geology lecture 18
TRANSCRIPT
Chapter 20
Atmosphere and Climate
Chapter 20
Outline• Atmosphere -what is it?
-Composition, coloration, P-T-density relationships-Relative humidity, latent heat, troposphere
• Circulation-Gradients and energy input-Movement, prevailing winds, high winds (jet streams)
• Weather-Air masses, fronts, clouds and precipitation-Storms (thunderstorms, tornadoes, hurricanes)
• Climate-Controls, belts, variability (El Nino example)
Chapter 20
Chapter 20
Introduction• Earth has a well-developed atmosphere (atm).
• gas mixture called
• Density & pressure variations cause• Atmosphere governs
• Temperature (T).• Pressure (P).• Moisture content.• Wind velocity. • Wind direction.
• Climate is
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Atmospheric Components• Present atmosphere comprised of a gas mix:
• Nitrogen 78%• Oxygen 21%• Other gases 1%
• Aerosols –
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Atmospheric Coloration
• Color due to• Light scattered passing through• Some light returns
• Why is the sky blue?• When the Sun is• Gases scatter
• Why is the sky red?• Setting Sun passes
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Pressure and Density• Air pressure –
• Greatest near• Decreases• 14.7 psi (1 atm) at sea level.
• Air density – • Maximum at• Decreases
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P and T Relations
• P & T conditions change with• P - higher near
• When air moves from higher to lower P, it…• Expands & cools.
• Moving from lower to higher P, it…
• Called
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Relative Humidity• Air has varying water amounts:
• Dry (desert) 0.3%• Humid (tropical rainforest) 4.0%
• Water content described by• Ratio (%) of measured• Dry air -
• Humid air -
• 100% relative humidity air is• Under-saturated air has <100%
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Relative Humidity• Moisture content changes with T.
• Cold air holds less; warm air more.
• Warm, under-saturated air becomes saturated as it cools.• Saturation T is the• Below dewpoint…
• Water forms
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Relative Humidity• Rising air cools (adiabatically) to form
• Common phenomena ->
• Clouds can dissipate by adiabatic
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Latent Heat• Water in air can
• With state changes, air T also• T change is not due to external energy; hence, “latent.”• Instead, derives from
• Evaporating water• Condensing water
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Atmospheric Layers• Atmosphere is thermally layered.
• Troposphere (0 - 9 to 12 km).• Mixing layer.
• All weather is here.
Chapter 20
Outline• Atmosphere -what is it?
-Composition, coloration, P-T-density relationships-Relative humidity, latent heat, troposphere
• Circulation-Gradients and energy input-Movement, prevailing winds, high winds (jet streams)
• Weather-Air masses, fronts, clouds and precipitation-Storms (thunderstorms, tornadoes, hurricanes)
• Climate-Controls, belts, variability (El Nino example)
Chapter 20
Chapter 20
Atmospheric Circulation• Troposphere experiences
• Wind velocities vary from• Wind circulation has both• Local –.
• Global –
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Pressure Gradients• Lateral pressure differences• Pressures mapped by
• Isobars cannot• Air flows from high to low P• Steeper the gradient,
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Energy Input• Air circulation is result of
• Warm air expands,• This air is replaced by
• Convection driven by• Solar energy =
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Energy Input• Solar energy bathing Earth is not
• Vertical Sun rays have• Oblique rays
• Tropics (vertical rays) receive• Poles (oblique rays) receive
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Seasons• Seasons due to• Earth orbits Sun, vertical rays
• More north• More south
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Seasons – January vs. July
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Atmospheric Movement• Troposphere divided into
• Hadley cells – • Ferrel cells – • Polar cells –
• Hadley cell – Rising
equatorial air creates
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• Rotation (via Coriolis effect), complicates• Cell airflow is deflected
• Forms• Cooling air
Atmospheric Movement
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Prevailing Winds• Result is • Called
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High Winds• Troposphere thickness changes
• Warm equatorial air• Cold polar air
• At given altitude, equatorial pressure• Causes equatorial high-altitude air• Air atop Hadley cells spill over top of Ferrel cells.
• Coriolis deflects
Chapter 20
High Winds• High-altitude pressure gradient
• Over
• High-altitude westerlies can
• Called
Chapter 20
Outline• Atmosphere -what is it?
-Composition, coloration, P-T-density relationships-Relative humidity, latent heat, troposphere
• Circulation-Gradients and energy input-Movement, prevailing winds, high winds (jet streams)
• Weather-Air masses, fronts, clouds and precipitation-Storms (thunderstorms, tornadoes, hurricanes)
• Climate-Controls, belts, variability (El Nino example)
Chapter 20
Chapter 20
Weather• Local-scale conditions of• Reflects
• Variation in• Land vs. sea.
• A weather system affects
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Air Masses
• Air packages with• >1,500 km across, • Characteristics reflect• Weather changes dramatically when
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Fronts• Fronts -
• Curved surfaces that lead
• Cold fronts:• Steep• Flow• Pushes up
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Fronts• Warm front:
• More gradual• Warm air• Pushes
• Incline reflects
• Warm air rising up the front causes
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Clouds and Precipitation• Water vapor in saturated air
• Condensing as• Precipitating as
• Condensation nuclei• Microscopic
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Clouds and Precipitation• Several air-lifting mechanisms.
