1. Chapter 4 Atmospheric and Oceanic Circulation

2. Air Pressure: the weight of a column of air at a location
Air pressure decreases with altitude, as there is less “weight” of air above

3. Barometers
Figure 4.2

4. Forces Creating and Affecting Air Movement
Pressure Gradient Force  
Coriolis Force  
Friction Force  
Gravity

5. Primary Cause of Wind: difference in pressure between two places (pressure gradient). Closer spacing of isobars indicates stronger presssure gradient.
one example of how pressure gradients are created:
At the surface, cold air, located next to warm air, is denser and has higher pressure.

6. Pressure Gradient
Fig. 4.7

7. The Coriolis Effect
Explanation: earth rotation “deflects” movement to the right in the N. Hemisphere, to the left in S. Hem. and
Effect is greater at higher latitudes
Friction reduces wind velocity, reduces Coriolis Effect
In upper atmosphere with negligible friction, wind flows nearly parallel to isobars (geostrophic wind)
Merry-go-round analogy

8. Coriolis Force
Figure 4.9

9. Pressure + Coriolis + Friction
Figure 4.8

10. Global High Pressure and Low Pressure Areas - Why they Develop:
Thermal (heating or cooling)
Great amounts of solar radiation promote heating, low pressure, and convergence (example: circum-equatorial area of low pressure known as “Inter -Tropical Convergence Zone” or ITCZ, or “Equatorial Low”)
Minimal or no solar radiation promotes cooling, high pressure, and divergence (Polar Highs)
Dynamic (mechanical forcing) results from movements of air masses (result in Subtropical Highs, Sub Polar Lows)
animation:

11. The Basic Oversimplified Model of Global Wind, Ignoring the Coriolis Effect:
The hot Equator and the cold Poles would create Equatorial low pressure (thermal low), Polar Highs (thermal high)
Surface winds would flow from poles to equator
Rising air at the Equator would flow to the north & south in the atmosphere and descend at the poles…BUT

12. But the Coriolis Effect will not allow this model to work on Earth
Winds from polar areas are deflected, becoming Easterlies rather than remaining northerlies – thus they never make it to the Equator to replace the warm, rising air there.
Therefore, a more complex model is needed…

13. This is your model to learn - worth at least 15 pts on exam: