Air Pressure and Winds

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Air Pressure and Winds
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Pressure Fluctuations
Figure 9.3
Solar heating of ozone gasses in the upper
atmosphere, and of water vapor in the lower
atmosphere, can trigger oscillating thermal tides
of sea-level pressure change.
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Pressure Scale Units
Many scales are used to record atmospheric
pressure, including inches of mercury (Hg) and
millibars (mb). The National Weather Service
uses mb, but will convert to metric units of
hectopascals (hPa). The conversion is simply 1
hPa 1 mb.
Figure 9.4
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Pressure Measurement
Figure 9.6
Changes in atmospheric pressure are detected by a
change in elevation of a barometric fluid or
change in diameter of an aneroid cell, which
indicates changing weather. Average sea level
pressure is 29.92 in Hg, or 1013.25 mb.
Figure 9.5
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Pressure Trends
Figure 9.7
Barographs provide a plot of pressure with time,
and are useful in weather analysis and
forecasting. Altimeters convert pressure into
elevation, and are useful in steep terrain
navigation or flying. Both use aneroid cells.
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Pressure Reading Reporting
Increase terrain elevation and decrease column of
air above. To remove the effect of elevation,
station pressure is readjusted to sea level
pressure at 10mb/100m. Isobars show geographic
trends in pressure, and are spaced at 4 mb
intervals.
Figure 9.8
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Single-Cell Circulation Model
The basis for average air flow around the earth
can be examined using a non-rotating, non-tilted,
ocean covered earth. Heating is more intense at
the equator, which triggers Hadley cells to
redistribute rising heat from the tropical low to
the polar highs.
Figure 11.1A
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Ecclesiastes 16   The wind goeth toward the
south, and turneth about unto the north it
whirleth about continually, and the wind
returneth again according to his circuits.
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ITCZ
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ITCZ
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ITCZ Limits
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Three Cell Circulation Model
A rotating earth breaks the single cell into
three cells. The Hadley cell extends to the
subtropics, the reverse flow Ferrel cell extends
over the mid latitudes, and the Polar cell
extends over the poles. The Coriolis force
generates westerlies and NE trade winds, and the
polar front redistributes cold air.
Figure 11.2A
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• HURRICANE IVAN DISCUSSION NUMBER 44 NWS
  TPC/NATIONAL HURRICANE CENTER MIAMI FL 5 AM EDT
  MON SEP 13 2004
• THE TRACK SCENARIO REMAINS THE SAME. THE
  HURRICANE IS FORECAST TO GRADUALLY TURN NORTHWARD
  OVER THE NEXT 72 HOURS INTO A WEAKNESS IN THE
  SUBTROPICAL RIDGE...AND THEN NORTHEASTWARD AS IT
  APPROACHES THE WESTERLIES.

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2002
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Observed Winds in January
Observed average global pressure and winds have
increased complexity due to continents and the
tilted earth. Differential ocean-land heating
creates areas of semi-permanent high and low
pressure that guide winds and redistribute heat.
Figure 11.3A
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Observed Winds in June
Global pressure and wind dynamics shift as the
Northern Hemisphere tilts toward the sun,
bringing the inter-tropical convergence zone, the
Pacific high, and blocking highs in the southern
oceans northward.
Figure 11.3B
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North American Winter Weather
Semi-permanent highs redirect North American
winds, such as cold interior southerly flow from
the Canadian high. The Polar front develops a
wave like pattern as air flows around lows.
Figure 11.4
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Global Precipitation Patterns
Global low pressure zones around the equator and
60 latitude generate convergence at the surface,
rising air and cloud formation. Zones of high
pressure at 30 and the Poles experience
convergence aloft with sinking, drying air.
Figure 11.5
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http//www.eoascientific.com/campus/earth/multimed
ia/coriolis/view_interactive
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Coastal Summer Weather
Figure 11.6
The semi-permanent Pacific high blocks moist
maritime winds and rain from the California
coast, while the Bermuda high pushes moist
tropical air and humidity over the eastern states.
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Coastal Winter Weather
During winter months, the Pacific high migrates
southward and allows for maritime winds with
moisture and rains to reach California. On the
east coast, precipitation is rather even
throughout the year.
Figure 11.7
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January Winds Aloft
Land-sea temperature differences trigger ridges
and troughs in the isobaric surface.
Figure 11.8A
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June Winds Aloft
Horizontal temperature gradients establish
pressure gradients that cause westerly winds in
the mid latitudes.
Figure 11.8B
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Jet Stream
Figure 11.10
Figure 11.9
High velocity Polar and subtropical jet stream
winds are located in the lower tropopause, and
they oscillate along planetary ridges and troughs.
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300 mb Winds Jets
300 mb pressure surface maps illustrate lines of
equal wind speed (isotachs) as the jets
meander. Jet streaks are the maximum winds,
exceeding 100 knots.
Figure 11.11
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Polar Jet Formation
Steep gradients of temperature change at the
Polar front trigger steep pressure gradients,
which then forces higher velocity geostrophic
winds. This is the trigger for jet stream flow.
