3. 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
4. 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
5. 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
6. 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
7. 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
8. 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
9. 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
10. Chapter 20
Relative Humidity
• Rising air cools (adiabatically) to form tiny water droplets
• Common phenomena -> form clouds
• Clouds can dissipate by adiabatic heating
11. 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
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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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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 –
15. 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
16. 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
17. 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
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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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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
21. 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
23. 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!
24. Chapter 20
High Winds
• High-altitude pressure gradient
• Over
• High-altitude westerlies can
• Called
26. 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
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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
29. 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
30. 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
31. 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
32. 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
33. Chapter 20
Cloud Types
• Clouds form in troposphere, controlled by:
• Air stability
• Elevation at which moisture condenses
• Wind conditions
36. Chapter 20
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
38. Chapter 20
Thunderstorms
• Lightning leader advances from
• Return stroke starts
• Connect to form
• Thunder is a
• Bolt heats air
• Air expands explosively.
47. 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.
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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.
51. Chapter 20
Climate Controls
• Climatic conditions governed by:
• Proximity to
• Land heats & cools faster than
• Near oceans have less
52. Chapter 20
Climate Controls
• Climatic conditions governed by:
• Proximity to ocean currents
• Warm currents produce
53. Chapter 20
Climate Controls
• Climatic conditions governed by:
• Proximity to
• Mountains alter air flow -
• Mountains modify
• Heavy precipitation on
• Rain shadow
54. Chapter 20
Climate Controls
• Climatic conditions governed by:
• Proximity to
• Latitudinally
• Govern
• Directly control
56. 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
57. Chapter 20
Climate Variability
• During El Niño, atmosphere-ocean circulation changes:
• Westerlies develop
• Low P zone moves
• Suppresses Peru
• Drought in