III. Circulation of the Atmosphere A. 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 to the Greeks. Aristotle’s treatise Meteorologica, “discourse on things above”.

III. Circulation of the Atmosphere A.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 Atmosphere A.Atmosphere, Weather and Climate Some historical examples: Dec London fog Los Angeles in the late 1950s

III. Circulation of the Atmosphere A.Atmosphere, Weather and Climate Some 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 Atmosphere A. Atmosphere, Weather and Climate Atmosphere: Gaseous layer from Earth’s surface to the “edge of space”. For us: Troposphere, Tropopause, Stratosphere Weather: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 Atmosphere A. Atmosphere, Weather and Climate B. Origin of the Atmosphere Earth’s early atmosphere very different than today’s….. Dense (10% of Earth’s mass) and composed of CO, H 2 S, N 2, H 2, H 2 0. 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 Atmosphere A. Atmosphere, Weather and Climate B. Origin of the Atmosphere Why doesn’t the atmosphere just drift off into space? Some of it does….but its only the lightest gases….hydrogen (H 2 ) and helium (He 2 ). The rest is held in place by gravity.

III. Circulation of the Atmosphere A. Atmosphere, Weather and Climate B. Origin of the Atmosphere C. State of the Atmosphere a. Composition (nearly constant) b. Temperature, Pressure, Humidity c. Winds

III. Circulation of the Atmosphere D. Movement of air 1. Vertical motion 2. Horizontal motion

IV. Circulation of the Atmosphere D. 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 Atmosphere D. 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 Atmosphere D. 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 Atmosphere D. 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 Atmosphere 5. 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 Zone Doldrums Horse Latitudes Tropical Easterlies Mid-latitude Westerlies, Polar Easterlies

IV. Circulation of the Atmosphere 5. General Circulation of the Atmosphere d. Consequences of large-scale circulation Distribution of major deserts, dust Distribution of rainfall Average zonal winds (Trades, Westerlies)

III. Circulation of the Atmosphere 6. Seasonal contrasts: Because Earth’s axis is tilted relative to its orbit around the Sun, we have seasons.

III. Circulation of the Atmosphere 6. 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 Atmosphere 6. 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 Atmosphere 6. Seasonal phenomena b. Hurricanes They are all the same thing (Atlantic) Typhoons in the Pacific Cyclones in the Southern Hemisphere

500 km 15 km

IV.Circulation of the Atmosphere 6. Seasonal phenomena b. Atlantic Hurricanes How and where do they start?

IV.Circulation of the Atmosphere 6. Seasonal contrasts b. Atlantic Hurricanes Start: As small low pressure disturbances off Africa Require: Warm water (>26 °C) (High evaporation rates) Stable troposphere (to develop the spiral) Track that keeps them over warm water To provide “fuel” through the release of latent heat of condensation Evaporation rate The 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 fuel that intensifies hurricane winds. Without latent heat there would be no hurricanes.

Northern Hemisphere Cloud Motion Southern Hemisphere Cloud Motion LOW pressure LOW pressure

IV.Circulation of the Atmosphere 6. Seasonal phenomena b. Atlantic Hurricanes Prediction? 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? Evaporation rate The rate of evaporation from the ocean is exponentially related to temperature of the water. Who cares?