Air Pressure and Wind

1
Air Pressure and Wind

• Chapter 6

2
Pressure

• Hear this term often in weather forecasts but
  what does it mean in the atmosphere?
• From earlier, its the weight of the air above
• How about weather?
• High pressure?
• Usually nice weather
• Low pressure?
• Associated with stormy weather

3
Wind

• Another weather element we deal with on a regular
  basis
• Anyone know why the wind blows?
• Turns out that wind and pressure are related
• In fact, wind blows due to horizontal differences
  in pressure
• If there is high pressure over one part of the
  country and low pressure over another part
• The atmosphere is out of balance and the wind
  blows in an attempt to restore balance

4
Air Pressure and Wind

• A couple of chapters ago we talked about
  temperature
• Before that, pressure and density
• In the atmosphere, these variables are all
  related such that a change in one will cause
  changes in the others
• Ex. If temperature changes, there will be a
  corresponding change in pressure and/or density

5
Air Pressure and Wind

• The Ideal Gas Law or Equation of State
  illustrates this
• Pressure is approximately equal to density x a
  constant x temperature
• We can ignore the constant and just go with..

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Air Pressure and Wind

• If the pressure doesnt change
• An increase in T results in a decrease in density
• Warm air is less dense and therefore rises
• A decrease in T results in an increase in density
• Cold air is dense and sinks
• Just like weve been saying all along

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Air Pressure and Wind

• If the temperature doesnt change
• An increase in pressure results in an increase in
  density
• A decrease in pressure results in a decrease in
  density
• At the same temperature, air at a higher pressure
  is more dense than air at a lower pressure

8
Atmospheric Pressure

• How does all of this stuff relate to the
  atmosphere?
• Weve said from the beginning that pressure is
  basically the weight the air above us.
• Like at the right
• Pressure at the surface is due to the weight of
  all air molecules in the colum

9
Atmospheric Pressure

• Simplified model
• Assumes air cant leave the column
• Columns of air have the same of molecules and
  are at the same temperature
• The pressure at the surface is the same
• What would happen if the temperatures of the air
  changed?
• Cool 1, Warm 2

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Atmospheric Pressure

• City 1, temp decreased so density increased
• City 2, temp increased causing density to
  decrease
• Just like the gas law said
• Pressure stayed the same
• Bottom line It takes a shorter column of cold
  air to exert the same amount of pressure as a
  taller column of warm air

11
Atmospheric Pressure

• Now lets go up to a certain height in the
  atmosphere
• At this level, in which column is the pressure
  greatest?
• So, relatively speaking, the pressure is high at
  this level in column 1 and low in column 2

H
L
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Atmospheric Pressure

• Points
• Pressure changes more rapidly w/ height in cold
  air masses
• Warm air aloft is associated w/ high pressure
  aloft
• Cold air aloft is associated w/ low pressure
  aloft
• Differences in temperature cause differences in
  pressure

H
L
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Atmospheric Pressure

• Finally, notice that the heights of pressure
  surfaces (500 mb in this example) are lower in
  the cold air column

500 mb
500 mb
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Atmospheric Pressure

• The difference in pressure establishes a force we
  call the pressure gradient force (directed from
  H to L)
• Now, if we remove the side barriers of the
  columns, air will rush from high to low pressure
  in order to equalize things .
• WIND!!

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Atmospheric Pressure

• Pressures at the surface will also change due to
  molecules moving
• Pressure will rise at City 1 and fall at City 2
• To make a long story short
• Differences in temp from place to place can cause
  differences in pressure resulting in the movement
  of air.

16
Sea-level Pressure
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Measuring Air Pressure

• Even though pressure is exerted on us at all
  times, its hard to detect small changes
• Can you tell difference between high and low?
• We can detect big changes in pressure though
• Like popping of ears in the mountains or in
  planes
• Air pressure equalizing inside/outside ears

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Measuring Air Pressure

• Mercury Barometer
• Just a large, hollow glass tube immersed in
  mercury
• As air pressure changes, mercury is forced up or
  down the tubepretty simple right
• On average, the height of the mercury would be
  29.92 inches (avg. sea level pressure)
• Or 1013.25 millibars

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Measuring Air Pressure

• Aneroid Barometer
• Has a hollow metal cell which expands or
  contracts as pressure changes
• Same type as in the 3-dial weather instruments
  people hang on walls

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Measuring Air Pressure
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Altimeters

• Just an aneroid barometer
• Calibrated to equate pressure to height
• Must be corrected constantly by pilots!

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Altimeters

• Or this might happen w/ poor visibility

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Atmospheric Pressure

• Seen something like this on TV right?
• Lines are called isobars
• These are lines of equal pressure (in millibars)
• Question
• If elevation varies across the US (and it does),
  and we know pressure changes quickly w/ height,
  then why are these types of maps nice and neat?
• Shouldnt they be really screwy looking?

