Cloud Development and Stability

1. Cloud Development and Stability Chapter 5

2. Stability Probably the hardest chapter for students So read!!! Ask questions if anything is unclear In general, clouds form as a result of warm air rising, cooling, and expanding So the questions are: Why do we have clouds sometimes and not others? Thunderstorms? Different cloud types (shapes/sizes)?

3. Stability All of these questions can be answered by examining the concept of atmospheric stability Stable Equilibrium - Something (air) pushed away from its original position wants to return Unstable Equilibrium - Something (air) pushed away from its original position wants to keep moving away For our purposes, we’re talking about air being pushed up or down (vertical air motions)

4. Equilibrium

5. Stability In the atmosphere, these “pushes” of air include: Heating Fronts Terrain

6. Air Parcel We used the term “parcel” when talking about moving air up or down in the atmosphere Just a balloon-like volume of air that does not mix with the surrounding air Defn. - Adiabatic - a process in which no heat is exchanged between an air parcel and the surrounding environment. If it rises, the air inside expands and cools If it sinks, the air inside compresses and warms Same amount of air, just at different pressures

7. Adiabatic Process The rate at which a parcel cools as it rises or warms as it sinks depends on whether or not the air is saturated (Avg. = 6.5º C per 1000 meters) If the air is unsaturated (RH<100%), this rate is 10º C per 1000 meters is is called the dry adiabatic lapse rate

8. Dry Adiabatic Lapse Rate

9. Moist Adiabatic Lapse Rate If an unsaturated parcel of air rises and cools, it will eventually cool to its dew point where it will be saturated (RH=100%) Further cooling results in condensation This is when a cloud begins to form Also, condensation represents a phase change of water from a gas to a liquid. Latent heat is released So if the air still continues to rise, will it still cool at the dry adiabatic rate?

10. Moist Adiabatic Lapse Rate No, the rate will be less due to the release of latent heat So, rising saturated air does not cool as quickly as rising unsaturated air In fact, it cools at an average rate of 6ºC per 1000 meters which is called the moist adiabatic lapse rate Same rate for warming if air is sinking

11. Lapse Rates

12. Determining Stability Now we know how air temperature will change as air rises or descends All we need to know is the air parcel’s temperature and moisture at some level (like at the surface) And we also know what mechanisms start air moving vertically (hot surface, fronts, terrain) We need to know one more thing in order to determine whether or not air will continue to rise on its own once started ????

13. Determining Stability Need to know if the air parcel is warmer or cooler than its environment if lifted So we need to know the temperature of the environment Defn. - environmental lapse rate - the actual rate of temperature change w/ height How do we do this? Radiosonde Once we know this, we can figure out what the stability properties of the atmosphere are

14. Determining Stability If a parcel rises and cools, and is then colder than the surrounding air, it will sink back to its original position - stable If the parcel is warmer than the surrounding air, it will continue to rise - unstable

15. A Stable Atmosphere Example 1 Environmental lapse rate is 4ºC/1000 m If air is unsaturated, it will cool at the dry adiabatic rate if lifted and will always be cooler than environment So, its tendency is to sink back to its original position

16. A Stable Atmosphere Example 1 If air is saturated, it will cool at the moist adiabatic rate if lifted and will always be cooler than environment So, its tendency is to sink back to its original position

17. A Stable Atmosphere So in the previous example, it didn’t matter whether or not the parcel was saturated It would always have been cooler than the surrounding environment if lifted This is called an absolutely stable atmosphere If air were to be forced to rise (mountains) in this type of situation, it would tend to spread our horizontally and result in thin, layered clouds - stratus, altostratus, etc.

