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Wind
•  Wind is the motion of the fluid we call the
atmosphere. To understand why this (and all other
fluids) move we must consider the FORCES that
act on the fluid.
•  There are several, but not all of them are
necessarily acting at a given time.
•  The major forces acting on the atmosphere are:
–  gravity
–  pressure gradient force
–  Friction
–  Coriolis Force (not a real force, but more on that later)
Force as a vector
•  Two things to remember about a force:
•  forces have a magnitude (how “strong” is
it?) and
•  forces have a direction (what direction does
it work in?)
•  We can represent this pictorially with an
arrow, with the length the magnitude
Weaker force
Stronger force
Pressure Gradient force
•  DEFINE PRESSURE
GRADIENT: a change in
pressure over a distance.
•  Any time a pressure gradient
exists in a fluid, there is a
pressure gradient force acting
toward the lower pressure:
The pressure gradient force
is the main cause of
atmospheric motion !!!
Just what is a force anyway???
•  DEFINE FORCE: some agent that causes an
object at rest to move, or alters the movement of
an object already in motion.
•  Examples:
•  gravity (causes motion towards center of earth)
•  magnetic (attracts iron objects toward magnet)
•  impulsive force (e.g. blow w/ a baseball bat,
explosion,... impulsive forces are often very
strong, but applied over a short amount of time)
Vectors
•  You are already familiar with other vector quantities like
velocity:
•  velocity magnitude is called “speed” (how fast?)
•  A more correct definition of FORCE might be to say that a
FORCE changes the DIRECTION and/or MAGNITUDE
of a body’s VELOCITY
•  NOTE: 1) If a body is moving at a constant velocity (no
change in speed or direction, i.e.,straight line), NO
FORCES ARE ACTING ON IT!
•  2) FORCES ADD UP! Two forces of the same magnitude
but opposite direction cancel each other out
Pressure gradient force (cont.)
•  Remember, nature usually acts in a way that
tries to eliminate a gradient!
•  An everyday example happens when you fill a
bathtub. Not all the fluid stays at one end...right?
Nature distributes the fluid in such a way that it
eliminates the pressure gradient. (it makes a flat
top surface... right?)
•  But... what about the vertical pressure gradient in
the atmosphere?
•  After all, pressure decreases with height, right?
•  Why doesn’t nature eliminate this gradient?
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Hydrostatic balance
•  in the vertical,
another force
(almost) perfectly
balances the
pressure gradient,
resulting in:
hydrostatic balance
Coriolis Force
500 mb
Fp
G
1000 mb
•  Most of the time, we are interested in horizontal,
rather than vertical winds. Also we are interested
in the horizontal pressure gradient force (PH)
Here gravity does not enter. (why...?)
•  The other important “force” for horizontal motion
is the coriolis force, which is not really a true
force, but kind of acts like one.
•  The coriolis force is simply a result of the fact
that our coordinate system (the latitude/longitude
pattern on Earth) is moving (rotating).
WHAT IS THIS FORCE ACTING IN THE VERTICAL?!!
Coriolis Force (cont.)
•  DEFINE CORIOLIS FORCE (Fc):
•  An apparent horizontal force, resulting from the
rotation of the planet, that always acts
perpendicular to the motion (velocity vector).
•  The magnitude of the coriolis force is proportional
to the speed (magnitude of the velocity vector).
Faster-moving objects (parcels) “feel” a stronger
coriolis force
Coriolis Facts
Fc acts
to the right in Northern Hemisphere
to the left in Southern Hemisphere
•  Fc never changes the magnitude of the velocity vector,
just the direction it points.
•  Fc is strongest at the poles and vanishes at the equator
•  Fc is a weak force but acts constantly on all moving
objects on the Earth
•  SO... it takes Fc a long time (several hours) to change the
flow. AND...
•  Fc is most important in processes with big time scales
(hours to days) and big space scales (100 km and up).
Figure 9.20
now we have the conceptual tools
to understand the winds!
•  DEFINE GEOSTROPHIC BALANCE: a balance
between the PH (hor. pressure gradient force) and the
coriolis force, Fc. The resulting so-called geostrophic wind
1) does not change speed and 2) goes in a straight line (has
no curvature).