24
Atmospheric Pressure

• This kind of map depicts sea-level pressure,
  not surface pressure
• We wouldnt always be able to tell where high and
  low pressure systems were otherwise
• How do we figure out what sea-level pressure is
  at each location where measurements are taken?

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Sea-level Pressure

• To get a sea-level pressure chart
• 1) Measure surface pressure
• 2) Correct for instrument error
• Temperature, gravity, materials of barometer,
  etc.
• 3) Correct for altitude
• 4) Draw isobars (usually at 4 mb increments)
• Connect the dots essentially
• 1) and 2) are easy. What about 3) and 4)?

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Altitude Correction

• Near the earths surface, pressure changes at
  10mb per 100m
• So, if a station is 300m altitude, 30mb needs to
  be added to the surface pressure to get sea-level
  pressure
• Once the altitude correction is done everywhere,
  we can draw the isobars

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Isobars

• 995 999 1005 1010
  
• 998 1002 1007
  1011 1014
• 1006 1010
  1015
• 1009 1013
  1020

996
1000
1004
1008
1012
1016
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Surface Chart

• End result is a sea-level pressure chart or
  surface chart
• Closed highs and lows show where centers of
  pressure systems are

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Pressure and Wind

• Northern Hemisphere surface winds blow clockwise
  and outward from high pressure (anticyclones)
• counter clockwise and inward around low pressure
  systems (cyclones)
• Note
• Winds cross isobars slightly
• Tightly packed - stronger winds
• Reversed flow in Southern Hemisphere

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Isobaric Map (Upper Air Chart)

• Shows the height of a pressure surface - constant
  pressure chart
• In meters (60m intervals)
• This one is 500mb
• From Monday - pressure surfaces are higher up in
  warm air
• So, the 500mb heights are higher toward the south

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Isobaric Map (Upper Air Chart)

• Where heights are low - cold air
• High heights - warm air
• Elongated areas of low heights/pressure are
  called troughs
• cold air
• Elongated areas of high heights/pressure are
  called ridges
• warm air

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Isobaric Map (Upper Air Chart)

• Other uses??
• Wind - shows us direction and speed
• Like surface map except winds tend to blow
  parallel to height lines
• Closer lines - stronger wind speeds
• Steering - upper level winds determine where
  surface systems go and whether or not they
  strengthen (more later)

33
Surface and Upper Air Charts

• Both surface and upper air charts are extremely
  valuable to meteorologists
• Surface charts identify where pressure systems
  are located and their intensities
• Upper air charts indicate where these systems
  will move and how they will strengthen/weaken in
  time

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Wind

• Now with all this background, we can determine
  why the wind blows
• More specifically, what the direction and speed
  will be
• Anything that moves does so due to the forces
  acting upon it
• Throwing a ball - pushing away by the hand,
  friction from the air, gravity, etc.
• Same thing for the wind

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Wind

• Actually 4 forces acting to influence wind speed
  and direction
• 1) Pressure gradient force
• 2) Coriolis force
• 3) Friction
• 4) Centripetal force

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Pressure Gradient Force

• Due to the difference in pressure over a distance
• Greater pressure gradients lead to a greater PGF
• like at the right
• Hurricanes are a good example
• Very low pressures at the center
• Pressure increases rapidly as you move away from
  the center
• Strong PGF

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Pressure Gradient Force

• ALWAYS directed from high to low pressure
• Direction is at right angles to the isobars
• This does not mean that wind blows directly from
  high to low though.other forces..

38
Pressure Gradient Force

• Wait just a second. Since pressure changes much
  faster w/height than horizontally, isnt the PGF
  incredibly strong in the vertical?
• Ive already said that vertical air motions are
  very small (usually) compared to horizontal
  winds..whats up with that?

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Pressure Gradient Force

• Gravity almost exactly balances the upward
  directed PGF - Hydrostatic balance

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High or Low Pressure?
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Coriolis Force (Effect)

• Tricky subject
• An apparent force due to the rotation and
  curvature of the earth
• Causes wind to deflect to the right in the
  Northern Hemisphere
• Left in SH
• Force is at a right angle to the wind
• Things drain differently in SH??? Umm..no

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Coriolis Force
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Coriolis Force

• Maximum at poles, zero at the equator
• Faster speeds - stronger Coriolis force
• Its why aircraft fly in circular paths

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Coriolis Force

• Summary
• Causes objects to deflect to the right of a
  straight path in the NH (left in SH)
• Amount of deflection depends on
• 1) Rotation of the earth
• 2) Latitude
• 3) Speed of object (wind, airplane, etc.)

