PowerPoint Presentation - Geophysics 189 Natural Hazards

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Principles of Weather
Hurricane Jeanne, 2004
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Outline

• Impact of severe weather
• Role of solar radiation
• Properties of water, air masses
• Effects from Earths rotation

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Severe Weather

• Includes thunderstorms, cyclones, floods,
  hurricanes, tornadoes
• Counting all natural hazard effects, severe
  weather leads to 75 of yearly fatalities and
  damage

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2004 - Hurricanes Jeanne, Ivan, Frances, and
Charley produced 150 fatalities, 41.5 billion
damage (from NOAA website)
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Weather vs. Climate

• Weather condition of the atmosphere at any
  particular time/place
• Made of several elements
• Air temperature and pressure
• Humidity
• Clouds
• Precipitation
• Visibility
• Wind

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Weather vs. Climate

• Climate Average weather
• Described for a particular region over a
  specified time

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Atmosphere

• Thin gas envelope that surrounds the planet
• Most within 30 km of the surface of the planet
• Mostly composed of
• Nitrogen
• Oxygen
• Smaller amounts of water, carbon dioxide, ozone
• Clouds with liquid water and ice
• Important shields from UV radiation, space junk

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Atmosphere

• Made of several layers
• Troposphere
• Stratosphere
• Mesosphere
• Thermosphere
• Weather contained within troposphere
• From surface to 11 km above, air molecules are
  well stirred, rising and falling in this layer

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Air pressure

• Air molecules have weight - exert force on
  surface (atmospheric pressure)
• Weight of air above, so pressure decreases with
  height
• Measured in units of pressure (millibars)
• Sea level atm. pressure 1013 mbar
• Commonly measured with barometer
• Height of mercury
• 1013 mbar 30 inches barometric pressure

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Air pressure and temperature

• Packet of air rises
• Air pressure lower, air expands
• Temperature related to air molecule speed
• Expansion lowers speed, lowers temp
• Rising air expands, cools
• Packet of air sinks
• Air pressure higher, air contracts
• Squeezing leads to more collisions, higher speed,
  higher temp
• Sinking air contracts, warms

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Role of Atmospheric Heating

• Sun provides significant amount of heat
• Reflected and absorbed
• Most radiation absorbed in equatorial belt
  (32ºN-34ºS)
• Polar latitudes have net cooling

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Fate of Heat

• Some trapped in rock at surface
• Some absorbed in water/water vapor in air
• Different amounts of heat lead to heat transport,
  moving air masses, storms

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Water and Heat

• High heat capacity
• Water and air can move heat around through
  convection
• Water changing phase - change in heat
• Ice to water latent heat stored in water
• Water evaporation latent heat of vaporization
• Vapor condensation latent heat of condensation

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Temperature and Water

• Amount of water in air humidity
• Higher temperatures mean more water vapor can be
  stored in the air (higher humidity)

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Back to Air Movement

• Convection warm air rises, cool air sinks
• Warm rising air -
• Pressure, temp lower, less water vapor it can
  hold. Condensation forms clouds.
• Cool sinking air -
• Pressure, temp higher, can hold more water vapor.
  Clear skies result.

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Vertical vs. Horizontal Motion of Air

• Vertical motions set up pressure differences at
  surface
• Hot air at surface wants to rise - lower
  pressures at surface
• Cold air sinking leads to higher pressures at
  surface
• Horizontal motion from high to low pressures
• If pressure gradient only force, winds move
  strictly from high to low pressure

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Examples of Surface Pressures

• Commonly find high pressures in southwest US in
  summer
• Arctic in winter - surface high pressures

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Effects from Rotation

• Toss a ball on a merry-go-round

Still straight-line motion, but merry-go-round
moves beneath it. Sitting on merry-go-round, it
appears like some force deflected the ball.
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Coriolis Effect

• Apparent force due to rotation of Earth (changes
  direction, not speed)
• Northern Hemisphere winds deflected to right
• Southern Hemisphere winds deflected to the left
• Effect greatest near poles, less at equator
• Important for paths of ocean currents, large
  winds, hurricanes

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Effects on Winds

• If just difference in solar radiation (no
  rotation), get simple convection cells (Hadley
  cell)

Too Simple!
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• Better description is a 3 cell model
• Also patterns influenced by continents, seasonal
  variations in radiation

Example fig
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Jet Streams

• Narrow high velocity winds that flow west to east
  
• Operate high in the atmosphere (10-14 km)
• 2 main jets
• Polar more impact on weather
• Subtropical

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Polar Jet Stream

• West-east motion occurs along boundary between
  polar cells and mid-latitude cells
• High temperature contrast leads to high velocity
  of the jet stream
• Path is variable, influences movement of large
  air masses
• Summer over Canada
• Winter over U.S.

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Common Polar Jet Stream Path in U.S.
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Characteristics of Large Air Masses

• Polar air masses cool
• Tropical air masses warm
• Land based air masses dry
• Water based air masses moist
• Over North America, dominant direction of motion
  is W-E

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Fronts

• Boundaries of air masses
• Sloping surface separating air masses of
  different temperatures, moisture content
• Typically where you get clouds, precipitation,
  severe weather

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Examples of Fronts
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Fronts

• Advancing cold front pushes warm air up
• Can produce clouds, thunderstorms

• Advancing warm front gentle slope, rises above
  cold front
• Produces widespread clouds

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• Cold front meeting warm front, bends rain

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Next Time

• Mid-Latitude Cyclones and Thunderstorms

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Midterm Exam
Mean 81.7 A 90-100 B 80-89 C 70-79 D 60-69 F
59 and below
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Problem Questions

• 2 Compressional forces lead to reverse faults
  (hanging wall moves up)
• 4 New Madrid region is an old rift zone
• 9 Earth is 4.6 billion years old (4600 million)
• 13 Magma viscosity is lowered by higher
  temperatures, lower crystal content, lower SiO2
  (answer all of these)
• 14 Lake Nyos event was a big carbon dioxide
  burp!

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Problem Questions

• 17 P waves can travel through solid, liquid,
  and gas
• Translational slides move down on weak planar
  surfaces
• Historical earthquakes in the Rio Grande rift
  (hint here) have been related to extensional
  forces, not compressional forces (answer false)

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Hints for Next Time

• Read each question carefully
• Look for hints in the question
• Attend class, particularly when video clips are
  presented (not posted on website)