1. Moisture, Clouds, and Precipitation
Chapter 18 Page 502
Moisture, Clouds, and Precipitation

2. A. Water in the Atmosphere
Introduction
Water vapor
Source of condensation and precipitation
Most important gas for atmospheric processes
Only 0-4% of gas
Precipitation – any form of water that falls from a cloud

3. 2. Water’s Changes of State a. Allows for the water cycle b
2. Water’s Changes of State a. Allows for the water cycle b. Changes with temperature and pressure c. Energy transfer in the form of heat must happen d. Solid to Liquid 1) Melting 2) Ice remains 0C until all ice is melted 3) Energy is stored in liquid water

4. e. Evaporation – process of going from a liquid to gas f
e. Evaporation – process of going from a liquid to gas f. Condensation 1) Process of going from gas to liquid 2) Causes clouds and fog

5. g. Sublimation – solid to gas h
g. Sublimation – solid to gas h. Deposition 1) Gas directly to solid 2) Frost

6. Phase Changes

7. 3. Humidity a. Amount of water vapor in the air b
3. Humidity a. Amount of water vapor in the air b. Saturation 1) Can’t hold any more water 2) Depends on temperature 3) Warm air holds more water

8. c. Relative Humidity 1) Ratio of air’s actual water-vapor content compared to amount of water vapor air could hold at that temp and pressure 2) Changed by a) Adding or removing water vapor i. Evaporation ii. Precipitation

9. b) Temperature i. Cooling can make nonsaturated air become saturated ii. Saturated air is cooled which causes condensation = clouds

10. d. Dew Point 1) Temp at which air needs to be cooled to become saturated 2) If cooled more = dew, fog, clouds 3) Higher dew points – moist air 4) Low dew points – dry air

11. B. Cloud Formation Air Compression and Expansion
Adiabatic Temperature Change
Happens without heat being added or subtracted
Happens with air compression and expansion
Expands = cools
Compressed = warms

12. b. Expansion and Cooling 1) Pressure decreases with altitude 2) Rising air expands and cools 3) Dew point is reached and clouds form 4) Opposite for sinking air

14. 2. Processes That Lift Air a
2. Processes That Lift Air a. Orographic Lifting 1) Caused from elevated terrain (mts.) 2) Rainshadow a) Windward side - air rises, expands, cools, and rains b) Leeward side – moisture gone, air sinks, compresses, warms = no rain

16. b. Frontal Wedging 1) Front – warm air and cold air collide 2) Cold air is denser 3) Warm air rises over it 4) Air expands, cools, reaches dew point, etc.

18. c. Convergence 1) Two air masses collide and are forced up 2) Air expands, cools, etc. 3) Florida

19. d. Localized Convective Lifting 1) Unequal heating of earth’s surface - Paved parking lot compared to wooded park 2) Warmer air rises, expands, cools, etc. 3) Thermals – parcels of warm rising air 4) Hawks/eagles use these for lift

21. 3. Stability a. Density Differences 1) Warm air – rises; cold air – sinks 2) Unstable air – air warmer than surroundings 3) Stable air – remains in original position

23. b. Degrees of Stability 1) Air is stable when temp decreases gradually with altitude 2) Temperature inversion - air temp increase with altitude 3) Most stable with temperature inversion a) Happens on clear nights b) Ground cools faster than air

24. c. Stability and Daily Weather 1) Stable air a) Resists vertical movement b) Wedging or convergence causes clouds c) Clouds are widespread and thin d) Little to no rain is produced e) Dreary overcast days

25. 2) Unstable air a) Frontal wedging or convergence form clouds b) Towering, thick clouds c) Thunderstorms and tornadoes

26. 4. Condensation a. Happens to saturated air b
4. Condensation a. Happens to saturated air b. Can be cooled to dew point or moisture added c. Types of surfaces 1) Needs something to condense on 2) Dew – grass, buildings, trees, etc.

