1. Atmospheric Stability
Adiabatic Processes
The concept of a parcel
Parcel and environmental lapse rates
Atmospheric dry stability
Determining stability

2. Air parcels A parcel is a “blob” of air
Small enough to have only one value of T, p, ρ, etc.
Large enough to contain a significant number of molecules. (Are there enough particles to talk about temperature as average kinetic energy, for example?)

3. Lapse Rates
Parcel lapse rate – the rate at which temperature changes as the parcel is lifted to a higher altitude
Environmental lapse rate – the rate at which the air surrounding the parcel changes as altitude increases

4. The Adiabatic Lapse Rate
An adiabatic process is one during which no heat is exchanged between the substance in question and its surroundings
Many atmospheric motions occur rapidly enough that parcels do not exchange a significant amount of heat with the environment
Examples:
rising air in a thunderstorm
Air rising over a topographic barrier (like a mountain)

5. Adiabatic Processes
(Chalkboard)

6. The Adiabatic Lapse Rate
The adiabatic lapse rate for DRY air on Earth is
Γd = g/cp
Γd = 9.81 m s-2 / 1004 J kg-1 K-1
Γd = K m-1
Γd = 9.77 K km-1

7. The Adiabatic Lapse Rate
This means that a rising(sinking) air parcel will cool(warm) at a rate of about 10 oC per km of ascent(descent) unless:
It exhanges significant mass or heat with the environment
It becomes saturated with respect to water vapor
It rises(sinks) so slowly that radiation heat transfer is possible

8. The Adiabatic Lapse Rate
What is the dry adiabatic lapse rate (Γd = g/cp) in these atmospheres?
Atmosphere
g (m s-2)
cp (J kg-1 K-1)
Venus
8.87
844
Mars
3.71
Titan
1.352
1039

9. The Adiabatic Lapse Rate
We have thus far only discussed the
DRY ADIABATIC LAPSE RATE
Water vapor condensation releases 2.5 MJ of energy for each kg of water condensed – this latent heat changes the adiabatic lapse rate for condensing air parcels to the
MOIST ADIABATIC LAPSE RATE

10. Atmospheric Stability

11. Atmospheric Stability
stable and unstable equilibria
air parcels
adiabatic process
adiabatic lapse rates
Stability does not control whether air will rise or sink. Rather, it controls whether rising air will continue to rise or whether sinking air will continue to sink.

12. Determining Stability
(Chalkboard)

13. A Stable Atmosphere environmental lapse rate absolute stability
stabilizing processes
Stable air provides excellent conditions for high pollution levels.

14. An Unstable Atmosphere
absolute instability
warming of surface air
destabilizing processes
superadiabatic lapse rates
Unstable air tends to be well-mixed.

15. Conditionally Unstable Air
conditional instability
dry and moist adiabatic lapse rates are different
Environmental lapse rate is between the two

16. Atmospheric Moisture
Twice now, we’ve mentioned moist adiabatic lapse rates.
Maybe we should talk about atmospheric moisture before we go down that road any further…

17. Humidity, Condensation and Clouds
Circulation of water in the atmosphere
Evaporation, condensation and saturation
Humidity
Dew and frost
Fog
Clouds

18. Circulation of Water in the Atmosphere

19. Circulation of Water in the Atmosphere
evaporation
condensation
precipitation
hydrologic cycle
The total amount of water vapor stored in the atmosphere amounts to only one week’s supply of precipitation for the planet.

20. Figure 4.1: The hydrologic cycle.
Fig. 4-1, p. 80

21. Figure 4.1: The hydrologic cycle.
Stepped Art
Fig. 4-1, p. 80

22. Evaporation, Condensation and Saturation

23. Evaporation, Condensation and Saturation
condensation nuclei
In very clean air, about 10,000 condensation nuclei are typically found in one cubic centimeter of air, a volume approximately the size of your fingertip.

