Ch. 8 Atmospheric Circulation Lecture Notes Page

Chapter 8: Atmospheric Circulation
Ocean & Atmosphere are intertwined: Gases & waters freely exchanged
Wind, Weather, Climate
Atmospheric Structure (not covered in textbook)
Consists of Layers Separated by Temperature
Thermosphere (furthest above Earth): Temperature increases with altitude
Mesosphere: 30+ mi. above earth
Stratosphere: (ozone layer) 11-30 miles above Earth
Temperature increases with altitude
Little circulation
Injections of volcanic eruptions
Pollution affects this layer
Troposphere (closest to Earth – you’re in it right now)
Temperature decreases with elevation
Where “weather” occurs
Pollution affects this layer
Composition and Properties of Air
Transparent, odorless gases & water vapor (up to ~4% of volume)
Condenses as Precipitation
Dust particles, pollution
Dry Air Compostion: 78.1% Nitrogen gas (N2), 20.9% Oxygen (O2), 0.9% other gases
(CO2, Argon Methane, Neon, etc.
Atmospheric Pressure
Measure of air density (mass/volume)
Pressure (density) increases when cooled (molecules packed) or when water vapor
content decreases
Density decreases when air is warmed
Standard pressure: 760 mm mercury (Hg)
Low pressure (< 760 mm Hg): Warm, less dense air
High pressure (>760 mm Hg): Cool, more dense air
Isobars = Areas of equal pressure
Vertical Movement of Air Masses
Warm air rises (less dense) & expands
Holds water vapor well
Cool air sinks (more dense) & compresses
Holds less water vapor
Atmospheric Circulation
Earth-Sun Dynamics
Solar Energy (light) converted to heat energy
Uneven Heating on Earth b/c:
 Earth is a sphere = Radiation (heat) varies with latitude
 Earth is tilted on its axis = Radiation varies with season
Why are poles colder than equator?
 Earth is a sphere (fat in middle, skinny at top & bottom): Light passes through
more atmosphere at poles, so there is increased light absorption in atmosphere
above poles, so less solar energy (heat) reaches land & water at poles
 Solar Radiation Varies with Latitude
o Highest at Equator (direct rays)
o Decreases As move toward poles (oblique/angled rays)
 Reflection (albedo)
o Weak near equator (dark land & sea absorb heat)
o Strong near poles (white ice cover on land & sea reflect light, so less heat
is absorbed)
Heat Budget for the Earth (Heat Coming In = Heat Going Out to atmosphere)
*check out the excellent diagram in the text (pg. 180 fig. 8.3)
 Reflected by atmosphere (clouds, gas, surface) = heat loss
 Absorbed by atmosphere (clouds, gas) & surface (H2O, land) = heat gain
 Reradiation (Heat leaves Earth): surface & H2O
 Transfer (surface to Atmosphere): Evaporation
Wind & Ocean Currents distribute heat
Poles: Lose more heat than gain (Oblique solar rays & reflection by white ice on land &
Equator: Gains more heat than it loses (Direct solar rays & absorption by dark land &
Earth is tilted on its axis 23.5˚
As Earth rotates around sun (365 days), One pole or other is closer to sun:
Earth’s Tilt Causes Uneven solar Heating – Seasons:
 Tilt toward sun at Summer Solstice (June 21-North)
o At poles = 24 hrs. day
 Tilt away from sun at Winter Solstice (21 Dec – N.)
o At Poles = 24 hr. Night
 Equator has little seasonal variation in temp.
Winds on a Non-rotating Earth Would Be Simple:
Warm air rises at equator
Cools as it rises & water vapor condenses as rain
Surface winds blow from equator to poles
Dry, cool air, then sinks at the poles
Warm Air Rises, Cool Air Falls = Convection Current
Atmosphere in Motion:
Without Rotation:
Air warmed & rises at Equator (less dense)
Air cools & sinks at poles (more dense)
2-celled convection system: 1 in North. Hemispher & 1 in S. Hemisphere
Rotation of Earth: Leads to deflection of air masses = Coriolis Effect
Coriolis Effect
Eastward rotation of Earth on its axis 15°/hr
Deflects moving objects (air, water) away from initial course
Earth spins faster at Equator (0°): 1700 km/hr
Than at higher latitudes: 1260 km/h at 43°
Both are at Same Longitude (79° W)
Rotation distance varies with latitude
1. Buffalo, NY (43°N) Higher Latitude: shorter rotation distance (Earth is skinnier near
2. Quito, Ecuador (Equator = 0°): Longer rotation distance (Earth is fattest at center –
Rotation speed depends on Latitude:
Fastest at Equator, Zero at poles
Quito: 1,658 km/h
Buffalo: 1260 km/h
Both disks must rotate 15° degrees in 1 Hr. Or Earth would rip itself apart!