27. 3) Air a) Condensation nuclei – dust, smoke, pollution, etc
3) Air a) Condensation nuclei – dust, smoke, pollution, etc. b) Condensation nuclei allow for condensation to form

28. C. Cloud Types and Precipitation
Types of clouds
Classification is based off height and shape
Cirrus
High, white, and thin
Wispy or feathery appearance
Could give halo

29. Cirrus Clouds
White and wispy
Made of ice crystals
Pictured Rocks, UP

30. c. Cumulus 1) Rounded individual cloud masses 2) Flat base w/rising domes 3) Cauliflower structure d. Stratus 1) Sheets or layers that cover entire sky 2) No individual cloud units
St. Petersburg, FL

31. Stratus
Ouachita, AK
Cumulus
Mackinac Island

32. e. High Clouds (3.5 miles +) 1) Cirrus 2) Cirrocumulus – fluffy 3) Cirrostratus – flat layers 4) Thin, white, ice crystals 5) No precipitation 6) Cirrus clouds followed by cirrocumulus or cirrostratus may mean approaching storms

33. f. Middle clouds (1.5 mi – 3.5 miles) 1) Use prefix alto- 2) Altocumulus – rounder and denser than cirrocumulus 3) Altostratus a) White to grayish sheet b) Sun visible as a bright spot 4) Infrequent and very light precipitation

34. g. Low Clouds 1) Stratus a) Gray and blanket most the sky b) Occasional light precipitation 2) Stratocumulus – long parallel rolls 3) Cumulus – fluffy clouds; fair weather

35. 4) Nimbostratus – dark gray blanket a) Rain clouds b) Long all day rains
Munising, MI

36. h. Clouds of Vertical Development 1) Base is low altitude but top is middle to high 2) Caused from unstable air 3) Cumulonimbus a) Heavy rains and thunderstorms b) Hail and tornadoes possible c) Cauliflower look

38. Sioux City, IA
Cauliflower top

39. 2. Fog a. Cloud with base very close to ground b
2. Fog a. Cloud with base very close to ground b. Caused by Cooling 1) Ground cools air close to ground 2) Air reaches dew point 3) Becomes denser and accumulates in low areas

40. Sioux City, IA
Normal
Fog

41. c. Caused by evaporation 1) Cool air moves over warm water 2) Water evaporates and air becomes saturated 3) Rising water vapor hits cold air 4) Cools and condenses 5) Rises immediately with air being warmed from below

43. Wall Cloud

44. Squall Line Produce a shelf cloud

45. SCUD Cloud Stratocumulus Under Deck
Result from high humidity near storm
Can be very low to the ground
NOT an indication of severe weather

46. Types of Clouds

47. 3. How Precipitation Forms
a. Cold Cloud Precipitation – Bergeron Process
1) Air is saturated w/water, which means it is supersaturated w/respect to ice
2) Water can’t coexist with ice crystals
3) Extra water becomes ice
4) Water continually evaporates
5) Crystals grow until heavy enough to fall

48. b. Warm Cloud Precipitation 1) Collision-coalescence process 2) Salt pulls vapor out of the air 3) Forms large drops 4) Drops move through cloud colliding and coalescing with smaller drops

49. 4. Forms of Precipitation a. Rain – liquid water falling b
4. Forms of Precipitation a. Rain – liquid water falling b. Snow – six-sided light, fluffy crystal c. Sleet 1) Falling clear/translucent ice 2) Warm air above freezing air

50. Sleet Formation

51. Dordt College Sioux Center, IA
d. Freezing rain 1) Supercooled water which freezes on contact 2) Warm air above freezing air

52. Freezing Rain Formation

53. Freezing Rain
Lansing, IL

55. Deicing Planes Water freezes on plane

56. 3) Grow by supercooled water 4) Updrafts keep hail in air
e. Hail
1) Cumulonimbus clouds
2) Begin as ice pellets
3) Grow by supercooled water
4) Updrafts keep hail in air

57. f. Graupel 1) “Soft hail” or “snow pellets” 2) Supercooled water freezes on a falling snowflake