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PGF - Coriolis Balance

• Above the friction layer near the surface, the
  PGF and CF roughly balance each other
• Thats why air aloft flows parallel to isobars
• Wind which flows at a constant speed parallel to
  evenly spaced isobars is called
• Geostrophic Wind

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Geostrophic Wind

• Always low pressure to the left and high pressure
  to the right (Northern Hemisphere)
• Speed depends on the packing or tightness of
  isobars
• loosely packed weak wind tightly packed
  strong wind

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Geostrophic Wind

• Only a theoretical wind but still, a good
  approximation of winds above the surface
• Why only theoretical?
• Isobars are rarely evenly spaced OR straight

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Gradient Wind

• In this case, the wind is called a gradient wind
• Basically the same as geostrophic wind except
  that it blows parallel to curved isobars
• Note both geostrophic and gradient winds refer
  to air flow well above the surface

49
Wind Review

• 4 forces (talked about 2 so far)
• 1) Pressure gradient force
• Directed from High to Low pressure
• Stronger PGF stronger wind
• 2) Coriolis force
• Deflects wind to right in NH
• Faster wind stronger CF
• Zero at equator, max at poles
• These two forces are roughly in balance above the
  surface
• Causes upper level winds to generally flow west
  to east in mid-latitudes (parallel to isobars or
  height contours)

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Friction

• Friction affects air flow near the surface
  (lowest 1 km or so)
• Slows down the wind (drag)
• If wind slows, what happens to the Coriolis
  Force??
• Weaker
• So, the PGF is now greater than the CF and flow
  is across isobars
• 30º angle

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Centripetal Force

• A little confusing so just remember it is
• The force required to keep an object (wind)
  moving in a circular path
• Directed inward toward the center in both high
  and low pressure systems

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Convergence and Divergence

• Now that we know a little about surface and upper
  level winds.
• How do they affect vertical air motions?
• By convergence and divergence patterns
• ex. Convergence Divergence

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Convergence and Divergence

• Remember what winds are like around high and low
  pressures?
• Winds diverge from high pressure and converge at
  low pressure
• If air converges at low pressure (at the surface
  for ex.), what must it do?
• Must rise (cant go into the ground right?)

54
Convergence and Divergence

• What about air diverging from a high pressure
  center (again at the surface)?
• Some air will have to sink from above to replace
  it
• This explains why we have clear weather w/ highs
  and cloudy weather w/ lows
• So far were just talking about the surface. But
  whats going on above the surface high and low
  pressure systems?

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Convergence and Divergence

• Air that is forced to rise due to convergence at
  the surface low eventually diverges aloft.
• Air converges aloft above the surface high and
  sinks to replace the diverging air at the
  surface.
• Think of it like columns of air above the surface
  pressure systems

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Convergence and Divergence

• In either of these examples, if convergence
  divergence, what happens to the surface pressure.
• Hint
• This means the of air molecules over the
  surface does not change.
• Pressure stays the same!

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Convergence and Divergence

• What if convergence and divergence are not
  equal??
• Over the low
• Divergence aloft gt Convergence at the surface?
• Net loss of air over low - pressure gets even
  lower
• Hurricanes?? (not Miami)
• Div lt Conv?
• Net gain of air - pressure increases

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Convergence and Divergence

• Over the high
• Convergence aloft gt Divergence at the surface?
• Net gain of air over high - pressure gets even
  higher
• Conv lt Div?
• Net loss of air - pressure decreases

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Convergence and Divergence

• In summary
• Pressure at the surface is largely dependent on
  wind patterns at both the surface and aloft
• This is why meteorologist actually care about
  what is happening above the surface of the earth
• If this seems a little fuzzy to you, look at
  figure 6.21 and convince yourself of it.

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Measuring Wind

• Described by
• 1) Direction
• Always direction its coming FROM
• Can be N/S/E/W or in degrees on a compass
• 2) Strength
• Usually mph, m/s (meters per second), knots
  (nautical miles per hour)
• NOTE Nautical mile gt statute mile

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Measuring Wind

• Wind directions
• Real easy, just think of a 360 circle
• East wind - 90
• South wind - 180
• West wind - 270
• North wind - 360
• Again, always described in terms of direction
  from
• ex. NW wind is out of the northwest, not toward
  the northwest

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Measuring Wind

• Wind vane
• Simple instrument which measures direction only
• Anemometers
• Measures speed only
• Example is of a cup anemometer

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Measuring Wind

• Aerovane
• Measures both wind speed and direction
• Will face into the wind giving direction
• Propellers rotate to yield wind speed
• Info is transmitted electronically
• One on top of the Love Building

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Measuring Wind

• Radiosonde
• Balloon is tracked from the surface
• Simple calculations to determine its speed (wind)
• RADAR
• Doppler in particular
• Can determine wind speed and direction by
  frequency changes in the emitted RADAR pulse
• Satellites
• Cloud drifts