Moisture, Clouds, and Weather

1. Moisture, Clouds, and Weather Chapter 19 (part 1 of 3)

2. From Water to Water Vapor The warmer the air, the more water vapor it can hold

3. Humidity • Absolute • Mass of water in a given volume (g/m3) • Air at 25ºC = 23 g/m3 • Air at 12ºC = 11.5 g/m3 • Relative actual quantity of H2O (%) = per unit of air x 100% maximum quantity at the same temperature

4. Relative Humidity (RH) • Saturation – 100% RH • Dew Point • If you are at 100% RH, and you cool it below, water vapor becomes liquid • Water condenses on soil, grass, airborne particles • Supersaturation and supercooling • Needs a nucleation site

5. How Does Air Reach the Dew Point?

6. Radiation Cooling • Heat lost by giving off energy (infrared / thermal) • Can be from atmosphere, land, or water

7. Contact Cooling • Warm, moist air cools against cold material • Dew • Frost • If dew point is below freezing • Formed directly from vapor

8. Cooling of Rising Air • Adiabatic temperature changes • Changes in T due to expansion or compression • Air cools between 10°C/1,000 m when dry and 5°C/1,000 m when wet • Adiabatic lapse rate • Why are they different? • Latent heat as gas becomes water

9. Rising Air and Precipitation Sierra Nevadas/ Panamint Range Death Valley Pacific Ocean Orographic Lifting

10. Rising Air and Precipitation Frontal Wedging Convection-Convergence (unequal heating)

11. What Controls Cloud Formation? • Normal Lapse Rate (6°C/1,000 m) • Air cools with elevation in troposphere • Varies with altitude, latitude, time of day, seasons • Dry air mass • “Wet” air mass • Latent heat keeps it “warm,” rises very high

13. Cirrus Clouds • “Wisp of hair” • High (6,000-15,000 m) • Made of ice crystals • Thin due to dry air

14. Stratus • “Layer” • Horizontal, sheet-like • Typical of… a cloudy day • Occur when condensation stops rising and spreads out • Nimbostratusif accompanied by rain or snow

15. Cumulus • “Heap” or “pile” • Think of it like an accumulation of clouds • Fluffy: display flat bottoms and billowy tops • Base of cloud at level of dew point in air

16. Cumulonimbus • Cumulus clouds which produce precipitation • Top sheared by winds, spreads at tropopause

18. Precipitation and Cloud Formation

19. The Formation of Rain • Droplets in a cloud are small (0.01 mm) • Coalescence of droplets • In clouds above freezing • 1 million to one “drop” • Ice is less dense than water • As air cools toward the dew point, water vapor may spontaneously form ice… not water • Ice then falls, remelts and makes rain

20. Surface Tension • Water wants to be a sphere!

21. Snow, Sleet… and Glaze? • If cloud is composed of ice and air near ground is cold…

23. Glaze

24. Hail • Hail—only occurs in thunderstorms—created by up/down drafts • Most frequent in late spring-early summer • Extreme T differences from surface to jet stream • Air forced upward over mountains Largest hailstone ever recorded: Diameter: 7.0 in (17.8 cm) Circumference: 18.75 in (47.6 cm) Weight: 756 g (1.5 lbs.)

25. Pressure and Wind • Warm air rises (~1 km/day), creates low pressure • Cold air sinks • Wind is caused by pressure differences

26. Pressure Gradient • Change in pressure difference with distance • Wind speed determined by pressure difference • Earth is unequally heated, pressure changes widely Large pressure difference

28. What Causes All the Variation • Solar heating from the Sun • Ex: Creates deserts at 30° N and S • Seasonal tilt of Earth • Continental character (e.g., mountains) • Ocean currents (temperature)

29. Cyclones and Anticyclones • Remember the Coriolis Effect

30. Air Masses and Fronts • Air mass: large body of air with ~uniform T and humidity at any given latitude • Ex: Atlantic O. above Gulf Stream circulation • Air masses collide along a front

31. Warm front • Air moves up slowly, light precipitation

32. Cold front • Air moves up rapidly, lots of precipitation

33. Occluded Fronts • Two cold air masses trap a warm air mass • Storm is short-lived; fast-moving cold air cuts off moisture supply

34. Stationary Front • If neither air mass is strong enough to replace the other, the system stalls

35. Formation of a Cyclone • Eventually (1-3 days), air rushing in to P zone equalizes pressure, storm dissipates

36. Thunderstorms

37. Thunderstorms • A weather phenomenon characterized by the presence of lighting (and thunder) • also--Heavy rain, high winds, hail, and-possibly- tornadoes • Common in areas of moist air • Equatorial regions • SE US • Can occur as • Single-cell or multi-cell • Squall line • Supercell—most dangerous

38. Thunderstorm development • Need: moisture, unstable air, lifting • Tend to follow three stages of development…

39. Lightning

41. Strikes • 1st, downward passage of negative charge • 2nd, bright return stroke (of light, not electrons)… like this • Heats air to 50,000ºF (28,000ºC) • Massive expansion of air at supersonic speed • Air moves at diff speed than sound it makes • Speed of light ~ 3 x 108 m/s, sound = 344 m/s • Divide # of seconds between flash and thunder by 5 • Safety: 30 s time difference = take cover

42. Do’s and Don’ts! • If inside • Don’t touch anything plugged in, or corded phones • Do not take a bath/shower or wash dishes • If outside • Seek shelter in a “safe” house (one with plumbing and wiring) • Avoid “unsafe” structures like patios, tarps, picnic pavilions, etc.. • Stay away from tall trees • Crouch on the balls of your feet • Shut off your car and touch nothing metal

43. Lightning Myths • Myth • Jewelry and metal belt buckles attract lightning • If outside, lay flat on the ground to decrease your height • Lightning never strikes the same place twice • Truth • Being isolated, tall and exposed make you more prone to being struck • Lying flat makes you more prone to ground current—lightning crouch • Lightning often strikes the same place repeatedly, especially if the object is tall • Ex. Empire State Building Lightning safety

45. Tornados

46. What is a Tornado? • Violently rotating, funnel-shaped cloud • Often associated with extremely violent thunderstorms--supercells

47. Tornado Facts • Tornados can occur almost anywhere in the world • Occur most frequently in US (tornado alley) • Duration: a few minutes (can last several hours) • Diameter (Avg.): 150’(can be >1/2 mile wide) • Length of path (Avg.): 4 miles (can be >200 miles) • Funnel can travel from 0 mph up to ~70 mph, usually travels at 30 mph • Winds 65->210 mph

48. When Tornados Occur A typical late afternoon tornado

49. Tornado Development—the supercell • A supercell—a thunderstorm containing a mesocyclone—a large, spinning updraft • Caused by motion of surface wind and wind aloft--vorticity

50. The Supercell Direction of storm motion