Hadley, Ferrel and polar cells; subtropical high কী?

The mean meridional circulation of each hemisphere has three cells. The Hadley cell rises near the equator, flows poleward aloft, sinks near $30{}^{\circ}$ and returns towards the equator at the surface. The Ferrel cell, between about $30{}^{\circ}$ and $60{}^{\circ}$, turns the opposite way: sinking near $30{}^{\circ}$ and rising near $60{}^{\circ}$, its surface flow poleward. The polar cell, the smallest and weakest, rises near $60{}^{\circ}$, sinks over the pole and returns towards $60{}^{\circ}$ at the surface. The subtropical highs are the belts of high sea-level pressure near $30{}^{\circ}$ N and S, under the sinking branches of the Hadley cells.