•  GEOSTROPHIC FLOW has the wind blowing parallel to
isobars (lines of constant pressure)
•  1) most straight-line large scale flow in the atmosphere is
nearly geostrophic.
•  2) geostrophic balance assumes no frictional forces.
•  This is OK well above the surface, but becomes less true as
you get closer to the ground
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Curved flow
At position 1), the parcel starts to accelerate down the pressure
gradient (purple vectors are wind velocity)
•  The main problem with geostrophic wind is
that it assumes straight line flow, and the is
NOT how the atmosphere usually works,
especially with big storm systems (low-pressure
systems).
•  NO PROBLEM!
•  Let’s just adjust our geostrophic wind results to
make curved flow.
•  Remember from the example of geostrophic
adjustment that to make the flow curve, we need
to apply a net force perpendicular to the velocity
At position 5), the parcel is in geostrophic flow (balance)
Here the purple arrows
represent the instantaneous
velocity of a ball on a string.
The heavy black arrows
represent the force always
towards the center of the circle
that the string must exert on the
ball to keep it going in a circle.
This force, always
perpendicular to the motion, is
called the centripetal force, in
this case caused by the tension
of the string. A similar force is
required to make air parcels
curve
Remember that CF (Coriolis Force) always acts to towards
the right of the flow.
So to have the flow curve to the RIGHT, we need CF
(coriolis force) to be a bit stronger than PGF (pressure
gradient force)
This slight imbalance gives us
the anticyclonic (clockwise)
flow we observe around highs
Notice that to make CF
stronger, we need to increase
the velocity from geostrophic
balance, which makes
anticyclonic flow
supergeostrophic in speed
Centripetal force
•  We have all the forces necessary to create a
centripetal force in the atmosphere, we just need
to not have the Pressure Gradient Force (PH) and
the Coriolis Force (Fc) not quite cancel each other
out, as they did in the case of geostrophic flow.
•  This slight imbalance between these 2 forces
provides a net force that can act as the centripetal
force to make the flow curve.
So to have the flow curve to the LEFT, we need CF (coriolis force) to
be a bit weaker than the PGF (pressure gradient force)
This slight imbalance
(PGF>CF ) gives us the
cyclonic flow (anticlockwise) we observe
around lows (cyclones)
Notice that to make CF
weaker, we need to decrease
the velocity from
geostrophic balance, which
makes cyclonic flow
subgeostrophic in speed
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Gradient Wind Balance
Gradient Winds (cont)
This cyclonic flow around low pressure centers and
anticyclonic around highs is called gradient wind balance
Points to remember about gradient flow:
•  1) In the southern Hemisphere, these directions are
reversed.
•  2) The coriolis force gets weaker near the equator, making
long-lasting lows and highs very rare in the tropics.
•  3) This result means that low-pressure systems have a hard
time “filling up”, since air cannot “get into them”, but
rather circles the low
•  4) remember that Fc takes a relatively long time to have an
effect, so phenomena like tornadoes, thunderstorms, etc.
are NOT affected by it.
•  5) In valleys btwn mountains, where flow is channeled, Fc
is not important and the wind will blow towards the low
pressure as much as possible. This means that valley winds
might well be perpendicular to the flow above ridge top!!!
•  6) Cyclones/anticyclones are seldom exactly circular, but
between gradient flow and geostrophic flow we have
conceptual models that work well in most situations.
Friction
•  We need to consider just one more important piece of the
puzzle, and that is adding in friction when we are near the
surface
•  All we need to do to add the effects of friction is to
remember that it always acts to slow things down!
•  Since CF gets weaker as the velocity decreases, this gives
a slight edge to the PGF and “unbalances” gradient
balance a bit.
•  The net effect of friction is that the wind tends to cross
isobars towards a low and away from a high
Figure 9.29a
CONVERGENCE, DIVERGENCE
and VERTICAL MOTION
•  All the air converging (flowing inward) towards
the center of the low has to go somewhere (like
upwards)
The top figures show flow aloft without the effect of friction
•  and diverging winds have to get their mass from
somewhere (like from above...)
So… friction-forced convergence causes upward
motion near the surface and divergence causes
downward motion (sinking or subsidence) near the
surface.
The bottom figures show the flow at the surface where friction is
important
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Figure 9.33
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