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 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

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 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

18. Chapter 20
Seasons
• Seasons due to 23.5 tilt to Earths rotation axis
• Earth orbits Sun, vertical rays
• More north
• More south

19. Chapter 20
Seasons – January vs. July

20. 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

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

22. Chapter 20
Prevailing Winds
• Result is regular wind directions
• Called prevailing winds
• May be locally modified

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

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 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

27. 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

28. 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

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

34. 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

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Storms
• Storms develop along
• Centered by
• Fueled by
• Result:

36. Chapter 20
Thunderstorms
• Local pulses of
• Rising air forms
• Latent heat released by
• Cumulus clouds
• Anvil head
• Heavy rains

37. Chapter 20
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.

39. Chapter 20
Tornadoes
• Near-vertical
• Air moves with
• Local winds up to
• Extremely

40. Chapter 20
Tornadoes
• Tornadoes develop along
• Strong W winds
• Strong SE surface winds
• Shear initiates
• Drafts tip the rotating

41. Chapter 20
Tornadoes
• Tornadoes prevalent in
• Proper conditions;
• Cold polar air from
• Warm moist air pushed
• Tornado-prone region called

42. Chapter 20
Hurricanes
• Huge low-P cyclonic storms from tropical Atlantic.
• Defined by
• Fueled by
• Originate in
• Do not form near

43. 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 –

44. Chapter 20
Hurricanes
• Storm “named” when winds exceed
• Named in
• Alternating male/female with
• Hurricane tracks move
• Landfall removes

45. Chapter 20
Hurricanes
• Hurricane-like storms outside the Atlantic are called…
• Typhoons –
• Cyclones –

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
• Long-term
• Trends include

50. 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.

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

55. Chapter 20
Climate Belts
• Climatic belts classified by

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