ES 1111

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ES 1111

• Winds and Pressure

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Wind

• The horizontal movement of the air is what we
  call wind
• Horizontal air motion is in response to
  horizontal variations in atmospheric pressure
• The wind direction that is reported is the
  direction from which the wind is blowing
• Units of wind speed are usually in knots (due to
  aviation use)

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Air Pressure

• Pressure is created by the moving gas molecules
  and atoms in the atmosphere
• Pressure is the force experienced per unit area
  due to the collisions with the atmospheric gases
• Pressure depends on the density and the
  temperature of the air
• One can think of air pressure as the weight of
  the air above you, but care must be made because
  air pressure can exist even if weight does not
  (outer space)

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Measurement of Pressure

• The instrument that measures air pressure is
  called a barometer
• The first barometer used a mercury column in a
  vacuum tube. As pressure increased, the height
  of the mercury column also increased
• Newer barometers, called an aneroid barometer,
  use an expandable diaphragm that changes shape
  depending on changes in air pressure

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Pressure Units

• One unit of pressure is inches of mercury, which
  is simply how tall the mercury column in the
  barometer is after corrections for temperature
  are made
• Another unit of pressure is called a millibar,
  which is more common in the US
• In your text, you will see hPa (hectopascals) as
  a unit of pressure. A hPa is the same thing as a
  millibar

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Pressure Variations

• At sea level, the standard atmospheric pressure
  is 1013.25 millibars
• Because density decreases exponentially with
  height, and temperature decreases, the air
  pressure decreases exponentially with height
• Surface air pressure is reduced to the value that
  the pressure would be at sea level in order to
  get rid of elevation influences in pressure

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Pressure Patterns

• The sea-level pressure for every station can be
  placed on a map, and contours of equal pressure
  (isobars) may be drawn
• In regions of high temperature, low pressure is
  likely and vice versa
• In regions of vertical ascent, low pressure is
  likely because air molecules are leaving the
  surface
• Regions of persistent high and low pressure are
  evident on weather maps. These are responsible
  for primary circulation features
• Some zones, like the mid-latitudes, have frequent
  weather disturbances called secondary circulation
  features

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Winter Pressure Patterns

• Figure 6.1(a), Page 96

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Summer Pressure Patterns

• Figure 6.1(b), Page 96

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Pressure Patterns Aloft

• Surface observations allow a dense network of
  pressure measurements
• Pressures aloft are measured by weather balloons
  and aircraft
• Pressure patterns aloft may be drawn on constant
  height maps, or on isobaric maps (where the
  height of that pressure level is plotted)

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Isobaric Chart (500 mb)

• Figure 6.2, Page 97

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Pressure Gradient Force

• The driving force for all air motions is
  variations in atmospheric pressure
• A force acts from high to low pressure, called
  the pressure gradient force
• PGF -(1/?) x (?P/?x)
• where ? is the density of the air and
• ?P/?x is the change in pressure over a
  horizontal distance
• Because the PGF increases as the change in
  pressure increases, wind speeds blow faster as
  isobars get packed closer together

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Vertical Pressure Gradient Force

• Since air pressure decreases exponentially with
  height, there is an upwardly directed pressure
  gradient force
• However, usually balancing the upward pressure
  gradient force is the downward force of gravity
• The balance between these two forces is called
  the hydrostatic balance

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Horizontal vs. Vertical Wind

• Because of the hydrostatic balance, the
  horizontal wind is much greater than vertical
  wind speeds
• Often the vertical component of the wind is
  neglected
• If the Earth did not rotate, the wind would blow
  directly from regions of highest pressure to
  lowest pressure

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Coriolis Force

• Because the Earth is rotating, air motions will
  appear to turn or deflect to the right of its
  path in the Northern Hemisphere
• This deflection is an apparent force, meaning
  it would not exist if it were not for the
  rotation of the Earth
• This apparent force is called the Coriolis force
• The Coriolis force depends on
• Earths rotation rate (assumed constant)
• Velocity of the moving object (Coriolis force
  increases as velocity increases)
• Latitude (Coriolis force is a maximum at the
  Poles, but is zero at the Equator)

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Coriolis Force

• Figure 6.4, Page 98

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Geostrophic Flow

• When the isobars (pressure contours) are
  straight, it is possible for the Coriolis force
  to be equal and opposite to the pressure gradient
  force
• This balance is called the geostrophic balance,
  and the wind that results is called the
  geostrophic wind
• In geostrophic flow, the wind blows parallel to
  the isobars with low pressure to the left

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Geostrophic Flow

• Figure 6.6, Page 99

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Curved Flow

• Rarely are isobars absolutely straight
• Cyclonic flow air has a counter-clockwise
  rotation around a low pressure area (in the
  Northern Hemisphere)
• Anticyclonic flow air has a clockwise rotation
  around a high pressure area

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Surface Winds

• Because of surface friction, the winds at the
  surface do not blow in geostrophic balance
• Friction slows the air down, and the slower
  velocity results in a weaker Coriolis force
• With the pressure gradient force now stronger
  than the Coriolis, the resultant winds blow
  across the isobars towards lower pressure

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Surface Winds

• Figure 6.8, Page 101

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Divergence Convergence

• Divergence is when a fluid extends its horizontal
  dimensions, or spreads out
• Convergence is when a fluid constricts its
  horizontal dimensions, or squeezes in
• To conserve mass in a vertical column of air,
  divergence at one level must exist with
  convergence at another level

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Mass Conservation

• Figure 6.12, Page 104

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Vorticity

• Vorticity is a fancy word that simply means
  spin
• There is a relationship between vorticity and
  convergence/divergence
• This relationship can be visualized by thinking
  of an ice skater
• Convergence (bringing arms in) results in an
  increase in vorticity (spins faster)
• Divergence (bringing arms out) results in a
  decrease in vorticity (spins slower)

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Vorticity and Convergence/Divergence

• Figure 6.13, Page 104