Figure 11.13A
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Winds Angular Momentum
Angular momentum is the product of mass,
velocity, and the radius of curvature and it must
be conserved. As northward-flowing air
experiences a smaller radius, it increases in
velocity and augments the jet stream flow.
Figure 11.14
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14 September 04 http//www.wunderground.com/US/Reg
ion/US/JetStream.html
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http//weather.unisys.com/upper_air/ua_500.html
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Smoothed Isobar Maps
Continental maps of station recorded sea-level
pressure are often smoothed and simplified to
ease interpretation. Smoothing adds error to
those already introduced by error in instrument
accuracy.
Figure 9.9A
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Constant Height Chart
Maps of atmospheric pressure, whether at sea
level or 3000 m above sea level, show variations
in pressure at that elevation.
Figure 9.10
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Constant Pressure Chart
Maps of constant pressure provide another means
for viewing atmospheric dynamics. In this
example, there is no variation in elevation for a
pressure of 500 mb.
Figure 9.11
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Variation in Height
Figure 9.13
Figure 9.12
Isobaric (constant pressure) surfaces rise and
fall in elevation with changes in air temperature
and density. A low 500 mb height indicates denser
air below, and less atmosphere and lower pressure
above. Contour lines indicate rates of pressure
change.
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Ridges Troughs
Figure 9.14
Upper level areas with high pressure are named
ridges, and areas with low pressure are named
troughs. These elongated changes in the pressure
map appear as undulating waves.
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Surface 500 mb Maps
Surface maps chart pressure contours, highs and
lows, and wind direction. Winds blow clockwise
around highs, called anticyclones. 500 mb maps
reveal patterns that on average are 5600 m above
the surface, where westerly winds rise and fall
across ridges and troughs.
Figure 9.15A
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Forces Motion
Pressure forces are only one influence on the
movement of atmospheric air. Air responds
similarly as water to this force, moving from
higher pressure to lower pressure. Centripetal,
friction, and apparent Coriolis are other forces,
however, determining winds.
Figure 9.16
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Pressure Gradient Force
Figure 9.17
Figure 9.18
Change in pressure per change in distance
determines the magnitude of the pressure gradient
force (PGF). Greater pressure changes across
shorter distances creates a larger PGF to
initiate movement of winds.
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PGF vs. Cyclonic Winds
Pressure gradient force (PGF) winds acting alone
would head directly into low pressure. Surface
observations of winds, such as the cyclonic flow
around this low, reveal that PGF winds are
deflected by other forces.
Figure 9.19
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Apparent Coriolis Force
Figure 9.20
Earth's rotation transforms straight line motion
into curved motion for an outside viewer. The
Coriolis force explains this apparent curvature
of winds to the right due to rotation. Its
magnitude increases with wind velocity and
earth's latitude.
Figure 9.21
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Geostrophic Wind
Figure 9.23
Winds have direction and magnitude, and can be
depicted by vectors. Observed wind vectors are
explained by balancing the pressure gradient
force and apparent Coriolis force. These upper
level geostrophic winds are parallel to pressure
contours.
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Wind Speed Pressure Contours
Figure 9.24
Just as a river speeds and slows when its banks
narrow and expand, geostrophic winds blowing
within pressure contours speed as contour
intervals narrow, and slow as contour intervals
widen.
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Isobars Wind Prediction
Upper level pressure maps, or isobars, enable
prediction of upper level wind direction and
speed.
Figure 9.25A
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Centripetal Acceleration Cyclones
Acceleration is defined by a change in wind
direction or speed, and this occurs as winds
circle around lows (cyclones) and highs
(anticyclones). Centripetal force is the term
for the net force directing wind toward the
center of a low, and results from an imbalance
between the pressure gradient and Coriolis forces.
Figure 9.26A
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Northern Southern Hemisphere Flow
Winds blow counterclockwise around low pressure
systems in the Northern Hemisphere, but clockwise
around lows in the Southern Hemisphere.
Figure 9.27A
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Meridional Zonal Flow
Figure 9.28
Wind direction and speed are indicated by lines,
barbs, and flags, and appear as an archer's
arrow. Upper level winds that travel a
north-south path are meridional, and those
traveling a west-east path are zonal.
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Friction Surface Winds
Surface objects create frictional resistance to
wind flow and slows the wind, diminishing the
Coriolis force and enhancing the effect of
pressure gradient forces. The result is surface
winds that cross isobars, blowing out from highs,
and in toward lows.
Figure 9.29A
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Surface Flow at Lows Highs
Figure 9.30
Figure 9.31
Southern Hemisphere flow paths are opposite in
direction to Northern Hemisphere paths, but the
same principles and forces apply.
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Sensing Highs Lows
The location of high and low pressure centers are
estimated by detecting surface wind direction and
noting pressure, Coriolis, and friction
forces. This figure illustrates the procedure
when standing aloft and at the surface.
Figure 9.32A
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Vertical Air Motion
Figure 9.33
Winds converging into a low pressure center
generate upward winds that remove the
accumulating air molecules. These updrafts may
cause cloud formation. Likewise, diverging air
molecules from a high pressure area are
replenished by downward winds.