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Contents. stabilitycloud developmentprecipitation. Clouds. Formation releases heatReflect, scatter, absorb IR radiationProduce precipitationVisually indicate the stability of the atmosphere. What do they do for us?. Questions. 1. How and why do clouds form on some days and not on others?2. Why does the atmosphere sometimes produce stratus clouds (thin layered) while other times we get cumulus, or cumulonimbus clouds to form?The answer is largely related to the concept of atmos30519
Chapter 5 - Title

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1. Chapter 5 - Title

2. Contents stability cloud development precipitation

3. Clouds Formation releases heat Reflect, scatter, absorb IR radiation Produce precipitation Visually indicate the stability of the atmosphere

4. Questions 1. How and why do clouds form on some days and not on others? 2. Why does the atmosphere sometimes produce stratus clouds (thin layered) while other times we get cumulus, or cumulonimbus clouds to form? The answer is largely related to the concept of atmospheric stability... The answer is largely related to the concept of atmospheric stability... The answer is largely related to the concept of atmospheric stability...

5. Cloud Development - Stable Environment Consider this simple situation of a marble in the bottom of a bowl If you push the marble up the side of the bowl, what happens? Stable air (parcel) - vertical motion is inhibited if clouds form, they will be shallow, layered clouds e.g. stratus Sisyphus = A cruel king of Corinth condemned forever to roll a huge stone up a hill in Hades only to have it roll down again on nearing the top. [Latin Sisyphus, from Greek Sisuphos.] Sisyphus = A cruel king of Corinth condemned forever to roll a huge stone up a hill in Hades only to have it roll down again on nearing the top.

6. Cloud Development - Unstable Environment If the marble is on the top of the bowl and you give it a little push, what happens? This is an unstable situation Unstable air (parcel) - vertical motion occurs commonly produces cummulus, cumulonimbus clouds So, the question becomes, how does one determine the stability of the atmosphere?

7. Determining Stability Compare temperature (Tp) of a rising air parcel to that of its surroundings (Te) Tp > Te : If rising air is warmer and less dense than the surrounding air, it will _____ until it reaches the same T as its surroundings. Tp < Te : If rising air is colder it will be more dense than the surrounding air and _____ back to the original level. Tp = Te : What happens? RISE SINKRISE SINK

9. As a parcel of air rises, it cools, but at what rate? rate of temperature change with height is called the lapse rate. Units of lapse rate are ?C /km Let's first consider an unsaturated parcel of air unsaturated parcels cool at a rate of 10?C /km - this is called the dry adiabatic lapse rate If the surface temperature is 40?C What will be a parcel's temperature at 1 km? What will be a parcel's temperature at 2 km? At 1 km, 30 ?C At 2 km, 20 ?CAt 1 km, 30 ?C At 2 km, 20 ?C

10. Moist Adiabatic Lapse Rate (MALR) Moist Adiabatic Lapse Rate (MALR) = less If air cools to T = TDP, RH = 100% Cloud forms and heat is released Latent heat added due to condensation offsets cooling due to expansion Air cools at a lesser rate (6?C / km) What will be a parcel's temperature at 3 km? (note: its now cooling at MALR) What will be a parcel's temperature at 4 km? 20 ? 6 = 14 ?C 14 ? 6 = 8 ?C 20 ? 6 = 14 ?C 14 ? 6 = 8 ?C

11. Absolutely Stable Air Normally, air T decreases with height Environmental Lapse Rate (ELR) Radiosonde ELR = 4 ?C/km (example) Stable if rising parcel cools more rapidly with height than surrounding environmental T. Always colder than surroundings

12. Absolutely Stable Air

13. Question How would you characterize the stability of an inversion layer?

14. Inversion Layer They are absolutely stable DALR > MALR > ELR Q: How do you form stable layers in the atmosphere? How do we make the ELR turn to the right?

