1. Lecture 5: General Circulation of the Atmosphere
EarthsClimate_Web_Chapter.pdf, p
Questions?
1. Why do winds blow around a high or a low pressure center?
Why does a low area mean more precipitation?
Why does a high mean a dry area?

2. 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
However, the tilt of the
Earth means this balance
is not maintained for each
latitude
SURPLUS
DEFICIT

3. 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

4. Global Wind Patterns
Westerlies
Polar easterlies
Hadley Cell

5. 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.

6. 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.

7. 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.

8. 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

9. 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.

10. Cyclones and Anticyclones
Three forces:
PGF
Coriolis
Frictional

11. 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.

12. 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

13. Coriolis Force and Nature’s Greatest Storms

14. Hurricane Movement Formation
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.

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