What causes circulation?
•Sun heats earth so equator is hot and
poles are cold
•This creates a convection cell
•Low pressure and warm temperatures at
the equator cause air to rise.
•High pressure and cold temperatures at the
poles cause air to sink.
However, it is not that simple.....
•
This general convection cell is split
up into three different smaller cells:
1) The Hadley Cell
2) The Ferrel Cell
3) The Polar Cell
Under idealized conditions
these three cells are found in
both hemispheres providing
mirror images of themselves
The Hadley Cell
Involves the most convection and therefore the most vertical
movement of air
Drives convection with the equator as a strong heat source
Warm air is picked up from equatorial low pressure regions and set
down in high pressure regions called horse or subtropical latitudes
(30˚north and south).
• The air brought down in these regions is
typically dry, which explains the formation of
deserts in south western United States, the
Middle East and the Sahara.
• Surface air, from this affect, flows towards the
equator.
The Polar Cell
Drives convection with the poles as “cold sinks”
Colder and dryer region than Hadley cell, but still enough
temperature difference to drive a convection cell.
Warm air is picked up at latitudes of 60˚ (north or south)
where pressure is low and brought to the cold high
pressure poles.
• Surface air, from this affect, flows towards
the equator
The Ferrel Cell
Acts as an intermediate cell between the polar and Hadley cells.
Variance in high and low pressures in the mid latitudes “zone of
mixing”
No strong heat source or cold sink make it less predictable.
Surface air, from this affect, flows towards the poles.
However, pressure and temperature are not
all that determines circulation......
The spin of the earth due to the coriolis force,
among other factors, causes easterly and
westerly winds.
Trade Winds
Easterlies or polar
easterlies are found
in the polar and
Hadley cells
Surface air in these
regions now moves
east and towards
equator
Anti-trade Winds
Westerlies are found
in the ferrel cell.
Since they move
against trade winds
rough storms occur
in this region often
forming cyclones
Intertropical Convergence Zone
What convection cells actually follow;
not the equator.
Presence of doldrums, not very much
wind in this low pressure region between
the north and south Hadley cells.
Shifts north and south due to earth’s
axial tilt from summer to winter.
This shift can create monsoons resulting
from the drastic changes in pressure
from season to season.
ITCZ in Winter
ITCZ in Summer