15. Formation of Stable Layers Air aloft warms Warm advection Sinking air (subsidence) surface cools Nighttime radiational cooling Cold air moving in at low levels Warm air moving over a cold surface (cold frontal passage)

16. Formation of Stable Layers Stable air strongly resists upward motion If clouds form in stable air they spread horizontally e.g. cirrostratus, altostratus, nimbostratus, stratus have flat tops and bases

17. Unstable Air Unstable if environmental air Temperature decreases with height more rapidly than a rising air parcel cools The rising parcel will become warmer than its environment and will rise

18. Unstable Air

19. Conditional Stability The unsaturated parcel will be cooler than then environment and will sink back to the ground The saturated parcel will be warmer than the environment and will continue to rise This is an example of conditional instability What is the condition? Whether or not the parcel is saturated.Whether or not the parcel is saturated.

20. Conditionally Unstable Air

21. Questions What causes the atmosphere to become more unstable? What causes air to rise so that clouds can form? ELR must become greater than DALR and MALR.ELR must become greater than DALR and MALR.

22. Formation of Unstable Layers Air T drops rapidly with height ELR steepens 1. Cooling of air aloft Winds bring in cold air Clouds emit IR to space 2. Warming of the surface Daytime surface heating Influx of warm air by wind Air moving over a warm surface

23. Formation of Unstable Layers 3. Warm air moving in at low levels This often occurs ahead of a cold front 4. Cold air moving over warm surface What is an example of this? Example over Lake Michigan Lake-Effect Snow.Lake-Effect Snow.

24. Stability Summary Clouds with rapidly rising air, especially cumulus congestus and cumulonimbus, indicate unstable conditions Stratus clouds indicate stable conditions

25. Review The air temperature in a rising parcel of unsaturated air decreases at the dry adiabatic rate, whereas the air temperature in a rising parcel of saturated air decreases at the moist adiabatic rate The dry adiabatic and moist adiabatic rate of cooling are different due to the fact that _______ ______ is released in a rising parcel of saturated air In a stable atmosphere, a lifted parcel of air will be colder (heavier) than the air surrounding it. Because of this fact, the lifted parcel will tend to sink back to its original position In an unstable atmosphere, a lifted parcel of air will be warmer (lighter) than the air surrounding it, and thus will continue to rise upward, away from its original position The atmosphere becomes more stable (stabilizes) as the surface air _______, the air aloft _______, or a layer of air sinks (subsides) over a vast area The atmosphere becomes more unstable (destabilizes) as the surface air ______, the air aloft ______, or a layer of air is lifted Layered clouds tend to form in a stable atmosphere whereas cumuliform clouds tend to form in a conditionally unstable atmosphere Latent heat. Cools, warms. Warms, cools. Latent heat. Cools, warms. Warms, cools.

26. Cloud Development and Stability Stages: Thermals breaks from surface and ?lifts? Expands and cools on lifting Cools to saturation point Moisture condenses Visible as cloud Can occur in 4 different ways:

27. Cloud Formation Mechanisms (c) And (d) will be discussed later.(c) And (d) will be discussed later.

28. (a) Convection The vertical extent of the cloud is largely determined by the stability of the environment...

29. Stability and Cumulus Clouds In an absolutely stable, environment, no clouds will likely form In a shallow conditionally unstable or absolutely unstable environment, one may expect clouds to develop, but their vertical growth will be limited...

30. Stability and Cumulus Clouds Stable above the cumulus Does not rise ?fair weather Cu? or cumulus humilis Unstable above the cumulus Towering cumulus congestus, then cumulonimbus

31. Questions Are the bases on convective clouds generally higher during the day or the night? Explain. 2. For least polluted conditions, what would be the best time of day for a farmer to burn agricultural debris? 1. At night, because of lower temperatures at the surface, the difference between the dew point and the surface temperature is smaller than during the day. Therefore an air parcel need not rise so high for it to become saturated. If convection is able to occur, the cloud bases will therefore be lower at night. 2. For least pollution the farmer should burn his debris at that time of day most favorable for convection, i.e., at that time when the atmosphere is least stable. That tends to be the case at the warmest time of day. Thus the farmer should burn his debris at the warmest time of day, probably from mid to late afternoon. 1. At night, because of lower temperatures at the surface, the difference between the dew point and the surface temperature is smaller than during the day. Therefore an air parcel need not rise so high for it to become saturated. If convection is able to occur, the cloud bases will therefore be lower at night. 2. For least pollution the farmer should burn his debris at that time of day most favorable for convection, i.e., at that time when the atmosphere is least stable. That tends to be the case at the warmest time of day. Thus the farmer should burn his debris at the warmest time of day, probably from mid to late afternoon.