18. A Stable Atmosphere The atmosphere is stable when the environmental lapse rate is small (or negative) Absolutely stable if less than moist adiabatic rate So, the atmosphere becomes more stable as the air aloft warms and/or the surface air cools Surface air cools by radiation (night), cold fronts, air moving over cold surfaces This is why thunderstorms usually die at night - surface cools

19. An Unstable Atmosphere Example 2 Environmental lapse rate is 11ºC/1000 m If air is unsaturated, it will cool at the dry adiabatic rate if lifted and will always be warmer than environment So, its tendency is to keep rising on its own

20. An Unstable Atmosphere Example 2 If air is saturated, it will cool at the moist adiabatic rate if lifted and will always be warmer than environment So, its tendency is to keep rising on its own

21. An Unstable Atmosphere So in example 2, again it didn’t matter whether or not the parcel was saturated It would always have been warmer than the surrounding environment if lifted This is called an absolutely unstable atmosphere If air is forced to rise in this type of situation, it would tend to keep rising on its own

22. An Unstable Atmosphere The atmosphere becomes unstable when the environmental lapse rate becomes large (cools quickly with height) Absolutely unstable if greater than dry adiabatic rate So, the atmosphere becomes more unstable when air aloft cools and/or air near the surface warms Surface air warms during the day, when a warm front passes, air moving over warm surfaces Almost never find absolutely unstable layers except near the surface on hot days

23. A Conditionally Unstable Atmosphere Example 3 Environmental lapse rate is 7ºC/1000 m Lies between the moist and dry lapse rates If an unsaturated parcel is lifted, it will always be cooler than the surrounding air and will tend to sink back to its original position Stable with respect to unsaturated air

24. A Conditionally Unstable Atmosphere Example 3 If a saturated parcel is lifted, it will always be warmer than the surrounding air and will tend to rise on its own Unstable with respect to saturated air

25. A Conditionally Unstable Atmosphere In example 3, stability is dependent upon whether or not rising air is saturated or unsaturated If unsaturated - stable If saturated - unstable This is called a conditionally unstable atmosphere Condition?? Whether or not the air becomes saturated

26. Environmental Lapse Rate of 9ºC

27. Stability Absolutely stable if the environmental lapse rate < moist adiabatic rate Absolutely unstable if the environmental lapse rate > dry adiabatic rate Conditionally unstable if the environmental lapse rate is between the dry and moist adiabatic rates

28. Cloud Development and Stability Touched on this briefly already 4 major ways air is forced to rise and produce clouds 1) Heating at the surface (convection) 2) Topography (mountains, hills, etc.) 3) Convergence of surface air (air flows come together) 4) Uplift along fronts 1st two for now, 3 & 4 later on

29. Cloud Development and Stability

30. Convection Talked about this already Hot surface heats air Warm air rises Cooler air from above sinks to replace it If the condensation level is low: One thermal may cause a cumulus cloud If high: May take several thermals

31. Convection on a Summer Day

32. “Cloud Streets”

33. Convection How much vertically a cumulus cloud grow will depend on stability If stable near cloud top - difficult time growing probably stay small If unstable or conditionally unstable through a deep layer - good chance for much vertical growth towering cumulus clouds If unstable over a very deep layer (several miles) - possible thunderstorm growth

34. Convection in an Unstable Atmosphere

35. Topography Basically, surface air must rise over a mountain range if the wind is coming from a certain direction This forced ascent over mountains is called orographic uplift Has a MAJOR effect on precipitation and temperature Rain shadows

36. Hawaii Rain Shadows

37. Precipitation We know some clouds produce rain, right? But why do some clouds precipitate but not others? And how do cloud droplets grow large enough to fall to the earth? From earlier, what do all cloud droplets need in order to form? Condensation nuclei Water vapor condenses on them

38. Precipitation Based on the sizes of CN, cloud droplets, and rain drops….do you think rain drops form just due to condensation??? It would take about 3 days for that to happen How long does it take for storms to pop up in the summer? 1 hour or less

39. Precipitation So, the process of condensation is not quick enough to produce raindrops How do they form then?? 2 processes

40. Collision and Coalescence 1) Collision and Coalescence Also called the “warm rain process” because it only involves liquid water Main thing needed for this process to work is droplets of different sizes How might that happen? 1) Simply random collisions 2) Different size CN Ex. Salt is larger than dust so its cloud drop will be larger too

41. Collision and Coalescence Since larger drops are heavier, they fall faster than smaller drops As they fall they collide and merge with smaller drops - coalescence

