Transcript 100Dec4

Connections
METR100-01 4 DEC2009
Last class we presented the horizontal
Pressure Gradient Force, which is the
driver for all winds. We only touched on
the cause of pressure gradients and did
not investigate what happens near the
surface of the Earth. These subjects will
be covered in today’s class.
All of this week’s presentations may be
found at my website
http://funnel.sfsu.edu/students/frankv/www/m100/
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First, however, a thought question
about a vertical column of air:
Earth’s gravity is acting on each parcel
of air. Why don’t they fall to the
ground?
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Air is compressible. An air parcel
develops pressure due to the weight of
all the air molecules on top of it. As
we showed two classes ago, air
pressure must increase with decreasing
altitude. This results in a vertical
pressure gradient, and that is what
overcomes the force of gravity.
The vertical forces on an air parcel
are more balanced than the
horizontal forces as evidenced by
the fact that up and down drafts are
usually much slower than
horizontal wind velocities and that
the extent of the troposphere is only
around 10 km (6 mi) or so.
So gravity is the cause of vertical
pressure gradients (which are much
greater than horizontal pressure
gradients.)
What causes horizontal pressure
gradients?
?
B&W Slides cut and pasted from a Metr100-02
concept map handout by Dr. Dempsey
Divergence
&
convergence
aloft
lead to:
•The fastest winds tend to occur where the PG is
largest, which is also where the isobaric surfaces
slope the most steeply (which is where height
contours on an isobaric map are closest together).
•The height of an isobaric surface aloft depends on
the (average) temperature below that isobaric
surface.
–Colder air in the lower troposphere creates
lower heights (lower pressure) aloft.
–Warmer air in the lower troposphere creates
higher heights (and higher pressure) aloft.
Wind and Pressure Patterns
• On horizontal surfaces (such as at sea level), pressure
varies from place to place.
– Maps with isobars drawn on them help us visualize
the spatial pressure pattern.
• Pressure differences between places create a net
force—the pressure-gradient (PG) force--on air,
pushing toward lower pressure. The PG force pushes
air into motion.
– The strength of the PG force is greater where the
pressure gradient (PG) is larger.
– On a weather map, the spacing of isobars allows us
to tell about the relative size of the PG and hence
the PG force.
•Once air is moving, the rotation of the earth
affects the motion by apparently trying to
deflect it. We account for this effect by
inventing a Coriolis force.
–The Coriolis force is stronger when the
wind is faster.
–The Coriolis force pushes on moving
objects (including air) to their right in the
N. Hem. and to their left in the S. Hem.,
but not at all at the equator.
When the travel time
of an air parcel is
short (for instance
local thunderstorm,
land/sea breeze, or
water flowing down a
drain) Coriolis Force
may be neglected,
however it cannot
when the time scale
is in hours. The
concept map to the
right is modified for
the Coriolis force.
•Together, the PG force and Coriolis
force tend to drive the wind close to
geostrophic balance.
–A wind where the balance is exactly
achieved is the geostrophic wind.
–The observed winds aloft are usually
close to the geostrophic wind.
•Winds aloft tend to blow toward the
east, northeast, or southeast.
Friction with the Earth’s surface
• Near the earth’s surface, friction is a third
important force (within the “friction layer”).
– Friction opposes the wind, trying to slow it
down.
– Friction is larger over land than over water.
(Land is “rougher” than water.)
• The 3-way combination of PG force, Coriolis
force, and friction drives winds across isobars
at an angle.
•As a result, surface winds tend to converge
into low-pressure areas and diverge out of
high-pressure areas. (We don’t see this aloft,
though!)
•As a result, air tends to move upward out of
surface low-pressure areas and sink (subside)
into surface high-pressure areas.
•Regardless of the combination of forces
acting on air, winds tend to be faster where
the PG (and hence PG force) is stronger.
Cold
Core
Low
(Height
variation
expanded)
Cold
Core
High
weakens
Warm
Core
Low
weakens
Warm
Core
High
strengthens