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III. Circulation of the AtmosphereA. Atmosphere, Weather and Climate
Some history: All cultures recognize impacts of climate, and want to predict and influence the weather.
Formal study of the atmosphere goes back tothe Greeks. Aristotle’s treatiseMeteorologica, “discourse on things above”.
III. Circulation of the AtmosphereA.Atmosphere, Weather and Climate
Some history: Initially humans most interested in extremes of weather: drought, flood, violent storms.
More recently, we are interested in how industrial activity influences weather and human health.
III. Circulation of the AtmosphereA.Atmosphere, Weather and ClimateSome historical examples:
Dec. 1952 London fog
Los Angeles in the late 1950s
III. Circulation of the AtmosphereA.Atmosphere, Weather and ClimateSome historical examples:
Local problems: original solutions…….
• Time release of pollutants with storms
• Build taller smokestacks
Effective locally, but using the atmosphere as an infinite sewer led to regional problems: acid rain, GHG, ozone depletion.
To understand how human activity might influence the atmosphere, weather and climate, we need to understand not only its structure, but why and how it moves.
III. Circulation of the AtmosphereA. Atmosphere, Weather and Climate
Atmosphere: Gaseous layer from Earth’s surface to the “edge of space”.
For us: Troposphere, Tropopause, StratosphereWeather:
Climate:
State of the atmosphere at a particular place or region, for a short period of time.
Average weather (for a place or region). Usually average of last 30 years.
III. Circulation of the AtmosphereA. Atmosphere, Weather and ClimateB. Origin of the Atmosphere
Earth’s early atmosphere very different than today’s….. Dense (10% of Earth’s mass) and composed of CO, H2S, N2, H2, H20.All was lost to the intense solar wind during earliest episode in Earth’s history.
Present atmosphere: Derived from outgassing of Earth’s hot interior. Initially different in composition from the modern atmosphere.
III. Circulation of the AtmosphereA. Atmosphere, Weather and ClimateB. Origin of the AtmosphereWhy doesn’t the atmosphere just drift off into
space?
Some of it does….but its only the lightest gases….hydrogen (H2) and helium (He2).
The rest is held in place by gravity.
III. Circulation of the AtmosphereA. Atmosphere, Weather and ClimateB. Origin of the AtmosphereC. State of the Atmosphere
a. Composition (nearly constant)b. Temperature, Pressure, Humidityc. Winds
IV. Circulation of the AtmosphereD. Movement of air
1. Vertical motion: buoyancy (density).
2. Horizontal motion: Air moves horizontally because of differences in pressure. Air always moves from high pressure to low pressure.
Rate of movement (wind speed) depends on the pressure gradient.
III. Circulation of the AtmosphereD. Movement of air
1. Vertical motion: buoyancy (density). 2. Horizontal motion: differences in pressure. 3. The Heat Engine
a) Sea Breeze, a simple heat engine.b) The Earth as a heat engine.
III. Circulation of the AtmosphereD. Movement of air
4. The Coriolis Effect: Tendency for fluids (air or water, or anything moving in them) moving across Earth’s surface to be deflected from a straight line path.
Not a real force
III. Circulation of the AtmosphereD. Movement of air
4. The Coriolis Effect: Coriolis Rules of thumb• NH to right; SH to left• Effect is small (cannot impact draining
bathtub, etc) • Biggest effect is on large objects.
• Impact increases as speed of object increases.
• Coriolis Effect is zero at the equator.
III. Circulation of the Atmosphere5. General Circulation of the Atmosphere
a. Hadley Cell: 0 to 30° on either side of the equator.b. Ferrel Cell: Indirect, 30 to 60° on either side of the equator.c. Polar High (or Polar Easterlies): 60-90°
ITCZ: Inter-Tropical Convergence ZoneDoldrumsHorse LatitudesTropical EasterliesMid-latitude Westerlies, Polar Easterlies
IV. Circulation of the Atmosphere5. General Circulation of the Atmosphere
d. Consequences of large-scale circulationDistribution of major deserts, dustDistribution of rainfallAverage zonal winds (Trades,
Westerlies)
III. Circulation of the Atmosphere6. Seasonal contrasts:
• Because Earth’s axis is tilted relative to its orbit around the Sun, we have seasons.
III. Circulation of the Atmosphere6. Seasonal contrasts:
Maximum flux of solar radiation shifts north and south of the equator over the annual cycle.
This exerts a powerful influence on atmospheric circulation, imparting a strong seasonal cycle over the large-scale patterns of atmospheric motion.
III. Circulation of the Atmosphere6. Seasonal phenomena
a. Temperature
Tied to the different properties of land vs ocean (just like our sea breeze analogy).
•
•
•
Albedo
Heat capacity
Convection (water) vs Conduction (solids)
(Continentality: range of temperature during a seasonal cycle)
III. Circulation of the Atmosphere 6. Seasonal phenomena
a. Temperature (Continentality: range of temperature during a seasonal cycle)
Tropical regions and maritime regions (coastal) have low continentality.
Mid-latitude large continents have high continentality.
Poles are lower.
III. Circulation of the Atmosphere6. Seasonal phenomena
b. Hurricanes
They are all the same thing
(Atlantic)Typhoons in the PacificCyclones in the Southern Hemisphere
SAFFIR/SIMPSON DAMAGE POTENTIAL SCALE
CATEGORY WINDSPEED (MPH) PRESSURE (MB) SURGE (FT)
1 74 – 95 > 980 4 - 52 96 – 110 965 – 979 6 - 83 111 - 130 945 – 964 9 - 124 131 - 155 920 - 944 13 - 185 > 155 < 920 > 18
IV.Circulation of the Atmosphere6. Seasonal phenomena
b. Atlantic HurricanesHow and where do they start?
IV.Circulation of the Atmosphere6. Seasonal contrasts
b. Atlantic HurricanesStart: As small low pressure disturbances off
AfricaRequire:
•••
Warm water (>26 °C)(High evaporation rates)Stable troposphere
(to develop the spiral)Track that keeps them over warm waterTo provide “fuel” through the release of latent heat of condensation
Eva
pora
tion
rat
eThe rate of evaporation from the ocean is exponentially related to
temperature of the water.
The role of water vapor in hurricanes
Release of latent heat is THE primary fuelthat intensifies hurricane winds.
Without latent heat there would be no hurricanes.
IV.Circulation of the Atmosphere6. Seasonal phenomena
b. Atlantic HurricanesPrediction?
Start First Principles
Basic circulation of the atmosphere
Coriolis
Why in Sept instead of July when hottest?
Will Global Warming cause more or stronger hurricanes?
What can we say from First Principles?E
vapo
rati
on r
ateThe rate of evaporation from the
ocean is exponentially related to
temperature of the water.
Who cares?