Global Wind Patterns
Caused by uneven heating
The circulations of the atmosphere
(winds) and oceans (currents) are
driven by sunlight hitting the earth’s
surface more directly near the
equator
a LOT of this sunlight energy hits
the tropics, between 23.50 North
and South of the equator.
InterTropical Convergence Zone
ITCZ
the “doldrums”
If Earth did not rotate, wind patterns would
be simply N to South along the surface
• Thermal
convection leads
to formation of
convection cell in
each hemisphere
• Heat Energy is
transported from
the tropics toward
poles in upper
atmosphere
• Cold air sinks and
shuttles back
toward equator
along surface
Coriolis effect
But the earth rotates, so there is a tendency
for objects moving over long distance (like
air masses) to curve due to “coriolis effect”
• Curve to right in N. Hemisphere
• Curve to left in S. Hemisphere
• This leads to winds generally blowing in
east/west directions and having spin to
them
CORIOLIS EFFECT
• THE ROTATION OF THE EARTH CAUSES
THE AIR TO MOVE AT ANGLES FROM THE
EQUATOR TO THE POLES AND BACK.
• http://uwf.edu/atc/projects/coriolis/main.swf
• http://www.classzone.com/books/earth_scienc
e/terc/content/visualizations/es1904/es1904p
age01.cfm?chapter_no=visualization
See coriolis effect as high pressure wind blows
into a low pressure air mass
• Wind blowing in curve to the right in N. hemisphere
• Thus, low pressure system has air spinning counterclockwise
• This pattern creates tornadoes and hurricanes as well
Heat Exchange in air masses
Warm, humid air rises, dry clear air sinks
Creates 3 huge convection/circulation cells in each hemisphere
Prevailing winds are created by coriolis effect
curving their paths
Add these prevailing winds
to your globe from north to
south:
Polar Northeasterlies
Westerlies
Northeasterly Tradewinds
Doldrums
Southeasterly Tradewinds
Westerlies
Polar Southeasterlies
Wind
• Wind blows from high pressure air masses to low pressure
• Winds named westerly wind IF blowing from the West
• Wind spead increases as pressure gradient increases (steeper
isobars)
Low pressure areas
•Warm, humid air rises, expands and condenses =
causing a low pres air mass at surface and
condensation above (lots of rain or snow)
•Equator and polar front, 00 and 600 N and S.
High Pressure Areas
• Dense, dry air (can by cold dense or hot
arid) sinks due to gravity = creating high
pressure at surface and outward blowing
wind, very little precipiation
• 30° N and S; 90° N and S at poles
dry
wet
dry
wet
dry
wet
dry
Notice location of major deserts
near 30 Latitude
Ferrel
cell
Hadley
Cells
Key features of three cell model
• Hadley cell takes heated air away from the tropics
- TROPICS: low pressure air rises near equator MUCH RAIN
-DESERT ZONE: condensed, high pressure air
descends near 30 degrees - clear skies!
• Ferrel Cell - driven by heat transports of eddies
- Polar air rises near 60 degrees and descends near 30
degrees
- explains surface westerlies from 30-60
• Weak (WUSSIE-BABIES) winds found near
– Equator (doldrums)
– 30 degrees (horse latitudes)
• Boundary between cold polar air and mid-latitude
warmer air is the polar front
1.
Understanding
the General Circulation
Unequal solar heating between the equator and
poles. Atmospheric general circulation moves heat
toward the north and south poles..
2. In a Hadley cell, warmer air rises and moves
poleward. Equator-to-pole Hadley cell is impossible.
The Earth rotates!
3. In the Northern Hemisphere, air is pushed to the right
as it moves; in the Southern Hemisphere, air is
deflected toward the left. The Coriolis effect.
- rotation produces trade winds; surface westerlies in
NH; upper tropospheric jet streams.
4. Ferrel cell runs backwards! Transports heat the wrong
way, towards the equator!
5. The Ferrel cell is weak, so does not move as much
energy as the other cells.
Global Wind and Pressure Patterns
Deserts: belts of persistent high
atmospheric pressure centered
at about lat. 30º N and S
Intertropical
Convergence
Zone (ITCZ):
Where
convergence of
air masses
along the
equatorial
trough
Polar Front: front lying between cold polar
air masses and warm tropical air masses
Hadley Cell: lowlatitude atmospheric
circulation cell with
rising air over the
equatorial trough and
sinking air over the
subtropical highpressure belts
Thermal Wind Balance
Produces Jet Streams
• Hot tropics and cold poles produce
westerly high altitude jet streams at
middle latitudes like ours.