Wider Quito disk: Must spin faster to keep up
Coriolis Effect: Earth’s Rotation deflects moving objects (air, cannonballs)
Deflected: Clockwise (North. Hemis.)
Counterclockwise (South Hemis.)
No Deflection at Equator
Hemispherical wind cells get divided into 6 smaller latitudinal systems
 Northeasterly & Southeasterly Trade Winds
 “Hadley” cells (0 - 30° latitude)
 Warm, Equatorial air rises up to Stratosphere
 Blocked from rising or sinking (by warm air below)
 Forced either north or south of equator
 Strong, steady wind system (warm & moist)
 At 30º N&S it cools & descends
Trades (NE & SE)
 Converge at: Intertropical Convergence Zone (ITCZ)
o a.k.a. Equatorial Doldrums
Low pressure belt of rising air (warm, humid)
No Coriolis deflection at equator = calm area of little winds
A lot of rain here
Bordered at high latitudes by a high pressure belt (dry, sinking air): “Horse
Northern & Southern Westerlies
 “Ferrel” cells (30 - 60° latitude)
 Intermediate belt between Hadley & Polar Cells
 Zone of Mixing:
o Warm air flowing North
o Cold air flowing South
 Winter storm systems typically ride this belt from west to east
 Westerlies (N & S): Meet colder, dense air flowing from poles to equator
o Converge at Polar Front
 Bordered at Low latitudes by a subtropical high pressure belt
Polar Cells (60°– 90° Latitude)
0 (Equator) Air rises
30 (N & S) Air falls
60 (N&S) Air rises
90 (N& S - poles) Air falls
Storms Created when Westerlies & Polar air masses collide
 Westerlies = warm moist air (low pressure system)
 Polar winds = cold, dry, dense air (High pressure system)
 2 air masses Will not mix:
o Cold air pushes up warm air
 Surface wind motion generated around high & low pressure systems Due to
Coriolis Effect
Frontal Storms: Spinning air masses linger for 2-5 days
Warm, moist air cools as it rises
Drops moisture as rain or snow (cold air doesn’t hold moisture well)
Cool air sinks, compresses, & warms
Mid-latitude weather: Ex: Nor’easters
Summer (Rainy):
Land warms in summer, air rises
Cold, dense air rushes to "fill the gap" caused by the rising warm air
Draws cooler, moist air from ocean
Winter (Dry):
Sea is warmer than land = warm, moist air rises above ocean
Cool continental air drawn towards ocean to “fill in the gap”
EX: S. India, S. Arizona (Gulf of California)
Coastal Areas (Land & Sea Breezes)
 Occurs daily
 Day: warm land air rises & is Replaced by cool sea air
o Onshore wind
 Sunset: winds shift (breezy)
 Night: Land cools (faster than sea)
o Warmer sea air rises & is Replaced by cool land air
o Offshore wind
Hurricanes (Typhoon)
Form from equatorial trade winds
Begin in warm water: >26°C (79°F)
Moist, low pressure
Convergence of rotating wind
Begin as Tropical Storms
Storm becomes hurricane at wind speeds: 119km (74 mi)
Dissipate over land or cold water
Move westward with Trade winds
Form between 10º-25º N & S Latitude
Hot, humid tropical air rises, drops moisture as it cools (as rises in altitude)
 Heat Energy released when water vapor converted to liquid
 Heat energy drives the cyclone
 2.4 trillion kw hours of power generated per day
Storm Surge: Elevated Seawater Driven by Hurricane [As high as 12 m (40 Ft)]
Onshore rush of water caused by high winds pushing on ocean's surface
Wind causes water to pile up above sea level
Shoreline bathymetry can have added effect
Particularly damaging if occurs at high tide
Combines effects of surge & tide
Hurricane Katrina (2005):
Greatest recorded storm surge (USA) = 9 meters (30 feet) high at Biloxi, Mississippi
23-31 Aug. 2005
Max wind -175 mph (at sea)
Most destructive U.S. storm
Hit land twice:
1st landfall: Cat. 4 (140 mph): Plaquemines Parish, LA
2nd landfall: Cat. 3 (125 mph): MS/LA border
Inland flooding causes most hurricane-related deaths
New Orleans: Built on an Estuary
Levees cut-off Mississippi River sediment supply
Sinking below sea level
Studying Hurricanes
 Unmanned aircraft: data on temperature, pressure, humidity, wind speed &
direction (200-500m above sea surface)
 Satellites: wind speed & direction, Rainfall, directions of motion
 Manned aircraft (1.5- 3.5km):
 Use Data collection devices that fall through a storm