32. (b) Topography Orographic uplift ? forced lifting along a barrier Forced lifting produces cooling Cloud on windward side Downwind side is warmer and drier why? Heat released during condensation on windward side. Water vapor condensed to form cloud on the windward side = rain shadowHeat released during condensation on windward side. Water vapor condensed to form cloud on the windward side = rain shadow

33. Topography

34. (c) Convergence If air converges to a given location near the surface: it can't "pile up" at that point it can't go downward, the ground is there it must go up! Common at the center of an extra-tropical cyclone (L pressure system)

35. (d) Frontal Lifting If air is lifted into a stable layer: stratus or nimbostratus clouds are often the result (common along warm fronts) If air is lifted into a conditionally unstable layer: cumulus or cumulonimbus are often the result (common along cold fronts)

36. Precipitation Q: How does precipitation form? not all clouds produce precipitation, why? Q: What determines the type of precipitation?

37. Rain Drops, Cloud Droplets, and CCN Formation of raindrops from cloud droplets is complex Cloud droplet: 0.02 mm Rain droplet: 2 mm Radius is 100 x greater, so volume is 100 x 100 x 100 = 1x 106 times greater CCN = Cloud Condensation Nuclei.CCN = Cloud Condensation Nuclei.

38. Precipitation Processes Condensation nuclei play a role far too slow 1 million cloud drops to 1 raindrop must be another process?

39. Rain in Clouds Without Ice (Warm Clouds) Collision and coalescence (sticky water) Most important factor is liquid water content Thickness of cloud Updrafts Electrical charge (+/-) (Smaller drops tend to bounce off one another) http://vortex.plymouth.edu/precip/terminalvelocity.htmlhttp://vortex.plymouth.edu/precip/terminalvelocity.html

40. Rain in Clouds Without Ice Explains why thin stratus only produces drizzle Towering cumulus with rapidly rising air can produce heavy showers

41. Questions 1. Why is a warm, tropical cumulus cloud more likely to produce precipitation than a cold, stratus cloud? 2. Clouds that form over water are usually more efficient in producing precipitation than clouds that form over land. Why? 1. Thicker! More chances for collisoin and coalescense. 2. Contain more water vapor.1. Thicker! More chances for collisoin and coalescense. 2. Contain more water vapor.

42. Rain in Clouds With Ice (Cold Clouds) At temperatures < freezing ice crystals coexist with liquid drops No. cloud droplets > No. ice crystals Liquid water exists as so called ?supercooled? water (-40?C < Temp < 0?C) only ice is found at altitudes above -40?C Why? Smaller the amount of pure water, lower the freezing point (less than 0 ?C) Few ice-forming nuclei (minerals, bacteria, leaf, ice crystals) Supercooled water is sometimes referred to as subcooled water.Supercooled water is sometimes referred to as subcooled water.

43. Supercooled Water Exp.

44. Results: Homogeneous Freezing Pure water drops do NOT freeze at 0?C it needs to be colder Larger water drops will freeze at warmer temperatures than smaller drops Smaller water drops require colder temperatures to freeze Hence, you will find more smaller drops than larger drops higher in the cloud

45. Rain in Clouds With Ice (Cold Clouds) Neither the liquid cloud drops or solid ice particles are large enough to fall as precipitation Given a mixed cloud containing water and ice, which type of particle (ice v.s. water) will grow more quickly and why?

