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.

Solar wind removes initial 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

III. Circulation of the AtmosphereD. Movement of air

1. Vertical motion2. Horizontal motion

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.

Flux of solar radiation less at higher latitudes; max. at equator

Earth

Sun

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

500 km

15 km

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.

Northern Hemisphere

CloudMotion

Southern Hemisphere

CloudMotion

LOW pressure

LOW pressure

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?