18 atmosphere climate_forstudents
TRANSCRIPT
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)
Chapter 20
Chapter 20
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
Chapter 20
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
Chapter 20
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
Chapter 20
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
Chapter 20
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
Chapter 20
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
Chapter 20
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
Chapter 20
Relative Humidity• Rising air cools (adiabatically) to form tiny water droplets
• Common phenomena -> form clouds
• Clouds can dissipate by adiabatic heating
Chapter 20
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
Chapter 20
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 constant motion (wind)
• Wind velocities vary from 0 to >100 km/hr• Wind circulation has both local and global aspects
• Local –.operates
• Global –
Chapter 20
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
Chapter 20
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
Chapter 20
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
Chapter 20
Seasons• Seasons due to 23.5 tilt to Earths rotation axis• Earth orbits Sun, vertical rays
• More north• More south
Chapter 20
Seasons – January vs. July
Chapter 20
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
Chapter 20
• 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
Chapter 20
Prevailing Winds• Result is regular wind directions • Called prevailing winds
• May be locally modified
Chapter 20
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!
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 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
Chapter 20
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
Chapter 20
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
Chapter 20
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
Chapter 20
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
Chapter 20
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
Chapter 20
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
Chapter 20
Cloud Types• Clouds form in troposphere, controlled by:
• Air stability • Elevation at which moisture condenses• Wind conditions
Chapter 20
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
Chapter 20
Storms• Storms develop along
• Centered by• Fueled by• Result:
Chapter 20
Thunderstorms• Local pulses of• Rising air forms• Latent heat released by• Cumulus clouds
• Anvil head• Heavy rains
Chapter 20
Thunderstorms• Lightning is
• Scientists do not• Cloud bases develop a• Result: buildup of• Air is a good insulator; prevents• Eventually, charge imbalance
Chapter 20
Thunderstorms• Lightning leader advances from• Return stroke starts• Connect to form• Thunder is a
• Bolt heats air• Air expands explosively.
Chapter 20
Tornadoes• Near-vertical• Air moves with
• Local winds up to• Extremely
Chapter 20
Tornadoes• Tornadoes develop along
• Strong W winds • Strong SE surface winds
• Shear initiates• Drafts tip the rotating
Chapter 20
Tornadoes• Tornadoes prevalent in
• Proper conditions;• Cold polar air from
• Warm moist air pushed
• Tornado-prone region called
Chapter 20
Hurricanes
• Huge low-P cyclonic storms from tropical Atlantic.• Defined by• Fueled by
• Originate in
• Do not form near
Chapter 20
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 –
Chapter 20
Hurricanes• Storm “named” when winds exceed
• Named in• Alternating male/female with
• Hurricane tracks move• Landfall removes
Chapter 20
Hurricanes• Hurricane-like storms outside the Atlantic are called…
• Typhoons – • Cyclones –
Chapter 20
Hurricanes
• Intensity is ranked • Category 1: Wind speed > 119 km/h; pressure > 980 mbars• Category 5: Wind speed > 250 km/h; pressure < 920 mbars
Chapter 20
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• Long-term• Trends include
Chapter 20
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.
Chapter 20
Climate Controls• Climatic conditions governed by:
• Proximity to• Land heats & cools faster than
• Near oceans have less
Chapter 20
Climate Controls• Climatic conditions governed by:
• Proximity to ocean currents• Warm currents produce
Chapter 20
Climate Controls• Climatic conditions governed by:
• Proximity to• Mountains alter air flow -
• Mountains modify• Heavy precipitation on
• Rain shadow
Chapter 20
Climate Controls• Climatic conditions governed by:
• Proximity to• Latitudinally
• Govern
• Directly control
Chapter 20
Climate Belts
• Climatic belts classified by
Chapter 20
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
Chapter 20
Climate Variability• During El Niño, atmosphere-ocean circulation changes:
• Westerlies develop
• Low P zone moves
• Suppresses Peru
• Drought in