24. Humidity

25. Mixing Ratio
The ratio of the mass of water vapor in air to the mass of dry air:
w = mv / md
Usually expressed in g kg-1
Some typical values:
Tropical marine boundary layer air: w ≈ 18 g kg-1
Polar air: w ≈ 1 g kg-1
Stratospheric air: w ≈ 0.1 g kg-1

26. Specific Humidity
The ratio of the mass of water vapor in air to the total mass of the air (dry air plus water vapor):
SH = mv / (md + mv)
w = SH / (1 – SH)
SH = w / (1 + w)

27. Vapor Pressure actual vapor pressure saturation vapor pressure
“Saturation” describes a condition of equilibrium: liquid water is evaporating at exactly the same rate that water vapor is condensing.

28. Vapor Pressure Actual vs. Saturation (or equilibrium) vapor pressure…
(Chalkboard)

29. Vapor Pressure Formula:
Saturation vapor pressure depends only on temperature…
Formula:
Saturation vapor pressure
Saturation vapor pressure at 273 K = 6.11 mb
Latent heat of vaporization = 2.5x106 J kg-1
Gas constant for water vapor = 461 J kg-1 K-1
273 K
Temperature

30. Vapor Pressure Formula:
Saturation vapor pressure depends only on temperature…
Formula:

31. Vapor Pressure
Saturation vapor pressure depends only on temperature…
Graph:

32. Relative Humidity definition of relative humidity
saturation and supersaturation
condensation
relative humidity and temperature
When the general public uses the term “humidity”, they mean “relative humidity.”

33. Relative Humidity
The ratio of the actual vapor pressure to the saturation vapor pressure.
rh = e / es
Since es depends on temperature, the relative humidity measures closeness to saturation, not actual water vapor content.

34. Figure 4.5: Saturation vapor pressure increases with increasing temperature. At a temperature of 10°C, the saturation vapor pressure is about 12 mb, whereas at 30°C it is about 42 mb. The insert illustrates that the saturation vapor pressure over water is greater than the saturation vapor pressure over ice.
Watch this Active Figure on ThomsonNow website at
Fig. 4-5, p. 83

35. Figure 4.7: When the air is cool (morning), the relative humidity is high. When the air is warm (afternoon), the relative humidity is low. These conditions exist in clear weather when the air is calm or of constant wind speed.
Fig. 4-7, p. 85

36. Relative Humidity and Dew Point
dew point temperature: the temperature to which air must be lowered to reach 100% relative humidity
dew point depression and relative humidity
The dew point temperature is useful for forecasting heat index, precipitation probabilities, and the chance of frost.

37. Measuring Humidity
psychrometers
hygrometers

38. Dew and Frost

39. Dew and Frost dew frost frost point and deposition
Frost is one of the few examples of deposition in nature.

40. Fog

41. Fog radiation fog advection fog upslope fog evaporation (mixing) fog
Fog is an extreme hazard to aircraft.

42. Clouds

43. Classification of Clouds
major cloud types
cloud appearance
cloud base
It’s easy to identify clouds, but it takes practice. The ability to identify clouds allows you to forecast many aspects of the weather using nothing but your eyes.

44. Table 4-2, p. 98

45. Cloud Identification high clouds middle clouds low clouds
clouds with vertical development

46. High Clouds cirrus cirrocumulus cirrostratus
Cirrostratus clouds can sometimes be quite thick.

47. Middle Clouds altocumulus altostratus
Altocumulus clouds are very pretty, especially just after sunrise or just before sunset.

48. Low Clouds nimbostratus stratocumulus stratus
Marine stratocumulus is the most common cloud type in the world.

49. Clouds with Vertical Development
cumulus
cumulus congestus
cumulonimbus
Not all cumulus clouds grow to be thunderstorms, but all thunderstorms start out as cumulus clouds.

50. Some Unusual Clouds lenticular clouds pileus mammatus clouds contrails
Several alleged ‘flying saucer’ reports have turned out to be lenticular clouds.

51. Cloud Development and Stability

52. Cloud Development and Stability
surface heating and free convection
uplift along topography
widespread ascent
lifting along weather fronts

53. Convection and Clouds thermals fair weather cumulus
Fair weather cumulus provide a visual marker of thermals.
Bases of fair-weather cumulus clouds marks the lifting condensation level, the level at which rising air first becomes saturated.

