tricellular model

  the basis of the modern understanding of the general circulation of the atmosphere as it relates to the troposphere. Air rises at the equator due to convection and is forced to move to higher latitudes when it reaches the tropopause. Having cooled it sinks over the tropics and drags down air adjacent to it. On reaching the surface, the air is again forced to split; some returning to the equator and some moving to the mid-latitudes. This warmed air meets cold polar air at about 60-of latitude and is forced to rise, again splitting at the tropopause; some returning to the tropics and some moving over the poles. This circulation forms three cells known as the Hadley, Ferrel and Polar respectively.