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Principles of Weather
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Hurricane Jeanne, 2004
Outline
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Impact of severe weather
Role of solar radiation
Properties of water, air masses
Effects from Earth’s rotation
Severe Weather
• Includes thunderstorms, cyclones, floods,
hurricanes, tornadoes
• Counting all natural hazard effects, severe
weather leads to ~75% of yearly fatalities
and damage
2004 - Hurricanes Jeanne, Ivan, Frances, and Charley produced ~150
fatalities, $41.5 billion damage (from NOAA website)
Weather vs. Climate
• Weather: condition of the atmosphere at any
particular time/place
• Made of several elements
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Air temperature and pressure
Humidity
Clouds
Precipitation
Visibility
Wind
Weather vs. Climate
• Climate: “Average weather”
• Described for a particular region over a
specified time
Atmosphere
• Thin gas envelope that surrounds the planet
• Most within 30 km of the surface of the planet
• Mostly composed of
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Nitrogen
Oxygen
Smaller amounts of water, carbon dioxide, ozone
Clouds with liquid water and ice
• Important: shields from UV radiation, space junk
Atmosphere
• Made of several layers
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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
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
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
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
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
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
Temperature and Water
• Amount of water in air: humidity
• Higher temperatures mean more water
vapor can be stored in the air (higher
humidity)
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.
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
Examples of Surface Pressures
• Commonly find high pressures in southwest
US in summer
• Arctic in winter - surface high pressures
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.
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
Effects on Winds
• If just difference in solar radiation (no
rotation), get simple convection cells
(Hadley cell)
Too Simple!
• Better
description
is a 3 cell
model
• Also
patterns
influenced
by
continents,
seasonal
variations in
radiation
Example fig
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
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.
Common Polar Jet Stream Path in U.S.
Characteristics of Large Air
Masses
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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
Fronts
• Boundaries of air masses
• Sloping surface separating air masses of
different temperatures, moisture content
• Typically where you get clouds,
precipitation, severe weather
Examples of Fronts
Fronts
• Advancing cold
front: pushes warm
air up
– Can produce
clouds,
thunderstorms
•Advancing
warm front:
gentle slope,
rises above cold
front
–Produces
widespread
clouds
• Cold front meeting warm front, bends rain
Next Time
• Mid-Latitude Cyclones and Thunderstorms
Mean 81.7
A: 90-100
B: 80-89
C: 70-79
D: 60-69
F: 59 and below
Midterm Exam
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!
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)
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)