46. Bergeron Process (the 3 Phase process) Bergeron or Ice-Crystal Process Supercooled liquid droplets surround each ice crystal Air is saturated, both are in equilibrium More water vapor molecules around liquid Escape from liquid surface easier Saturation V.P. above water droplet is greater than the ice Water vapor molecules diffuse from droplet to ice crystal Ice crystals grow larger at the expense of the water droplets Also Accretion Aggregation http://vortex.plymouth.edu/precip/precip2aa.htmlhttp://vortex.plymouth.edu/precip/precip2aa.html

47. Accretion, Fracturing and Aggregation

48. Rain in Clouds With Ice (Cold Clouds) Homogeneous nucleation means without condensation nuclei: Homogeneous nucleation occurs when the water vapor molecules condense and form a cloud droplet. To do this requires an environmental temperature of -40C and saturated air, or relative humidity of several hundred percent. Heterogeneous nucleation means with condensation nuclei: It turns out that saturating the air is not always enough to form a cloud. The water vapor molecules need a site to condense on. This site is called a Condensation Nuclei and the process referred to as heterogeneous nucleation. Cloud condensation nuclei (CCN) are about 1 micron is size. Homogeneous nucleation means without condensation nuclei: Homogeneous nucleation occurs when the water vapor molecules condense and form a cloud droplet. To do this requires an environmental temperature of -40C and saturated air, or relative humidity of several hundred percent. Heterogeneous nucleation means with condensation nuclei: It turns out that saturating the air is not always enough to form a cloud. The water vapor molecules need a site to condense on. This site is called a Condensation Nuclei and the process referred to as heterogeneous nucleation. Cloud condensation nuclei (CCN) are about 1 micron is size.

49. Cloud Seeding Man-made: Inject nuclei e.g. silver iodide or dry ice Cloud particles become large enough to fall as rain Not very effective!

50. Questions 1. Suppose that a thick nimbostratus cloud contains ice crystals and cloud droplets all about the same size. Which precipitation process will be most important in producing rain from this cloud? Why? 2. When cirrus clouds are above a deck of altocumulus clouds, occasionally a clear area, or "hole," will appear in the altocumulus cloud layer. What do you suppose could cause this to happen? 1. Bergeron process. Ice crystals grow in preference to cloud droplets. 2. Ice crystals from the Cirrus cloud act as ice nuclei, seeding the altocumulus.1. Bergeron process. Ice crystals grow in preference to cloud droplets. 2. Ice crystals from the Cirrus cloud act as ice nuclei, seeding the altocumulus.

51. Review Cloud droplets are very small, much too small to fall as rain Cloud droplets form on cloud condensation _____. Hygroscopic nuclei, such as salt, allow condensation to begin when RH is less than 100 % Cloud droplets, in above freezing air, can grow larger as faster-falling, bigger droplets collide and coalesce with smaller droplets in their path In the ice-crystal (Bergeron) process of rain formation, both ice crystals and liquid cloud droplets must coexist at below-freezing temperatures. The difference in saturation vapor pressure between the liquid and ice causes water vapor to diffuse from the _________ _________ (which shrink) toward the ________ ________ (which grow) Most of the rain that falls over middle latitudes results from melted snow that formed from the Bergeron process Cloud seeding with _________ __________ can only be effective in coaxing precipitation from clouds if the cloud is supercooled and the proper ratio of cloud droplets to ice crystals exists Liquid droplets Ice crystals Silver IodideLiquid droplets Ice crystals Silver Iodide

52. Precipitation Types Summary Much rain starts as snow through the ?ice crystal process? and melts as it falls Rain drops are not tear shaped. They are round or flattened Water freezes into different shapes at different temperatures A warm layer above a cold surface can result in sleet or freezing rain Hail requires strong updrafts to be supported

53. Precipitation Types - Rain Rain ? drop diameter > 0.5 mm (0.02 in) Drizzle ? drop diameter < 0.5 mm (most from stratus) Virga ? rain leaving cloud base and evaporates before hitting the ground often visible as evaporating streaks of precipitation Rain Events: Showers - localized, sometime heavy rain events usually associated with cumulonimbus sometimes called a "cloud burst? Continuous rain - from nimbostratus

54. Are Raindrops Tear-Shaped? Which shape more accurately depicts a rain drop? Surface tension tends to squeeze drop to a shape that has the smallest surface area for its volume

