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)