Hadley cell theory (Held-Hou 1980): Rising air at equator, poleward flow aloft conserves angular momentum → u = Ω a sin²φ / cosφ → subtropical jet. Hadley cell extent limited by baroclinic instability: φ_H ≈ (5 ΔH gH / 3 Ω²a²)^(1/2) where ΔH is equator-pole temperature contrast. Beyond φ_H, eddies drive the Ferrel cell (thermally indirect — cold air rises in midlatitudes). The polar cell closes the circulation. Trade winds (NE/SE) result from Coriolis deflection of Hadley return flow. Wider cells → stronger jet streams.