http:www'srh'noaa'govsrhjetstreamsynopticwind'htm

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Wind, Air masses, and Fronts
Clare 102 Weather and Climate Change Spring 2009
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Key question can we predict weather?
Never, no matter what may be the progress of
science, will honest scientists who care for
their reputation venture to predict weather.
Francois Arago, 1786 -1853
We shall know more and more by degrees.
Robert FitzRoy, 1850s
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Heres what it takes

• Vilhelm Bjerknes - first modern scientific
  forecaster. In a 1904 publication he argued that
  weather prediction requires
• sufficiently accurate knowledge of the current
  state of the atmosphere
• sufficiently accurate knowledge of the laws
  according to which one state of the atmosphere
  develops from another.
• Cox. Stormwatchers. 154

Critics complained that his method of forecasting
storms took longer than just waiting to see how
the storms actually developed. What he needed
was ?
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Computing power
About 1 calculation per second
About 5,000 calculations per second
1.7 quadrillion calculations per second
http//campus.udayton.edu/hume/Computers/The20Ea
1.jpg
http//strongtowersoftware.com/eniac1.gif
http//en.wikipedia.org/wiki/FileRoadrunner_super
computer_HiRes.jpg
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A brief history of weather prediction

• Aristotle (384-322 B. C.) Meteorologica. He
  discussed winds, earthquakes (which he thought
  were caused by underground winds), thunder,
  lightning, rainbows, and meteors, comets, and the
  Milky Way which he thought were atmospheric
  phenomena). http//www.ucmp.berkeley.edu/history
  /aristotle.html
• Jacques Charles and others (starting 1783)
  Charles invented the gas balloon made the first
  atmospheric ascent with thermometer and
  barometer. Hydrometer added later. Cox.
  Stormwathers. 21
• Elias Loomis (1837) attempted to reconstruct
  the wind speeds in a tornado. Collected data on
  individual storms from dozens of stations. First
  synoptic weather maps. Cox. Stormwathers.
  41-49

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A brief history (2)

• Joseph Henry (1849) as Secretary of the
  Smithsonian he established a telegraph network of
  weather stations. Permitted first
  near-simultaneous tracking of storms.Cox.
  Stormwathers. 47
• Robert FitzRoy (1850s) first national weather
  forecasts (he coined the phrase). Beginning
  in Aug. 1861 his forecasts featured in several
  London newspapers. Optimistic about meteorology
  (We shall know more and more by degrees) Cox.
  Stormwatchers. 80
• William Ferrel (1858) first complete
  mathematical description of global circulation
  patterns.

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Ferrels paper
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A brief history (3)

• Francis Capen (1863) - tried to convince Pres.
  Lincoln of the value of meteorology for wartime
  strategies. Lincoln commented, It seems to me
  that Mr. Capen knows nothing about weather He
  told me three days ago that it would not rain
  again till the 30th of April or the 1st of May.
  It is raining now and has been for 10 hours. I
  can not spare any more time for Mr. Capen. Cox.
  Stormwatchers. 63
• Cleveland Abbe (1870) established first U. S.
  network of weather stations, under Army Signal
  Corps. Transferred to U. S. D. A. renamed the
  Weather Bureau (1891) Cox. Stormwatchers.
  Chapt. 12

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A brief history (4)

• Vilhelm Bjerknes (1904) - first fully scientific
  approach to weather forecasting. Applied
  Newtonian physics to atmospheric science.
  Founded the Bergen (Norway) school of
  meteorology. Critics complained that his method
  of forecasting storms took longer than the
  weather itself. Cox. Stormwatchers. 154
• Jacob Bjerknes (1918-1960s) Vilhelm s son.
  Theories of fronts and life cycle of cyclones
  (in his 20s). Foundation of all modern weather
  forecasting. First clear explanation of El Nino
  (1969, age 71) Cox. Stormwatchers. Chapt. 12

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Who to study

• Know
• Be aware of
• Appreciate

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Air pressure and isobars

• iso means equal
• bar means pressure
• So isobar lines join places with air

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Isobars show high and low pressure systems
Left Ahrens. Meteorology Today. Fig.
8.16 Right Lutgens Tarbuck,. The Atmosphere.
Fig. 11-4a
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Pressure gradient force
Like this
http//www.eoearth.org/article/Wind
http//www.deskpicture.com/DPs/Sports/Skier.jpg
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The Coriolis Effect
http//www.srh.noaa.gov/srh/jetstream/synoptic/win
d.htm
In the Northern Hemisphere, objects in motion are
deflected
Originally suggested by Hadley (1735)
Mathematics worked out by Ferrel (1858)
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Try it on a globe
Try it N -gt S or S -gt N The deflection is always
(in the Northern hemisphere)
http//luna.tau.ac.il/tomer/staff/morris/NewSite/
Chapter3/Coriolis_Effect.htm
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East-west motions are also deflected
Ahrens. Meteorology Today. 8th ed. Fig. 8.22
The deflection is greatest at the poles and zero
at the equator
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Why do winds spiral into low pressure zones?

• Atmospheric circulation involved a of forces
• The net force causes a
• circulation pattern around a low pressure
  system, called cyclonic flow

Ahrens. Meteorology Today. Fig. 8.26a
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Cyclones and Anticyclones
Ahrens. Meteorology Today. Fig. 8.26
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Hurricanes other tropical storms are cyclones
which way do they spin?
http//www1.ncdc.noaa.gov/pub/data/nexradviewer/ka
trina/klix-landfall-anim_med.gif
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Converging air masses

• Air moves into a Low pressure zone (much as
  water moves toward a sink drain)
• Air cant just pile up there, so it is
• As the air rises, it . Often it
• This is why Low pressure often leads to

http//www.srh.noaa.gov/srh/jetstream/synoptic/win
d.htm
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Diverging air masses

• Air moves way from a High pressure zone (much as
  water flows off of a hill)
• Air from above must sink to replace it. As the
  air sinks, it
• Warmer air can support more evaporation, so its
  relative humidity
• This is why High pressure often leads to

http//www.srh.noaa.gov/srh/jetstream/synoptic/win
d.htm
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Fronts

• a line of contact between
• Vilhelm Bjerknes (1920) used military language to
  describe frontsThe warm is victorious to the
  east of centre. The cold, which is pressed hard,
  escapes to the west, in order suddenly to make a
  sharp turn towards the south, and attacks the
  warm air in the flank it penetrates under it as
  cold West wind. Cox. Stormwatchers. 166-7

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Fronts seen from above
http//www.physicalgeography.net/fundamentals/7s.h
tml
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Anatomy of a warm front
http//www.physicalgeography.net/fundamentals/7r.h
tml
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Weather associated with warm fronts
http//www.physicalgeography.net/fundamentals/7r.h
tml
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Anatomy of a cold front
http//www.physicalgeography.net/fundamentals/7r.h
tml
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Weather associated with cold fronts
http//www.physicalgeography.net/fundamentals/7r.h
tml
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Sometimes fronts collide
http//www.physicalgeography.net/fundamentals/7r.h
tml
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What caused this weather?
Source SBU Weather Station
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What caused this weather?
Source SBU Weather Station
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What caused this weather?
Source SBU Weather Station
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Friday, 26 December 2008
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Saturday, 27 December 2008
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Sunday, 28 December 2008
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Monday, 29 December 2008