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Three primary circulation cells organize atmospheric winds: the tropical Hadley cell (equator to 30° lat, driven by differential solar heating), the mid-latitude Ferrel cell (30° to 60°, driven by baroclinic eddies), and the polar cell (60° to pole, driven by pole-equator temperature gradient). The Coriolis effect deflects these flows, creating trade winds, mid-latitude westerlies, and polar easterlies.
You already know from studying the Hadley cell that the tropics receive more solar energy than the poles, creating a temperature gradient that drives atmospheric circulation. You also know that the Coriolis effect deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The three-cell model extends these ideas to explain the full pattern of surface winds across the planet — why the tropics have steady easterly trade winds, mid-latitudes have prevailing westerlies, and polar regions have weak easterlies.
The Hadley cell is the most straightforward. Intense heating near the equator causes air to rise vigorously at the Intertropical Convergence Zone (ITCZ), creating a belt of low pressure, clouds, and rain. This air flows poleward at high altitude, but the Coriolis effect progressively deflects it eastward. By about 30° latitude, the upper-level flow has turned nearly parallel to the latitude lines and can no longer continue poleward efficiently. It piles up, sinks, and compresses — creating the subtropical high-pressure belts that produce the world's great deserts (Sahara, Arabian, Sonoran). The descending air splits: some flows back toward the equator as the surface trade winds (deflected westward by Coriolis, so they blow from the northeast in the Northern Hemisphere), completing the Hadley cell. The rest flows poleward, forming the mid-latitude surface winds.
The Ferrel cell occupying roughly 30°–60° latitude is fundamentally different from the Hadley cell. It is not a simple thermally driven convection loop. Instead, it is maintained by the churning of mid-latitude weather systems — the extratropical cyclones and anticyclones (baroclinic eddies) that transport heat poleward through their chaotic swirling. The net effect of these eddies produces surface winds that blow generally from the southwest in the Northern Hemisphere — the prevailing westerlies. At the boundary between the Ferrel and Hadley cells (~30°), air descends; at the boundary between the Ferrel and polar cells (~60°), air rises along the polar front, where cold polar air meets warmer mid-latitude air. This convergence zone is where most mid-latitude storms develop.
The polar cell is the simplest and weakest. Cold, dense air sinks over the poles, flows equatorward along the surface, and is deflected by Coriolis into the polar easterlies. When this cold polar air meets the warmer westerlies near 60° latitude, the contrast generates the polar front and its associated jet stream. The boundaries between cells are not rigid walls — they shift seasonally as the sun's direct rays migrate between the Tropics of Cancer and Capricorn. In summer, the Hadley cell expands poleward, pushing subtropical highs and dry conditions into higher latitudes. In winter, it contracts, and the polar front dips equatorward, bringing storm tracks to lower latitudes. This seasonal migration explains much of the world's climate seasonality beyond simple temperature changes.