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)

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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.

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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)

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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

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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)

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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)

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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