1. Chapter 20
Atmosphere and Climate

2. 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

3. Chapter 20
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

4. Chapter 20
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

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.

13. 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

14. Chapter 20
Atmospheric Circulation
• Troposphere experiences
• Wind velocities vary from
• Wind circulation has both
• Local –.
• Global –

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

18. Chapter 20
Seasons
• Seasons due to
• Earth orbits Sun, vertical rays
• More north
• More south

19. Chapter 20
Seasons – January vs. July

20. Chapter 20
Atmospheric Movement
• Troposphere divided into
• Hadley cells –
• Ferrel cells –
• Polar cells –
• Hadley cell – Rising
equatorial air creates

21. Chapter 20
• Rotation (via Coriolis effect), complicates
• Cell airflow is deflected
• Forms
• Cooling air
Atmospheric Movement

22. Chapter 20
Prevailing Winds
• Result is
• Called

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

25. 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

26. Chapter 20
Weather
• Local-scale conditions of
• Reflects
• Variation in
• Land vs. sea.
• A weather system affects

27. Chapter 20
Air Masses
• Air packages with
• >1,500 km across,
• Characteristics reflect
• Weather changes dramatically when

28. Chapter 20
Fronts
• Fronts -
• Curved surfaces that lead
• Cold fronts:
• Steep
• Flow
• Pushes up

29. Chapter 20
Fronts
• Warm front:
• More gradual
• Warm air
• Pushes
• Incline reflects
• Warm air rising up the front causes

30. Chapter 20
Clouds and Precipitation
• Water vapor in saturated air
• Condensing as
• Precipitating as
• Condensation nuclei
• Microscopic

31. Chapter 20
Clouds and Precipitation
• Several air-lifting mechanisms.
• Convective lifting –
• Frontal lifting –
• Convergence lifting –
• Orographic lifting –

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

34. Chapter 20
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

35. Chapter 20
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

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

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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

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)

45. Chapter 20
Hurricanes
• Hurricane-like storms outside the Atlantic are called…
• Typhoons – Western Pacific Ocean
• Cyclones – Northern Indian Ocean

46. 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

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.

48. 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

49. 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

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

55. Chapter 20
Climate Belts
• Climatic belts classified by T, precipitation, and vegetation

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