctic/Antarctic Circles (66.5°N/S), these belts are zones of rising air formed by the convergence of westerlies and polar easterlies. In the northern hemisphere, they include the Icelandic Low and Aleutian Depression. In the southern hemisphere, they encircle Antarctica. Polar jet streams form in these regions due to temperature contrasts between cold polar air and warmer mid-latitude air.
Polar High-Pressure Belts:
Situated between 80° and 90° latitudes in both hemispheres, these regions experience descending cold, dry air, forming high-pressure zones. Air flows from the poles toward the subpolar low-pressure areas as polar easterlies.
General Circulation of the Atmosphere:
The global movement of winds, influenced by pressure gradients caused by uneven heating of Earth, is called the general circulation. Winds are classified into:
- Primary circulation: Planetary winds formed by global pressure belts (trade winds, westerlies, polar easterlies).
- Secondary circulation: Cyclones, anticyclones, and monsoons.
- Tertiary circulation: Local winds influenced by topography and sea-land interactions.
Planetary Winds (Primary Circulation):
These winds blow consistently from high-pressure to low-pressure areas. Trade winds converge at the ITCZ (Doldrums), rise to the upper troposphere, move polewards, and descend at 30°N/S forming subtropical highs. Air moving toward the equator forms easterlies, while mid-latitude westerlies arise from sinking cold air from poles meeting rising subtropical air. Polar easterlies flow from poles to subpolar lows.
Hadley Cell (Tropical Cells):
Proposed by George Hadley (1735), this model describes convection cells in each hemisphere driven by equatorial heating. Warm air rises at the equator, releasing latent heat via cumulus clouds, and moves polewards in the upper troposphere, descending at 30°N/S to form subtropical highs.
Ferrel Cell (Polar Front Cells):
Located between 30° and 60° latitudes, these cells exhibit westerlies influenced by the Coriolis effect. Winds are variable due to migratory cyclones, aiding heat distribution in mid-latitudes.
Polar or Subpolar Cells:
Between 60° and 90° latitudes, air descends from the upper troposphere, forming high-pressure zones. Cold polar easterlies move toward subpolar lows, converging with warmer westerlies to create the Polar Front. Upper troposphere winds flow polewards from 60° latitude, maintaining heat balance.