Atmospheric Circulation

Six Cell Circulation Model

	Hadley Cells Warm air rises at equator, where it expands, cools, water vapor condences to form clouds and rain, and forms a low pressure system (aka the ITCZ). Goes across the upper atmosphere to 30° N/S. Meets air in Ferrel Cell moving towards 30° N/S. Air sinks at 30° N/S. As it does the air compresses, heats up and dries out, forming a high pressure system called the horse latitudes. Goes across the surface of the earth towards the equator (trade winds). Meets air in other Hadley Cell moving towards the equator, where the . . .

	Ferrel Cells Air sinks at 30° N/S. As it does the air compresses, heats up and dries out, forming a high pressure system called the horse latitudes. Goes across the surface of the earth towards 30° N/S (westerlies) Meets air in Polar Cell moving towards 60° N/S Warm air rises at 60° N/S, where it expands, cools, water vapor condences to form clouds and rain, and forms a low pressure system Goes across the upper atmosphere towards 30° N/S Meets air in Ferrel Cell moving towards 30° N/S, where the . . .

	Polar Cells Warm air rises at 60° N/S, where it expands, cools, water vapor condences to form clouds and rain, and forms a low pressure system Goes across the upper atmosphere to the poles Meets air in the other Polar Cell moving towards the poles Air sinks at the poles. As it does the air compresses, heats up and dries out, forming a high pressure system called the horse latitudes. Goes across the surface of the earth towards 60° N/S (polar easterlies) Meets air in Ferrel Cell moving towards 60° N/S, where the . . .

Below is a graphic showing a cross section of the circulation cells. Note how low pressure systems with rain form where are is rising, and high pressure systems with dry weather form where air is descending. Also, jet streams (upper altitude winds) form at the boundary between each cell. These jet streams help to move around air masses and storms.

Pressure Systems

As the air in each of these circulation cells moves towards 0°, 30° N/S, 60° N/S and 90° N/S, it starts to move up or down before reaching those latitudes. The air is actually not moving (or, at least, not due to the circulation cells) at each of those latitudes. Now, imagine you are on a wind-powered boat like all of the explorers were on during the 16th through 19th centuries. You and your crew are sailing along just fine . . . until the wind stops. This can either be a temporary setback or a complete disaster depending upon which latitude you've stalled at.

Doldrums
"Doldrums" is a term that was coined by sailors. It refers to the calm equatorial waters where two Hadley cells meet. Getting becalmed in this area was an annoyance, but not necessarily a deadly one due to the frequent rains found in this area. This region is now also called intertropical convergence zone (ITCZ)

Horse Latitudes
The Horse Latitudes occur at 30º latitude where arid air is descending, heating up and drying out. Evaporation is high and precipitation low in this region, which is a problem if you are in a wind-driven boat in an area where there is no wind. Sailors called this area the "Horse Latitudes" because ships would be becalmed for weeks. Competition for food and water would become an issue, and horses (and people) would start to die. So this is the latitude where the horses would be tossed overboard. :o(

Subpolar Low
Like the doldrums, this region is inbetween two areas of rising air which generate a low pressure system and lots of rain. Few sailors ventured into these areas during the age of exploration, so there is no colorful term to describe this latitude.

Wind Belts

As the air in each circulation cell moves across the surface of the earth, we feel that as winds. In each cell there is a permanent wind belt in which the winds are constantly blowing.

Trades – 15ºN & 15ºS latitude

Westerlies – 45ºN & 45ºS latitude

Polar Easterlies – 75ºN & 75ºS latitude