Climate, Climate Change
Nuclear Power and the
Alternatives
PHYC 40050 Environmental Physics
Climate, Climate Change
Nuclear Power and the
Alternatives
PHYC 40050
Peter Lynch
Meteorology & Climate Centre
School of Mathematical Sciences
University College Dublin
PHYC 40050 Environmental Physics
Lecture 4
Atmospheric Forces and Wind
The General Circulation
PHYC 40050 Environmental Physics
FORCES THAT MOVE THE AIR
Gravitational force:
 Is directed downward , normal to the
earth
 Equal to the mass of the air times the
gravitational acceleration: mg
Pressure gradient force
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Pressure
gradient
force
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PRESSURE GRADIENT FORCE
 Is a function of the pressure difference and air
density:
F = (1/ρ) ∂p/∂n
where




F = Pressure gradient force per unit mass
ρ = Density of air
p = Pressure
n = Distance normal to isobars
 Newton’s law states:
force=mass times acceleration
 F is force per unit mass, equivalent to acceleration.
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APPARENT FORCES
IN THE ATMOSPHERE
 Coriolis Force
 Centrifugal Force/Centripetal
acceleration
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CORIOLIS
EFFECT
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CORIOLIS EFFECT
 Weather map shows that winds go
around a low pressure area, not across
the isobars.
 This deviation is a result of the Earth's
rotation and has been named the
coriolis effect (coriolis force)
 This force always acts at right angles to
the direction of motion.
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CENTRIFUGAL FORCE
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THE MECHANISM OF GEOSTROPHIC FLOW
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GEOSTROPHIC FLOW
 A balance is reached between the
coriolis force and the pressure
gradient force.
 If there is no friction, this occurs
when the parcel of air is parallel to
the isobars.
 At this point there is no net force on
the air parcel - no acceleration - it
now moves with constant velocity.
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GEOSTROPHIC FLOW …
 Under these idealized conditions the
airflow is said to be in geostrophic
balance.
 Wind generated is called the
geostrophic wind.
 Normally only applies to winds aloft.
 Surface winds are subject to friction.
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Fig. 6-15, p. 172
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GRADIENT WINDS
 Winds around centers of high or low
pressure follow curved paths in order to
stay parallel with the isobars.
 These winds are called gradient winds.
 Low pressure centers are called
cyclones - rotation is counter-clockwise
– the same as the earth.
 Centers of high pressure are called
anti-cyclones.
In southern hemisphere the directions of
flow are reversed.
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Fig. 6-24,
p. 181
SURFACE WINDS
 Friction affects winds only close to the
earth's surface.
 Here we must balance three forces coriolis, pressure gradient and friction.
 Net effect is to induce a net inflow around a
cyclone, an effect known as convergence.
 Around an anticyclone we get a net outflow,
divergence.
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The General Circulation
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HADLEY CELL
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GLOBAL CIRCULATION
 George Hadley first suggested in 1735
the general concept of atmospheric
circulation – a single cell, to explain the
existence of the easterly winds at the
surface
 Cold air at pole - high pressure at
surface. Warm air at equator - low
pressure at surface. Pressure gradient
force at surface will move air from pole
to equator at surface. Return path at
high altitudes.
 Coriolis force produces easterlies
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Composite (clouds, surface temperature (colors))
image. Note the line of clouds along the ITCZ
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Fig. 7.10
Three
cell
model
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GLOBAL CIRCULATION
 But in reality we have three cells, with
boundaries at about 30º and 60º latitude.
 This results in sinking air at 30ºN and 30ºS.
But sinking suppresses cloud development
and precipitation. Hence most of the worlds
deserts occur along these latitudes.
 These are the horse latitudes.
 Circulation at the surface moves from 30º
to the equator – easterly winds.
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GLOBAL CIRCULATION
 Between 30º and 60º the circulation
at the surface is from 30º to 60º,
giving rise to westerly winds.
 Between 60º and the Poles, the flow
is from the Pole to 60º, leading to
easterly winds at the surface.
 Upwelling at the equator and at 60º –
precipitation.
 The Doldrums.
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JET STREAMS
Conservation of Angular Momentum:
 As the parcel of air moves from the equator to 30º at
high altitudes, its velocity increases to conserve
angular momentum.
 At the same time the coriolis force acts to produce a
strong westerly wind – the sub-tropical jet stream
 A similar jet forms at 60º – the polar jet, or polar
front jet
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Cloud band from Pacific to Florida follows Sub-tropical jet
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SUBTROPICAL AND POLAR JET STREAMS IN
RELATION TO THE THREE CELLS
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WESTERLIES
 In the upper troposphere there is high
pressure over the equator, and low pressure
over the poles.
 This produces a net flow from the equator to
the poles.
 This flow plus the coriolis force produces
westerlies.
 Winds are geostrophic
 Pressure gradient increases with altitude.
Thus so does the wind speed
 Jet streams are part of the westerlies
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DISH-PAN EXPERIMENT
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Fig. 7-14, p. 199
500 MB WINDS
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(A) Zonal flow pattern – air flows nearly parallel to
latitudes.
(B) Meridional flow pattern.
(C) Combination of the two flows.
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Fig. 7-16, p. 200
WAVES IN THE WESTERLIES
 Dish pan experiment.
 C. G. Rossby.
 Waves along the jet streams are known
as Rossby waves.
 Three to six of them around the globe.
 The air flow along the edge of the
waves can be rapid, however the waves
move slowly – about 15º per day.
 Higher jet stream speeds in the winter.
 Jets shifts south in the winter, north in
the summer.
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WESTERLIES AND THE HEAT BUDGET
 Major function of atmospheric dynamics is
to move heat from the equator to the poles.
 But how can winds move heat when the
predominate wind direction is zonal (E to W,
or W to E)?
 The meanderings of the jet streams
continually mix cold and warm air, thus
transporting heat.
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POLEWARD TRANSPORT OF HEAT BY THE
OCEANS AND ATMOSPHERE
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Mean position of the ITCZ
in January and July
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THE SW MONSOON
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PRECIPITATION PATTERNS AND
TOPOGRAPHY
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End of Lecture 4
PHYC 40050 Environmental Physics