Transcript chapter 4a

Chapter 4
Atmospheric and Oceanic Circulation
Atmospheric & Oceanic Circulation
Major things you need to know:
• What causes wind to happen
• Global pressure areas and wind
patterns, and explanations for them
• What causes ocean currents
• Global ocean current geography
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
Forces Creating and Affecting
Air Movement
•
•
•
•
Pressure Gradient Force
Coriolis Force
Friction Force
Gravity
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.
Pressure Gradient
Fig. 4.7
The Coriolis Effect
• Explanation: earth rotation “deflects”
movement to the right in the N. Hemisphere, to
the left in S. Hem.
• Effect is greater at higher latitudes
• Friction reduces wind velocity, thus reduces
Coriolis Effect
• In upper atmosphere with negligible friction,
wind flows nearly parallel to isobars
(geostrophic wind)
• Merry-go-round analogy
Coriolis Force
Figure 4.9
Pressure + Coriolis + Friction
Figure 4.8
Global High Pressure and Low
Pressure Areas - Why they Develop:
1.
2.
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: http://www.geography.hunter.cuny.edu/tbw/wc.notes/7.circ.atm/animations/GlobalWind.html
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
But the Coriolis Effect will not allow
this model to work
• 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…
This is your model to learn - worth at least 15 pts on exam:
Barometers
Figure 4.2