• Convective lifting – • Frontal lifting – • Convergence lifting – • Orographic lifting –
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Clouds and Precipitation• Rain, snow, sleet form in 2 ways,
• Collison & coalescence – • Drops fall when
• Typical raindrops are
• Drops >5 mm
• Cold air near ground turns rain
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Cloud Types• Clouds form in troposphere, controlled by:
• Air stability• Elevation at which moisture condenses• Wind conditions
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Cloud Types• Clouds described by shape:
• Cirrus – wispy, thin, feathery• Cumulus – puffy, cottony• Stratus – stable, layered
• Prefixes narrow cloud types.• Cirro – high-altitude• Alto – mid-altitude• Nimbo – rain-producing
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Storms• Storms develop along
• Centered by low pressure• Fueled by warm, moist air• Result: lightening, wind, rain, hail/sleet/snow
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Thunderstorms• Local pulses of intense rain, wind, lightning• Rising air forms cumulus clouds• Latent heat released by condensing water warms air• Cumulus clouds build upward
• Anvil head develops• Heavy rains ensue
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Thunderstorms• Lightning is electrical charge separation in clouds
• Scientists do not fully understand why this happens• Cloud bases develop a negative charge• Result: buildup of positive group charge• Air is a good insulator; prevents charge dissipation• Eventually, charge imbalance overwhelms air
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Thunderstorms• Lightning leader advances from cloud base• Return stroke starts from ground• Connect to form the bolt• Thunder is a direct result
• Bolt heats air 8K to 30K degrees C• Air expands explosively.
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Tornadoes• Near-vertical rotating funnel-shaped vortex cloud• Air moves with violent speed about a rotation axis
• Local winds up to 500 km/h (300mph)• Extremely destructive
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Tornadoes• Tornadoes develop along steep P gradients
• Strong W winds carry polar air• Strong SE surface winds carry warm moist air
• Shear initiates horizontal rotation• Drafts tip the rotating cylinder upright
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Tornadoes• Tornadoes prevalent in Midwest US
• Proper conditions; March to September• Cold polar air from Canada sweeps south
• Warm moist air pushed north from Gulf of Mexico
• Tornado-prone region called “Tornado Alley”
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Hurricanes• Huge low-P cyclonic storms from tropical Atlantic.
• Defined by sustained winds >119 km/hr (74 mi/hr)• Fueled by warm ocean winds (>27 degrees C)• Originate in low latitudes (<20 degrees N) with warm water
• Do not form near equator (insufficient lateral winds)
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Hurricanes• Hurricanes develop in summer & late fall.• Form over warm tropical ocean waters off W. Africa
• Cyclonic low-P “tropical disturbances” pull air inward• Air rises, cools, condenses; releases latent heat• Heat buoys air, creates lower P, pulls in more air• Over time, storm gains size and strength• Size range – 100 to 1500 km
• Strength – >250 km/hr
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Hurricanes• Storm “named” when winds exceed ~60 km/hr
• Named in alphabetical order• Alternating male/female with varying national origin
• Hurricane tracks move W and N, often crossing land• Landfall removes fuel (warm, moist air)
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Hurricanes• Hurricane-like storms outside the Atlantic are called…
• Typhoons – Western Pacific Ocean• Cyclones – Northern Indian Ocean
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Hurricanes
• Intensity is ranked • Category 1: Wind speed > 119 km/h; pressure > 980 mbars• Category 5: Wind speed > 250 km/h; pressure < 920 mbars
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Hurricanes• 2005 hurricane season set records:
• Most named storms (26) – previous record 21 in 1933.• Most hurricanes (13) – Previous record 12 in 1969.• Most category 5s (3) – Previous record 2 in 1960 and 61.• Most major hurricanes (Cat. 3 or higher - 7).• Most major hurricanes in the U.S. (4).
Increased stormy trend likely reflects climate change.
Chapter 20
Outline• Atmosphere -what is it?
-Composition, coloration, P-T-density relationships-Relative humidity, latent heat, troposphere
• Circulation-Gradients and energy input-Movement, prevailing winds, high winds (jet streams)
• Weather-Air masses, fronts, clouds and precipitation-Storms (thunderstorms, tornadoes, hurricanes)
• Climate-Controls, belts, variability (El Nino example)
Chapter 20
Chapter 20
Climate
• “climate” refers to persistent weather patterns • Long-term (30+ years) regional trends• Trends include maxima, minima, ranges, timing, etc. in:• T,P, humidity, precipitations, wind conditions, storms
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Climate Controls• Climatic conditions governed by:
• Latitude – N or S position.• Determines insolation
• Hotter near equator
• Colder near poles
• Seasonally varies
• Altitude – Height above SL. • Elevation linked to T
• For same latitude:• Lower elevations warmer
• Higher elevations colder
• ~6oC/km lapse rate.
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Climate Controls• Climatic conditions governed by:
• Proximity to water- influences T stability• Land heats & cools faster than oceans
• Near oceans have less T extremes (smaller T ranges)
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Climate Controls• Climatic conditions governed by:
• Proximity to ocean currents influences T conditions• Warm currents produce warmer climates
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Climate Controls• Climatic conditions governed by:
• Proximity to mountains• Mountains alter air flow – funneling/blocking winds
• Mountains modify moisture patterns• Heavy precipitation on windward side
• Rain shadow(desert) on leeward side
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Climate Controls• Climatic conditions governed by:
• Proximity to semi-permanent high and low P cells• Latitudinally controlled
• Govern prevailing winds
• Directly control humidity
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Climate Belts
• Climatic belts classified by T, precipitation, and vegetation
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Climate Variability• Climate can change in cyclic patterns.
• Example: El Niñ o – Oscillation (ENSO) -> air/water circulation off Peru.
Normal circulation is:• Easterlies push Peru coast surface water west
• Upwelling deep, cold, nutrient-rich water replaces flow
• Rain in west Pacific
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Climate Variability• During El Niñ o, atmosphere-ocean circulation changes:
• Westerlies develop in west Pacific
• Low P zone moves out over east Pacific
• Suppresses Peru coastal upwelling
• Drought in west Pacific