55. Precipitation Types - Snow Snow - Often visible as fall streaks associated with high cirrus More likely to reach surface when cold Snow Events: Flurries - weak, intermittent - produced from developing Cu Snow squalls - brief, heavy snow fall - produced from Cu Steady Snow - continuous for hours - produced from Nb Blizzard - low temperatures, strong winds, blowing snow

56. Snow Warm, moist air, thin film of water acts as glue = giant snowflakes Cold, dry air = powdery flakes Branching ?dendrite? pattern Aggregates lead to many complex patterns

57. Other Types of Frozen Precipitation The vertical variation of temperature near the ground can have a dramatic influence on the type of precipitation that is observed at the ground

58. Precipitation Types - Sleet If a deep freezing layer exists at low levels, sleet may form Snow will melt in warmer air layers Surface layer will re-freeze into ice pellet known as sleet British sleet = snow and rain Layer may be too shallow to re-freeze, supercooled liquid reaches surface and instantly freezes = frezing rain (glaze)British sleet = snow and rain Layer may be too shallow to re-freeze, supercooled liquid reaches surface and instantly freezes = frezing rain (glaze)

59. Meteograms Source: http://vortex.plymouth.edu/precip/precip2d.html is a typical profile for sleet.? You can see how the layer of colder air goes up higher in the atmosphere than it does in B, which is a typical profile for freezing rain.? The layer of cold air in B is much more shallow and therefore, for a falling rain droplet, it would not have time to freeze while it is still in the air, but instead would freeze as it makes contact with the below freezing surface of the earth. This is a Meteogram, which is a more complicated version of the graphs shown in these slides.Source: http://vortex.plymouth.edu/precip/precip2d.html is a typical profile for sleet.? You can see how the layer of colder air goes up higher in the atmosphere than it does in B, which is a typical profile for freezing rain.? The layer of cold air in B is much more shallow and therefore, for a falling rain droplet, it would not have time to freeze while it is still in the air, but instead would freeze as it makes contact with the below freezing surface of the earth. This is a Meteogram, which is a more complicated version of the graphs shown in these slides.

60. Precipitation Types - Freezing Rain If a shallow freezing layer exists at low levels, freezing rain, or glaze may form Supercooled fog or cloud freezing = rime

61. Precipitation Types - Snow Grains and Pellets Snow grains ? small opaque grains of ice (drizzle sized from St cloud) Snow pellets (graupel, soft hail) ? white opaque grains of ice size of a rain drop (break/bounce on impact)

62. Precipitation Types - Hail Graupel, frozen rain, insects act as accumulation points ?embryos? Grow by accretion in Cb clouds

63. Measuring Precipitation Rain gauge contains a funnel to magnify the effective rain depth EXAMPLE:? 0.1 inches of rain would fill the inside tube with 1 whole inch of water, and a scaled ruler would measure this as 0.1 inches of actual rainfall. *Imagine how hard it would be to measure to the nearest .01 of an inch in a regular bucket of water with a standard ruler! Radio detection and rangingRadio detection and ranging

64. Measuring Precipitation A tipping bucket gauge ? a pair of buckets seesaw, alternately fill and dump rain every 0.01? The amount of precipitation can be recorded along with the time of occurrence and intensity (amount of rain per unit of time).

65. Radar ra(dio) d(etecting) a(nd) r(anging) Radars transmit electromagnetic waves that bounce off of precipitation particles Radar waves are scattered by rain or snow Almost no scattering by cloud droplets (too small) Strength of radar echo is related to rain or snowfall rate Better radar in 1990?s: Doppler radar measures everything a non-Doppler measures, plus speed of approaching or receding objects) A palindrome is a word, verse, or sentence that reads the same backward or forward. A palindrome is a word, verse, or sentence that reads the same backward or forward.

66. Doppler Radar Doppler radar = radar that can measure ?Doppler shift? (change in frequency (or ?) of waves caused by object approaching or receding) http://www.mnsu.edu/weather/wx_WALTER/Modules/Radar/radar_edu.htmlhttp://www.mnsu.edu/weather/wx_WALTER/Modules/Radar/radar_edu.html

67. Doppler Radar Draw radar waves on the following image:

68. Answer


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