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 Relative 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
Nimbostratus if 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: in (17.8 cm)
Circumference: 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 Heats air to 50,000ºF (28,000ºC)
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 If outside
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 Truth
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
Distributed by the Disaster Team
Supercourse (
Developers
Developed by Randall Schindler while at Steven F. Austin University
Distributed by the Supercourse Tornado Team, Harold Brooks, NOAA, Daniel Mccarthy, NOAA, Schaefer, NOAA
Eric Noji, CDC, Scott Lillianbridge, Uni. Texas
Francois Sauer, Supercourse, Kansas City
Faina Linkov, Mita Lovalekar, Ronald LaPorte, Supercourse Pittsburgh
Doug Barrett, Janet Winterton, Fraser, Kevin Maney, USA Today

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
Anytime of the year- usually in the spring, summer, and fall
Most tornados occur during late spring in the month of May
Between the late afternoon and early evening is when most tornados are spawned
The most dangerous time for formation during evening hours

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

51. Tornado Formation Mesocyclone changes position
Development of rear flank downdraft—gust front
Tornado develops at occlusion point between RFDD and FFDD

53. Tornado Life Cycle Dust whirl Organizing Mature Shrinking
Decaying or “rope” stage A
Note: The time it takes to go from one stage to the next can vary considerably…not every tornado goes through all the stages C

54. Tornado Indicators
A greenish colored sky (cause by light scattering--hail)
Mammatus clouds
A sudden drop in barometric pressure
Large hail of at least .75 in. diameter
Strong winds > 60 mph
Frequent and intense lightning
A rotating wall cloud or a cloud that appears to hang from the sky
A loud rumbling noise- seek shelter!

55. Mammatus clouds
Green sky

56. Early Warning Systems Tornado watch Tornado warning
Tornado development is possible; take shelter and remain alert
Tornado warning
Radar indication or direct observation of a Tornado on the ground

57. Tornado Safety If inside If outside
Small, interior, windowless room on lowest level of building
Basement or storm shelter
Bathtubs and pillows—know what is above you
If outside
Seek shelter indoors
If no shelter available, find low spot and cover head
NEVER take shelter in a car