42. Collision and Coalescence The absolute most important factor in this process is liquid water content Must have water!! Other important factors include Range of droplet sizes some larger than others Cloud thickness Updrafts in a cloud Electric charge of droplets opposites attract

43. Collision and Coalescence This cumulus cloud has a good chance of becoming a thunderstorm and producing rain: It has lots of water It’s thick It has updrafts What about nimbostratus clouds?? Thin - means small if any updraft and not much water Drizzle at best

44. Ice Crystal Process 2) Ice Crystal Process Occurs in clouds comprised of both liquid water and ice crystals - like thunderstorms

45. Ice Crystal Process All water at low levels All ice at very high levels In between - both water and ice How does this happen if temp is below freezing? Two reasons 1) Small cloud droplets freeze at lower temps 2) Ice crystals need “ice nuclei” on which to form Not many of these things Clay, bacteria, etc.

46. Ice Crystal Process Remember saturation? In the middle of the cloud (ice and water), the air is saturated But, since water is warmer, it takes more water vapor molecules to saturate the air around it So, is the saturation vapor pressure greater around the drop or the ice??

47. Ice Crystal Process Around the water since there are more molecules Things are out of balance now and vapor molecules will move toward the ice to equalize the pressure But if they move into already saturated air over the ice, what happens???

48. Ice Crystal Process Molecules must condense out of the air onto the ice Ice grows Also, now the air is unsaturated over the water So more molecules evaporate from the drop to replace those lost to the ice

49. Ice Crystal Process Bottom line: The ice crystals grow in size at the expense of the water drops Ice crystals get heavy and fall

50. Ice Crystal Process Falling crystals collect super-cooled water droplets - accretion The icy product is called “graupel” melts to form rain

51. Icy Crystal Process Falling crystals can also fracture as they collide with other crystals Happens in colder clouds If the pieces stick together…..

52. Ice Crystal Process Snowflakes This process of ice crystals forming snow is called “aggregation” So the thunderstorms we see around here have all kinds of things going on in them ice, water, snow, graupel, hail, etc Does a snowflake really look like this?

53. By the way, what shape is the correct one for a rain drop??

54. Types of Precipitation I’m not going into all of the types (maybe too boring) We’ll concentrate on rain, snow, sleet, and freezing rain How do these differ and why do we get each type? Has to do with the vertical profile of temperature

55. Snow and Sleet Snow is easy - freezing from cloud down to the surface Sleet - happens when there is a deep freezing layer near the surface and an above-freezing layer below the cloud

56. Freezing Rain and Rain Freezing rain - a lot like sleet except the sub-freezing layer near the surface is shallow - so rain stays in liquid form until it hits the surface Rain - occurs when there is a deep above-freezing layer from the surface upward

57. More Frozen Precipitation Occurs mostly in the spring and summer?? Hail Lethal stuff Can damage crops, cars, property $2K to my car a couple of years ago Even responsible for killing livestock Big hail is pretty heavy (1/2 lb or so) and falls quickly

58. Hail Forms in T-storms w/ strong updrafts Begin as “embryos” usually ice particles but sometimes bugs too updrafts cause embryo to come in contact with super-cooled water which freezes on it 5-10 minutes to become golf ball size

59. Measuring Precipitation 2 primary methods 1) Gauges 2) Radar

60. Rain Gauge Old fashioned gauge Funnel on top channels water into a tube How can a person possible measure rain with this thing if only .05 of an inch fell? Top of funnel is 20 cm wide (10X width of tube) So rainfall is amplified 10X in the tube allowing for precision

61. Tipping Bucket Rain Gauge Bucket holds .01 inches of rain before it tips Need to have at least .01 inches to use the term “rainfall” If less - “trace”

62. RADAR Stands for RAdio Detection And Ranging How it works: Transmitter sends out a microwave pulse If the pulse hits something (rain), some of it is reflected back toward the radar How much comes back indicates the intensity of the rain Advantage: We can “see” and estimate rainfall in previously inaccessible regions

63. Doppler Storm Total Rainfall Estimates total rainfall over a period of time based on intensity during that period