Lecture 5: General Circulation of the Atmosphere
EarthsClimate_Web_Chapter.pdf, p. 15-22
Questions?
1.
Why do winds
blow around a
high or a low
pressure
center?
2.
Why does a
low area mean
more
precipitation?
Why does a
high mean a
dry area?
The Global Energy Budget: Driver of Atmospheric Motion
A balance exists between
the incoming solar and
outgoing longwave energy
averaged over the globe
and the year
SURPLUS
DEFICIT
However, the tilt of the
Earth means this balance
is not maintained for each
latitude
• To compensate for this energy
imbalance, winds in the
atmosphere and currents in the
oceans transport cold air and
water toward the equator
• About 1/3 of this transport
occurs from the evaporation of
tropical waters and subsequent
transport into high latitudes,
where it condenses and
releases latent heat
• About 1/3 occurs from the
poleward transport of warm
waters by ocean currents
• The remaining 1/3 occurs from
middle latitude cyclones and
anticyclones
Global Wind Patterns
Polar easterlies
Westerlies
Hadley Cell
Hadley Cell
Westerlies
Polar easterlies
Atmospheric Pressure
The pressure at the
base of this column
of air results from
the weight of the
gasses above.
The earth's
atmosphere has a
greater density of
gases at its base due
to gravity.
Forces & Motion
Air responds
similarly as water to
this force, moving
from higher pressure
to lower pressure.
Pressure forces are
only one influence on
the movement of
atmospheric air.
PGF vs. Cyclonic Winds
Pressure
gradient force
(PGF) winds
acting alone
would head
directly into low
pressure.
Surface
observations of
winds, such as
the cyclonic
flow around
this low, reveal
that PGF winds
are deflected by
other forces.
Playing catch on a merry-go-round
Clockwise
Blue,
Yellow,
Red,
Green
Straight-line,
watch from
above
Curved to the
left, watch by
the green on
board
Coriolis Force
Counterclockwise
Earth's rotation transforms straight line
motion for an outside viewer in space
into curved motion for an viewer on
Earth.
The Coriolis force explains this apparent
curvature of winds to the right due to
rotation.
Its magnitude increases with wind
velocity and earth's latitude.
Cyclones and Anticyclones
Three
forces:
PGF
Coriolis
Frictional
Vertical Air Motion
Winds converging into a low pressure center generate upward
winds that remove the accumulating air molecules.
These updrafts may cause cloud formation.
Likewise, diverging air molecules from a high pressure area are
replenished by downward winds.
Northern Hemisphere Flow
Winds blow
counterclockwise
around low pressure
systems in the
Northern
Hemisphere, but
clockwise around
lows in the Southern
Hemisphere.
Figure 9.27A
Coriolis Force and Nature’s Greatest Storms
Hurricane Movement
Formation
1.
2.
3.
4.
5.
6.
Low-pressure areas
(easterly waves)
Warm, moisture,
unstable area
Ocean surf. temp. >
26–27°C
Low wind shear
Sufficient Coriolis
force (5-10 degrees
away from the
equator)
Strong upper-level
anticyclone.
Their movement is
guided by winds
around
subtropical high
pressure systems.
R. Anthes http://www.ucar.edu/governance/meetings/oct05/followup/meeting_summary.html
R. Anthes http://www.ucar.edu/governance/meetings/oct05/followup/meeting_summary.html
R. Anthes http://www.ucar.edu/governance/meetings/oct05/followup/meeting_summary.html
R. Anthes http://www.ucar.edu/governance/meetings/oct05/followup/meeting_summary.html
Can you answer these questions at the
end of this lecture?
• What causes atmospheric surface pressure gradient and
change?
• What atmospheric forces drive winds?
• Why is cyclonic flow associated with cloudy and rainy weather,
whereas anticyclonic flow is associated with clear and cooler
weather?