Pressure, and the forces that explain the wind

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Chapter 6

• Pressure, and the forces that explain the wind

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Dz
area
Hydrostatic balance The upward pressure
gradient force is equal and opposite to the
gravity
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aneroid barometer
mercury barometer
aneroid barograph
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What is the typical SLP?How much does it vary ?
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Average air pressure in Laramie
780
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We need to reduce station pressure to a standard
height, for instance sea level Why?
Because winds are driven by horizontal pressure
differences
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Isobars and pressure patterns
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Where are you more likely to find a pressure
value of 994 mb? At A or B ?
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Becoming acquainted with contouring and frontal
analysis

• http//cimss.ssec.wisc.edu/wxwise/contour/index.ht
  ml
• http//cimss.ssec.wisc.edu/wxwise/fronts/fronts.ht
  ml

Defining patterns on a surface weather chart

• lows and highs
• trofs and ridges
• saddle

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ridge
trof
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examine the current weather analysis
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What drives the wind?
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Pressure gradient force (PGF) and wind
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here, Dx100 km and Dp4 mb
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The PGF is directed from high to low pressure,
and is stronger when the isobars are more tightly
packed
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in reality, winds do not blow from high to low,
at least not along the shortest path
so there must be other force(s)
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Coriolis force
Geostrophic wind balance a balance between the
PGF and the Coriolis force
link
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L
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Buys-Ballot law

• When you face downwind, the low will be on your
  left
• Vice versa in the southern hemisphere

you (seen from above)
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The geostrophic wind blows along the isobars
(height contours), counterclockwise around lows
(in the NH), and at a speed inversely
proportional to the spacing between the isobars
(height contours)
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in the southern hemisphere, the low is on your
right when you look downwind
L
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There is a third force, important only near the
ground
Friction slows the wind
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1008
1004
1000

• Interplay between 3 forces
• Pressure gradient force
• Coriolis force
• Friction (near the ground)
• Check out how they affect the wind!

Guldberg-Mohn balance
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Trajectories spiral out of a high, and into a
low

• 10 over oceans
• 30 over land
• gt 30 near mountainous terrain

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finally, a fourth force centrifugal force
CFF
PGF
Coriolis
PGF
Coriolis
CFF
faster-than-geostrophic wind (supergeostrophic)
slower-than-geostrophic wind (subgeostrophic)
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The jet stream wind is subgeostrophic in trofs,
and supergeostrophic in ridges
slow
fast
fast
slow
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Where does the air, spiraling into a low, end up?
height
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subsidence leads to clear skies
rising motion leads to cloudiness and
precipitation
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Fig. 10.11
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300 mb height, 9 Nov 1975, 7 pm
Find the trofs
Fig. 10.13
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fast
upper-level divergence, low-level convergence
slow
surface low
300 mb height, 9 Nov 1975, 7 pm
Fig. 10.13
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Todays surface weather analysis
http//www.rap.ucar.edu/weather/surface/sfc_den.gi
f http//weather.uwyo.edu/surface/front.html
Todays upper-air maps
http//weather.uwyo.edu/upperair/uamap.html
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Upper-level winds,and upper-level charts
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Upper level charts are NOT plotted at constant
height, eg 18,000 ft. Rather, they display the
topography of a pressure surface, eg 500 mb
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Approximate conversion of pressure level to
altitude
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1000 mb near sea level
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850 mb - 5,000 ft
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700 mb - 10,000 ft
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500 mb - 18,000 ft
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300 mb - 30,000 ft
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200 mb 40,000 ft
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pressure at a fixed height (sea level)
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elevation of the 1000 mb surface
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contours sea-level pressurecolor fill 1000 mb
height
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Why do isobar and height contour charts look
(almost) the same?
1560 m
height
low
high
1500 m
pressure surface
sea level
New York
Boston
Pressure decreases with height at about 10 mb
every 100 m
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Locate the trofs
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Thickness and temperature
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thickness between 2 material surfaces (1000- 500
mb)
temperature
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L
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Pop quiz why is their a pit in the 500 mb
surface over Antarctica?
- because it is much colder there than over
Australia and other surrounding places - because
of the ozone hole - because there is less
sunshine - I give up
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calm
calm
L
Jet stream is due to the cold pool
below (circumpolar vortex)
calm
calm
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Jet stream

• why does it exist?
• why does it vary in strength?

