Chapter 20

Atmosphere and ClimateAtmosphere 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 air motion • Atmosphere governs physical conditions of weather

• Temperature (T).• Pressure (P).• Moisture content.• Wind velocity. • Wind direction.

• Climate is long term weather behavior

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Atmospheric Components• Present atmosphere comprised of a gas mix:

• Nitrogen 78%• Oxygen 21%• Other gases 1%

• Aerosols – tiny suspended particles • Liquid droplets solid dust particles

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Atmospheric Coloration

• Color due to light energy dispersion• Light scattered passing through atm• Some light returns to space

• Why is the sky blue?• When the Sun is overhead..• Gases scatter blue light

• Why is the sky red?• Setting Sun passes through thicker

atmosphere• Only red is left, blue scattered to space

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Pressure and Density• Air pressure – force due to weight of overlying air

• Greatest near surface• Decreases upward• 14.7 psi (1 atm) at sea level.

• Air density – mass of air/vol• Maximum at sea level• Decreases upward

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P and T Relations

• P & T conditions change with elevation• P - higher near surface; lower above

• When air moves from higher to lower P, it…• Expands & cools.

• Moving from lower to higher P, it…• Sinks, shrinks, and warms• Called adiabatic cooling and heating

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Relative Humidity• Air has varying water amounts:

• Dry (desert) 0.3%• Humid (tropical rainforest) 4.0%

• Water content described by relative humidity • Ratio (%) of measured water content to max possible

• Dry air –low relative humidty

• Humid air – high relative humidity

• 100% relative humidity air is saturated• Under-saturated air has <100% relative humidity

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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 dewpoint• Below dewpoint…

• Water forms dew or frost

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Relative Humidity• Rising air cools (adiabatically) to form tiny water droplets

• Common phenomena -> form clouds

• Clouds can dissipate by adiabatic heating

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Latent Heat• Water in air can change its state

• Liquid >gas or visa versa

• With state changes, air T also changes• T change is not due to external energy; hence, “latent.”• Instead, derives from state change alone

• Evaporating water absorbs heat, cools air• Condensing water releases heat, warms air

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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)

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Atmospheric Circulation• Troposphere experiences constant motion (wind)

• Wind velocities vary from 0 to >100 km/hr• Wind circulation has both local and global aspects

• Local –.operates

• Global –

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Pressure Gradients• Lateral pressure differences drive horizontal winds• Pressures mapped by isobars-line of equal P

• Isobars cannot cross• Air flows from high to low P perpendicular to isobars• Steeper the gradient, faster the airflow

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Energy Input• Air circulation is result of heat movement

• Warm air expands, becomes less dense> rises• This air is replaced by sinking, colder, denser air

• Convection driven by differential solar heating (insolation)• Solar energy = insolation, or incoming solar radiation

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Energy Input• Solar energy bathing Earth is not evenly disturbuted

• Vertical Sun rays have more energy • Oblique rays

• Tropics (vertical rays) receive• Poles (oblique rays) receive

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Seasons• Seasons due to 23.5 tilt to Earths rotation axis• 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 6 N-S motion cells

• Hadley cells – low latitude • Ferrel cells – mid latitude • Polar cells – high latitude

• Hadley cell – Rising

equatorial air creates low P,cools and rains

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• Rotation (via Coriolis effect), complicates N-S flow • Cell airflow is deflected E or W depending on flow direction

• Forms convergent and divergent zones• Cooling air sinks, warm air rises

Atmospheric Movement

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Prevailing Winds• Result is regular wind directions • Called prevailing winds

• May be locally modified

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High Winds• Troposphere thickness changes with latitude

• Warm equatorial air expands it• Cold polar air thins it

• At given altitude, equatorial pressure will be higher• Causes equatorial high-altitude air to flow towards poles

• Air atop Hadley cells spill over top of Ferrel cells.

• Coriolis deflects these winds too!

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High Winds• High-altitude pressure gradient

• Over

• High-altitude westerlies can

• Called

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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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Weather• Local-scale conditions of T,P wind speed, humidity• Reflects prevailing winds plus local variations

• Variation in topography and vegetation• Land vs. sea.

