5. Chapter 20
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
6. Chapter 20
Pressure and Density
• Air pressure –
• Greatest near
• Decreases
• 14.7 psi (1 atm) at sea level.
• Air density –
• Maximum at
• Decreases
7. Chapter 20
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
8. Chapter 20
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%
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
• Below dewpoint…
• Water forms
10. Chapter 20
Relative Humidity
• Rising air cools (adiabatically) to form
• Common phenomena ->
• Clouds can dissipate by adiabatic
11. Chapter 20
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
12. Chapter 20
Atmospheric Layers
• Atmosphere is thermally layered.
• Troposphere (0 - 9 to 12 km).
• Mixing layer.
• All weather is here.
15. Chapter 20
Pressure Gradients
• Lateral pressure differences
• Pressures mapped by
• Isobars cannot
• Air flows from high to low P
• Steeper the gradient,
16. Chapter 20
Energy Input
• Air circulation is result of
• Warm air expands,
• This air is replaced by
• Convection driven by
• Solar energy =
17. Chapter 20
Energy Input
• Solar energy bathing Earth is not
• Vertical Sun rays have
• Oblique rays
• Tropics (vertical rays) receive
• Poles (oblique rays) receive
23. Chapter 20
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
24. Chapter 20
High Winds
• High-altitude pressure gradient
• Over
• High-altitude westerlies can
• Called
32. Chapter 20
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
33. Chapter 20
Cloud Types
• Clouds form in troposphere, controlled by:
• Air stability
• Elevation at which moisture condenses
• Wind conditions
35. Chapter 20
Storms
• Storms develop along
• Centered by low pressure
• Fueled by warm, moist air
• Result: lightening, wind, rain, hail/sleet/snow
36. Chapter 20
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
37. Chapter 20
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
38. Chapter 20
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.
39. Chapter 20
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
40. Chapter 20
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
41. Chapter 20
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”
42. Chapter 20
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)
43. Chapter 20
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
44. Chapter 20
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)
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.
50. Chapter 20
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
• ~6o
C/km lapse rate.
51. Chapter 20
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)
52. Chapter 20
Climate Controls
• Climatic conditions governed by:
• Proximity to ocean currents influences T conditions
• Warm currents produce warmer climates
53. Chapter 20
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
54. Chapter 20
Climate Controls
• Climatic conditions governed by:
• Proximity to semi-permanent high and low P cells
• Latitudinally controlled
• Govern prevailing winds
• Directly control humidity
56. Chapter 20
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
57. Chapter 20
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
Editor's Notes
Think about main themes- plate tectonics
100 questions- 50 (fourth exam) 50 (fair game)
Study like previous exams- smaller amounts of material and think about summary of ideas from the ends of different chapters
More extreme changes in weather patterns causes more storms
