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 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– 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
– 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– 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
• 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
Midterm ExamMean 81.7
A: 90-100
B: 80-89
C: 70-79
D: 60-69
F: 59 and below
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)
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