What set the atmosphere in motion

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What set the atmosphere in motion?
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Review of last lecture

• Earths energy balance at the top of the
  atmosphere and at the surface. What percentage of
  solar energy is absorbed by the surface?
• Atmospheric influences on radiation (3 ways)
• What cause the greenhouse effect? What are the
  major greenhouse gases? What is the wildcard?
• The three types of atmospheric scattering. What
  causes the blue sky? Why causes the
  reddish-orange sunsets? What causes the colors of
  rainbow?
• Basic characteristics of global temperature
  distribution (T decreases poleward, isotherm
  shifts seasonally, T over land gt over ocean in
  summer).

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Atmospheric Thickness

• No defined top to the atmosphere
• The atmosphere is very shallowand is less than
  2 of the Earths thickness

Over 90 of atmosphere in the lowest 16km is
where nearly all weather occurs
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Temperature Layers
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Troposphere

• Depth varies based largely on temp.
• Thinnest layer
• Thickest at equator
• Temperature decreases with height (heat source is
  from sfc, farther you are from sfc, colder it is)
• Nearly all weather occurs here, planes fly here,
  all mountain peaks are in troposphere
• Contains 80 of the atmospheric mass

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Pressure Essentials

• Pressure force exerted/unit area (weight above
  you)
• units - Pascals (Pa) or millibars (mb) (1 mb
  100 Pa)
• Average surface pressure over globe 1013.2
  mb.
• Atmosphere is mixture of gases -gt partial
  pressure.
• Daltons Law sum of partial pressures equals
  total pressure
• Pressure gradient (pressure difference between
  two locations/distance) gives rise to a force
  (pressure gradient force), which sets the air in
  motion.

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The Equation of State (Ideal Gas Law)
Pressure density x temperature x 287 J kg-1
K-1 p ?TR

• Describes relationships between pressure,
  temperature, and density (Start w/ molecular
  movement in sealed container ? Pressure
  proportional to rate of collisions between
  molecules and walls).
• At constant temperatures, an increase in air
  density will cause a pressure increase (Add more
  molecules ? increase density ? increase rate of
  collisions ? raise pressure)
• Under constant density, an increase in
  temperature will lead to an increase in pressure
  (Raise temperature ? increase speed of molecules
  ? increase rate of collisions ? raise pressure)

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Vertical pressure distribution and vertical
pressure gradient

• Pressure decreases with height
• Pressure decreases non-linearly w/ height (Why?
  Because air is compressible, so denser near the
  surface)
• Vertical pressure gradient

At sea level, p 1000 mb At 10 km, p 300 mb ?
Therefore Gradient (1000mb-300mb)/10km 70
mb / km !!
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So why dont we have huge vertical
winds??Answer Hydrostatic Equilibrium

• explains why air doesnt continuously blow upward
  or get pulled downward
• The downward force of gravity (weight of parcel)
  is balanced by a strong vertical pressure
  gradient (VPG) ? creates hydrostatic equilibrium
• ?p/?z?g

?p/?z
Vertical Pressure Gradient
weight of parcel
?g

• denser atmosphere experiences greater
  gravitational force
• force mass (density) x acceleration (gravity)
• to maintain hydrostatic equilibrium balanced by
  greater VPG
• P decreases upwards, z increases upwards ? ?p/?z
  is negative ? VPG directed upwards
• Local imbalances initiate various up- and
  downdrafts

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Horizontal pressure distribution and horizontal
pressure gradient

• Pressure maps depict isobars, lines of equal
  pressure
• Through analysis of isobaric charts, pressure
  gradients are apparent
• Steep pressure gradients are indicated by closely
  spaced isobars
• Typically only small gradients exist across large
  spatial scales (4 variation at continental
  scale), smaller than vertical gradients

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Horizontal Pressure Gradients and wind

• The pressure gradient force initiates movement of
  atmospheric mass, wind, from areas of higher to
  areas of lower pressure
• Horizontal wind speeds are a function of the
  strength of the pressure gradient

SLP and winds plotted on same chart ? Notice the
strong winds in Ohio due to tight pressure
gradient
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Forces Affecting the Speed and Direction of the
Wind

• Horizontal pressure gradients responsible for
  wind generation
• Three factors affect wind speed and/or direction
  (velocity)
• Pressure Gradient Force  (PGF)
• Coriolis Effect  (CE)
• Friction Force (FF)

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1. Pressure GradientForce(PGF)

• pressure gradient high pressure ? low pressure
• pressure differences exits due to unequal
  heating of Earths surface
• spacing between isobars indicates intensity of
  gradient
• flow is perpendicular to isobars

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2. The Coriolis Effect

• objects in the atmosphere are influenced by the
  Earths rotation
• Rotation of Earth is counter-clockwise looking
  down from N. Pole.
• results in an apparent deflection (relative to
  surface)
• deflection to the right in Northern Hemisphere
  (left in S. Hemisphere)
• Greatest at the poles, 0 at the equator
• Increases with speed of moving object and
  distance
• CE changes direction not speed

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Winds in the upper airGeostrophic Balance
Friction is very small in the upper air

• Now the wind speed/direction is simply a balance
  between the PGF and CE. This is called
  GEOSTROPHIC BALANCE.
• Upper air moving from areas of higher to areas
  of lower pressure undergo Coriolis deflection
• Air will eventually flow parallel to height
  contours as the pressure gradient force balances
  with the Coriolis force

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Winds near the surface
The third term (friction) must be considered

• Friction slows down wind speed and reduces
  Coriolis deflection
• Friction is important for air within 1.5 km of
  the surface (the so-called planetary boundary
  layer). It varies with surface texture, wind
  speed, time of day/year and atmospheric
  conditions. Friction above 1.5 km is often small
  (often called the free atmosphere), except over
  regions with storms and gravity waves.

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Pressure Gradient Coriolis Friction Forces
w/out Friction (gradient wind)
w/ Friction
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Cyclones, Anticyclones, Troughs and Ridges

• High pressure areas (anticyclones) ? clockwise
  airflow in the Northern Hemisphere (opposite flow
  direction in S. Hemisphere)
• Characterized by descending air which warms
  creating clear skies
• Low pressure areas (cyclones) ? counterclockwise
  airflow in N. Hemisphere (opposite flow in S.
  Hemisphere)
• Air converges toward low pressure centers,
  cyclones are characterized by ascending air which
  cools to form clouds and possibly precipitation

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Cyclones, Anticyclones, Troughs and Ridges on
weather charts

• Isobars usually not closed off at highest levels
  Troughs (low pressure)/Ridges (high pressure)
• Isobars usually closed off at lowest levels
  Cyclones, Anticyclones

Highest? Level
?Lowest Level
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The unanswered questions (challenges) in
atmospheric circulation

• How large is the boundary layer friction under
  different weather conditions?
• How large is the upper air friction due to
  storms? (called cumulus friction or convective
  momentum transport)

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Summary

• Four layers of the atmosphere, what separate
  them?
• Definition of pressure and its unit.
• Definition of pressure gradient. Pressure
  gradient sets the air in motion.
• Equation of state (Relationship between P, ?, and
  T)
• Vertical Pressure Distribution. How does pressure
  change with height? What is the hydrostatic
  equilibrium?

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Summary (cont.)

• Know 3 Forces that affect wind speed /direction
• Especially work on Coriolis force, as this is the
  hardest to understand. Which direction is air
  deflected to by Coriolis force?
• What is the geostrophic balance? At which level
  is it valid? Difference between upper level and
  surface winds
• Does cyclones correspond to high or low surface
  pressure? Is the air moving clockwise or
  counter-clockwise around them? How about
  anticyclones?