The jet stream is the result of a horizontal
temperature gradient and thus a thickness
gradient
thickness 20.3 Tmean thickness is in meters
between 1000 and 500 mb Tmean is the layer-mean
temperature in Kelvin
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5000
100
pretty flat
5200
pretty steep
5400
5600
150
5800
1000 mb height (m)
500 mb height (m)
near the ground weak PGF, weak wind
near 18,000 ft strong PGF, strong wind

• Where is the 1000-500 mb thickness lower? Where
  is it higher?
• Where is the colder airmass where is the
  warmer one?

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5000
B
5100
B
100
5200
5400
5600
150
5800
A
A
1000 mb height (m)
500 mb height (m)
Calculate thickness at A and B

• at A Z500-Z1000 5850-150 5700 m
• at B Z500-Z1000 5100-100 5000 m

answer the lower atmosphere is less thick at B
up north
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indeed, it is colder where the air is less thick
B
A
700 mb mean temperature (C)
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Relation between wind and temperature ...

• Key colder air is less thick, therefore upper
  level winds will blow cyclonically around cold
  pools

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For instance, look at the pole-to-pole variation
of temperature with height (in Jan)
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Around 30-45 N, temperature drops northward,
therefore westerly winds increase in strength
with height
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The N-S temperature gradient is large between
30-50N and 1000-300mb
Therefore the westerly wind increases rapidly
from 1000 mb up to 300 mb
J
J
cold
cold
warm
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thermal wind

• The increase of wind with height parallel to the
  isotherms, cyclonically around cold pools

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Illustration compare the 300 mb height over the
northern hemisphere ...
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to the temperature
700 mb
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Question

• Why, if it is colder at higher latitude, doesnt
  the wind continue to get stronger with altitude ?

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There is definitively a jet ...
stratosphere
troposphere
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Pop quiz Why is there a jet maximum in the upper
troposphere?

• because the air is too thin in the stratosphere
• because it is warmer over the poles than over the
  equator, in the stratosphere
• because of the ozone hole
• because there is too much friction with outer
  space in upper layers of the atmosphere.

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Answer above 250 mb, it is no longer colder at
higher latitudes...
60 kft
stratosphere
troposphere
18 kft
pole
equator
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Now explain why a jet stream is found above a
frontal zone
wind speed (kts)
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The jet stream is there because of low-level
temperature differences
polar front jet (PFJ)
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Pop quiz why is the jet stream stronger in
winter?

• because the north-south temperature gradient is
  larger
• because cold air is lighter and can be blown
  around easier
• because there is less sunshine
• because there are fewer thunderstorms that act as
  obstacles to the upper-level flow.

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Pop quiz why is the jet stream stronger in
winter?
because the north-south temperature gradient is
larger because cold air is lighter and can be
blown around easier because there is less
sunshine because there are fewer thunderstorms
that act as obstacles to the upper-level flow.
Change the equator-to-pole temperature gradient,
and see what happens to the jet stream!
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Pop quiz according to climate change models and
observations, the arctic is warming up faster
than low latitude regions. What does this imply
about the strength of the jet stream and the
intensity of storms spawned by the jet stream?

• they weaken
• they strengthen
• it can go either way
• I give up

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Summary

• There are four key forces driving the wind
• pressure gradient force (to start the motion)
• Coriolis force
• friction (only near the ground)
• centrifugal force
• As a result the wind blows counterclockwise
  around lows (in the NH)
• friction makes the low-level wind spiral into
  lows
• the centrifugal force slows the wind in trofs,
  and speeds it up in ridges
• Weather changes (as we know it) is the result of
  passing jet streams, with
• rising motion clouds ahead of a trof, with a
  low at the surface
• sinking motion clear skies upstream of a trof,
  with a high at the surface
• the deep vertical motion is due to changes in
  wind speed in the jet, as the wind in trofs
  (ridges) is slower (faster) than expected from
  geostrophic balance
• The jet stream tends to occur above regions with
  a large temperature difference
• The jet blows counterclockwise around cold pools
  (in the NH)

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Lets cover chapter 7 (global winds) and skip
chapter 8 (air-sea interaction) then we ll do
chapter 9 (air masses and fronts) andchapter 10
(mid-latitude weather)