• A weather system affects a region for a short time

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Air Masses

• Air packages with unique recognizable properties • >1,500 km across, they flow over a region for days• Characteristics reflect origin and changes • Weather changes dramatically when air masses changes

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Fronts• Fronts – boundaries between air masses

• Curved surfaces that lead air masses

• Cold fronts:• Steep T and P gradients –move fast• Flow beneath warm air masses

• Pushes up warm, humid air, and creates storms

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Fronts• Warm front:

• More gradual P and T gradient – move slow • Warm air climbs up over colder air

• Pushes cold air into wedge

• Incline reflects less steep T/P gradients

• Warm air rising up the front causes broad cloud cover/precip

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Clouds and Precipitation• Water vapor in saturated air changes states by..

• Condensing as water droplets• Precipitating as ice crystals

• Condensation nuclei help initiate this change• Microscopic solid or liquid particles

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Clouds and Precipitation• Several air-lifting mechanisms.

• Convective lifting – warmed air rises• Frontal lifting – air is carried upward along fronts• Convergence lifting – converging windds force air upward• Orographic lifting – air must raise to pass over mountains

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Clouds and Precipitation• Rain, snow, sleet form in 2 ways,

• Collison & coalescence – small droplets collide/merge• Drops fall when too large to suspend

• Typical raindrops are 2mm

• Drops >5 mm break up

• Cold air near ground turns rain to sleet

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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 – wipsy, 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• Fueled by• Result:

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Thunderstorms• Local pulses of• Rising air forms• Latent heat released by• Cumulus clouds

• Anvil head• Heavy rains

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Thunderstorms• Lightning is

• Scientists do not• Cloud bases develop a• Result: buildup of• Air is a good insulator; prevents• Eventually, charge imbalance

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Thunderstorms• Lightning leader advances from• Return stroke starts• Connect to form• Thunder is a

• Bolt heats air• Air expands explosively.

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Tornadoes• Near-vertical• Air moves with

• Local winds up to• Extremely

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Tornadoes• Tornadoes develop along

• Strong W winds • Strong SE surface winds

• Shear initiates• Drafts tip the rotating

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Tornadoes• Tornadoes prevalent in

• Proper conditions;• Cold polar air from

• Warm moist air pushed

• Tornado-prone region called

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Hurricanes

• Huge low-P cyclonic storms from tropical Atlantic.• Defined by• Fueled by

• Originate in

• Do not form near

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Hurricanes• Hurricanes develop in summer & late fall.• Form over warm tropical ocean waters

• Cyclonic low-P “tropical disturbances”• Air rises, cools, condenses; • Heat buoys air, creates• Over time, storm gains

• Size range –

• Strength –

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Hurricanes• Storm “named” when winds exceed

• Named in• Alternating male/female with

• Hurricane tracks move• Landfall removes

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Hurricanes• Hurricane-like storms outside the Atlantic are called…

• Typhoons – • Cyclones –

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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

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Climate

• “climate” refers to• Long-term• Trends include

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Climate Controls• Climatic conditions governed by:

• Latitude – N or S position.• Determines

• Hotter near

• Colder near

• Seasonally

• Altitude – Height above SL. • Elevation linked to

• For same latitude:• Lower elevations

• Higher elevations

• ~6oC/km lapse rate.

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Climate Controls• Climatic conditions governed by:

• Proximity to• Land heats & cools faster than

• Near oceans have less

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Climate Controls• Climatic conditions governed by:

• Proximity to ocean currents• Warm currents produce

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Climate Controls• Climatic conditions governed by:

• Proximity to• Mountains alter air flow -

• Mountains modify• Heavy precipitation on

• Rain shadow

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Climate Controls• Climatic conditions governed by:

• Proximity to• Latitudinally

• Govern

• Directly control

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Climate Belts

• Climatic belts classified by

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Climate Variability• Climate can change in cyclic patterns.

• Example: El Niño – Oscillation (ENSO) -> air/water circulation off Peru.

Normal circulation is:• Easterlies push Peru

• Upwelling deep, cold,

• Rain in

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Climate Variability• During El Niño, atmosphere-ocean circulation changes:

• Westerlies develop

• Low P zone moves

• Suppresses Peru

• Drought in