Global atmospheric circulation is divided into three circulation cells by the latitudinal distribution of solar radiation and the Coriolis force. These cells are the source of prevailing winds.
Basic Knowledge
Near the equator, solar radiation is strong, and heated air rises. It moves toward higher latitudes aloft, descends near latitude 30°, and returns to the surface in a large circulation called the Hadley cell. At the surface of the Hadley cell, winds deflected by the Coriolis force are the trade winds (Northern Hemisphere: northeast trade winds; Southern Hemisphere: southeast trade winds).
The area near latitude 30° is a zone of descending air and high pressure (the subtropical high-pressure belt). The area near latitude 60° is a zone of rising air (the subpolar low-pressure belt), and the Ferrel cell (surface: westerlies) forms between 30° and 60°. From 60° to the poles, the polar cell (surface: polar easterlies) exists.
The westerlies (latitudes 30–60°) are the main belt of prevailing west winds and are closely connected with mid-latitude cyclones, fronts, and changes in Japan's weather.
Hadley cell (equator–30° latitude; rising at the equator and sinking at 30°; surface = trade winds)
Ferrel cell (30–60° latitude; rising at 60° and sinking at 30°; surface = westerlies)
Polar cell (60–90° latitude; rising at 60° and sinking at the poles; surface = polar easterlies)
Trade winds (northeast and southeast winds blowing toward the equator; surface part of the Hadley cell)
Westerlies (prevailing west winds in the mid-latitudes; move mid-latitude cyclones eastward)
Intertropical Convergence Zone (ITCZ) (low-pressure belt where the northern and southern trade winds converge and rise; shifts seasonally)
Further Study (Background and Significance)
The Ferrel cell is thermodynamically an indirect circulation (a circulation that receives energy) and is maintained between the Hadley and polar cells (direct circulations). The Ferrel cell is a theoretical concept; in reality, it forms on average through random disturbances caused by mid-latitude cyclones and anticyclones.
The subtropical jet stream (altitude 10–15 km, near latitude 30°, speed 200–300 km/h) is a belt of strong winds that blows where the north–south temperature difference is greatest and also affects aircraft flight times. It shifts northward in summer and southward in winter.
Because the Arctic warms strongly through Arctic amplification, the north–south temperature difference decreases, and the westerlies meander. This is thought to increase extreme weather, including heat waves, cold waves, and prolonged rain.
- Three cells: Hadley (0–30°) / Ferrel (30–60°) / polar (60–90°)
- Surface of the Hadley cell = trade winds (northeast and southeast)
- Surface of the Ferrel cell = westerlies (west → east)
- Surface of the polar cell = polar easterlies
- Subtropical high-pressure belt (30°) = clear weather → concentration of deserts
- Subpolar low-pressure belt (60°) = much rain and cloud
- Meandering westerlies can cause extreme weather
Points of Caution (Often Confused)
① Do not confuse the locations of rising and sinking air in the three cells (Hadley: rising at the equator and sinking at 30°; Ferrel: rising at 60° and sinking at 30°). ② Trade winds blow from east to west (Northern Hemisphere: northeast → southwest). ③ Westerlies blow from west to east, which is why ordinary weather changes arrive from the west. ④ The Ferrel cell is an indirect circulation and is actually an accumulation of disturbances.
Practice
- Give the names of the three cells in order from low to high latitudes.
- At approximately what latitude is the subtropical high-pressure belt distributed?
- In which direction do the westerlies blow in the Northern Hemisphere? Answer east or west.