54. Topography and Clouds orographic uplift rain shadow
The rain shadow works for snow too. Due to frequent westerly winds, the western slope of the Rocky Mountains receives much more precipitation than the eastern slope.

55. Conditional Stability
Environmental lapse rate between wet and dry adiabatic lapse rates
Lifting Condensation Level (LCL)
Level of Free Convection (LFC)
(Chalkboard)

56. Summary of Atmospheric Stability
Absolute Stability = environmental lapse rate greater than both wet and dry adiabatic lapse rates
Absolute Instability = environmental lapse rate less than both wet and dry adiabatic lapse rates
Conditional Stability = environmental lapse rate less than dry but greater than we adiabatic lapse rate. The environment is stable to moist but unstable to dry disturbances.

57. Precipitation Processes

58. Collision and Coalescence Process
terminal velocity
coalescence
warm clouds
A typical cloud droplet falls at a rate of 1 centimeter per second. At this rate it would take 46 hours to fall one mile.

59. Figure 5. 9: Conditionally unstable atmosphere
Figure 5.9: Conditionally unstable atmosphere. The atmosphere is conditionally unstable when unsaturated, stable air is lifted to a level where it becomes saturated and warmer than the air surrounding it. If the atmosphere remains unstable, vertical developing cumulus clouds can build to great heights.
Watch this Active Figure on ThomsonNow website at
Stepped Art
Fig. 5-9, p. 116

60. Ice Crystal Process cold clouds supercooled water droplets
saturation vapor pressures over liquid water and ice
accretion
The upper portions of summer thunderstorms are cold clouds!

61. Figure 5. 22: The ice-crystal process
Figure 5.22: The ice-crystal process. The greater number of water vapor molecules around the liquid droplets causes water molecules to diffuse from the liquid drops toward the ice crystals. The ice crystals absorb the water vapor and grow larger, while the water droplets grow smaller.
Watch this Active Figure on ThomsonNow website at
Fig. 5-22, p. 124

62. Figure 5. 22: The ice-crystal process
Figure 5.22: The ice-crystal process. The greater number of water vapor molecules around the liquid droplets causes water molecules to diffuse from the liquid drops toward the ice crystals. The ice crystals absorb the water vapor and grow larger, while the water droplets grow smaller.
Watch this Active Figure on ThomsonNow website at
Stepped Art
Fig. 5-22, p. 124

63. Cloud Seeding and Precipitation
silver iodide
It is very difficult to determine whether a cloud seeding attempt is successful. How would you know whether the cloud would have resulted in precipitation if it hadn’t been seeded?

64. Precipitation in Clouds
accretion
ice crystal process

65. Precipitation Types

66. Rain rain drizzle virga shower
Virga is much more commonly observed in the western US, because the humid climate of the eastern US reduces the visibility.

67. Snow snow fallstreaks dendrite blizzard
Snowflake shape depends on both temperature and relative humidity.

68. Sleet and Freezing Rain
rime
Sleet makes a ‘tap tap’ sound when falling on glass.

69. Snow Grains and Snow Pellets
graupel

70. Hail updraft cycles accretion
A hailstone can be sliced open to reveal accretion rings, one for each updraft cycle.

71. Figure 5.35: Hailstones begin as embryos (usually ice particles) that remain suspended in the cloud by violent updrafts. When the updrafts are tilted, the ice particles are swept horizontally through the cloud, producing the optimal trajectory for hailstone growth. Along their path, the ice particles collide with supercooled liquid droplets, which freeze on contact. The ice particles eventually grow large enough and heavy enough to fall toward the ground as hailstones.
Watch this Active Figure on ThomsonNow website at
Stepped Art
Fig. 5-35, p. 134

72. Measuring Precipitation

73. Instruments standard rain gauge tipping bucket rain gauge
It is difficult to capture rain in a bucket when the wind is blowing strongly.

74. Doppler Radar and Precipitation

75. Figure 5. 39: A microwave pulse sent out from the radar transmitter
Figure 5.39: A microwave pulse sent out from the radar transmitter. The pulse strikes raindrops and a fraction of its energy is reflected back to the radar unit, where it is detected and displayed, as shown in Fig
Stepped Art
Fig. 5-39, p. 135