Lab 6

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Lab 6

• Pressure and Wind

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Pressure

• Weather maps are usually in millibars (mb) but on
  television meteorologists typically mention
  inches of mercury.
• Average sea level pressure is 1013.25 mb or 29.92
  inches of mercury.
• Pressure typically drops by 10 mb for every 100
  meters you ascend.
• When looking at a map of pressure, meteorologists
  need to pick out areas of high and low pressure.
• Observations taken at higher altitudes will
  always appear to be an area of low pressure,
  although their pressure is lower all the time so
  a low may or may not be present.
• Therefore, each stations pressure is converted
  to sea level pressure so comparisons can easily
  made.
• By comparing readings that have been normalized
  to sea-level, it is easy to pick out areas of
  high and low pressure on the map.

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Converting to sea level pressure

• Mt. Whitney, California has an elevation of 4,418
  m.
• The barometer read 551.3 mb. Is this reading
  indicative of higher or lower than average
  pressure? It is hard to tell until you correct it
  to sea level pressure.
• What is the corrected sea level pressure?
• Add 10 mb for every 100 m that Mt. Whitney is
  above sea level.
• Corrected pressure (mb) actual pressure (mb)
  elevation(m)/10(m/mb)
• Corrected pressure 551.3 mb4418m/10m/mb
• 551.3 mb 441.8 mb
• 993.1 mb
• Pressure is lower than 1013.25 mb

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High Pressure

• High Pressure Centers (anticyclones)
• A high pressure center is where the pressure has
  been measured to be the highest relative to its
  surroundings. That means, moving in any direction
  away from the "High" will result in a decrease in
  pressure. A high pressure center also represents
  the center of an anticyclone and is indicated on
  a weather map by a blue "H".

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Low Pressure

• Low Pressure Centers (cyclones)
• A low pressure center is where the pressure has
  been measured to be the lowest relative to its
  surroundings. That means, moving in any
  horizontal direction away from the "Low" will
  result in an increase in pressure. Low pressure
  centers also represent the centers of cyclones.

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Pressure Gradient Force

• The atmosphere is continually trying to reach a
  state of equilibrium.
• Just like lakes where water flows from higher
  surfaces to lower surfaces, air flows from
  regions of high pressure to regions of low
  pressure.
• The force that causes wind to move from regions
  of higher pressure to regions of lower pressure
  is called the pressure-gradient force.
• ForceMass x Acceleration

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Pressure gradient force (cont)

• The force is largest where the gradient is
  highest
• Gradient change in some property from one point
  to another, divided by the distance between the
  points.
• Gradient can be increased by
• Increasing the relative magnitudes of the high
  and/or low pressure
• Decreasing the distance between the high and low
• Areas where the gradient is highest have strong
  winds
• Ex. A hurricane with a central pressure of 950
  mb, and a radius of 200 km. Very strong pressure
  gradient leads to very strong winds.

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Hurricane Pressure Gradient
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Pressure Gradient Force
Strong Pressure Gradient Stronger Winds
Weak Pressure Gradient Weaker Winds
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Hydrostatic balance

• Remember pressure is highest at the earths
  surface, and decreases until it reaches 0 on the
  edge of the atmosphere.
• Remember pressure gradient in the vertical is
  much stronger than in the horizontal.
• Since wind blows from high to low pressure,
  shouldnt there be a wind that blows straight
  up???
• Luckily for us, the pressure gradient force in
  the vertical is approximately in balance with
  gravity. (in other words, a neutral parcel of air
  has no force acting on it in the vertical. A
  force of some other kind is needed to cause it to
  move vertically)
• This balance is called hydrostatic balance.

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Coriolis Force

• Coriolis Force is an apparent force that causes
  objects (and winds) in the northern hemisphere
  tend to move to their right due to the rotation
  of the earth.
• If the earth were not spinning, there would be no
  coriolis force.
• As an object moves over the surface of the Earth,
  the Earth continues spinning beneath it. This
  makes the object appear to deflect from its
  original path, despite the fact that the path
  actually remains straight.
• Coriolis force takes effect the minute objects
  not resting on the earths surface begin to move.

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Coriolis Force contd

• The strength of the coriolis force depends on
  both the wind speed and the latitude.
• Measurement of latitude 0 at equator, 90 at
  north pole, -90 at south pole
• Due to the equation below, we can see that the
  Coriolis Force is 0 at the equator and maximum at
  the poles.
• The longer an object is exposed to the influence
  of Coriolis Force, the greater the displacement.

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Coriolis Force
Image courtesy of Department of Atmospheric
Sciences (DAS) at the University of Illinois at
Urbana-Champaign
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Illustration of the coriolis force

• Show video

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How does coriolis force affect circulation

• For the northern hemisphere

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Coriolis in the southern hemisphere

• In the southern hemisphere, the latitude is taken
  to be negative (0 to -90)
• Sine of an angle between 0 and -90 is negative
• Coriolis effect is opposite in the southern
  hemisphere (turns wind to the left)
• This causes the circulations to be opposite in
  the southern hemisphere.

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Circulations in southern and northern hemisphere
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Buys-Ballots Law

• In the northern hemisphere, if you stand with
  your back to the wind, then the lower pressure
  will be to your left.
• Why is this true?

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Buys-Ballots law
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Geostrophic Balance (figure 6.4)

• With no other forces acting on the wind, pressure
  gradient force and coriolis force approximately
  balance each other
• For the most part, winds will be parallel to
  isobars
• Approximation is better in upper levels of the
  atmosphere
• Upper levels wind speeds are faster therefore
  coriolis force is greater. Here, coriolis force
  is large enough to balance pressure gradient
  force.

Image courtesy of Department of Atmospheric
Sciences (DAS) at the University of Illinois at
Urbana-Champaign
Geostrophic Balance
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Friction

• Winds over a smoother surface are faster than
  over a rough surface (ex. They are faster over
  water than over land)
• Winds within about 1 km are affected by surface
  friction (boundary layer)
• Above this layer, wind is not really affected by
  the surface. Wind speeds increase as you go up in
  the troposphere (usually)

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Frictions Effect on Wind

• Lower levels (within 1km of surface) wind
  speeds are slower due to friction, coriolis force
  is weaker and not large enough to balance
  pressure gradient force.
• Pressure gradient force is slightly larger, and
  pulls the winds across the isobars towards the
  low pressure area
• The angle at which the wind crosses the isobars
  is usually around 30 degrees, but varies based on
  the roughness of the terrain.

Image courtesy of Department of Atmospheric
Sciences (DAS) at the University of Illinois at
Urbana-Champaign
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Convergence lifting (figure 6.5)

• Convergence lifting is one of the lifting
  mechanisms mentioned in chapter 4
• Air flows from high towards low pressure
• Air flows into the area of low pressure and is
  forced to rise
• Rising air cools and precipitation may occur
• This is why low pressure systems have
  precipitation associated with them.
• air flows away from the center of a high, and is
  replaced by air sinking from upper levels.
• The air sinking compresses and warms. This lowers
  the relative humidity and leads to clear skies
  and generally clear conditions.
• This is why high pressure systems are typically
  associated with clear skies and fair weather