1. MADISON’S CURRENT WEATHER
Madison Weather at AM CDT 18 JUL 2002
Updated twice an hour at :05 and :25
Sky/Weather: MOSUNNY
Temperature: 80 F (26 C)
Dew Point: 69 F (20 C)
Relative Humidity: 69%
Wind: W6 MPH
Barometer: 29.98S ( mb)

2.                                                                                                                                                                     
                                                                                                                                                                    
Last 24 hrs in Madison
FOG

3. Current Surface Weather Map with Isobars (“iso” = equal & “bar” = weight), Fronts and Radar

4. Current Temperatures (°F) & Isotherms (“iso” = equal +”therm” = temperature)

5. Current Dewpoints (oF)

6. Tomorrow AM Forecast Map

7. D. BUILDING A RAINDROP (con’t.)
Need to add water to increase cloud droplet size to raindrop size
But… amount needed is proportional to cube of radius
Factors opposing Precipitation
Cloud-producing updrafts greater than droplet’s terminal velocity;
Dry atmosphere below cloud base may evaporate droplet to form VIRGA. where ...

8. D. BUILDING A RAINDROP (con’t.)
Terminal velocity
Dependent upon size
Amount of water needed depends upon:
Volume of spherical droplet;
or equivalently, the cube of the radius (Since V = 4/3 r3, where r = radius).

9. D. BUILDING A RAINDROP (con’t.)
So if rcloud = 20 m, an increase to rrain = 200 m:
then a tenfold increase in radius (200 m Vs. 20 m) causes a Volume increase that would be = 1000 times larger.

10. D. BUILDING A RAINDROP (con’t.)
Factors opposing Precipitation
Cloud-producing updrafts greater than droplet’s terminal velocity;
Dry atmosphere below cloud base may evaporate droplet to form VIRGA.

11. Virga

12. A Convective Rainshower

13. E. POSSIBLE PRECIPITATION MECHANISMS (Growth of Liquid rain drops)
The current proposed mechanisms:
Simple Condensation by Diffusion Process
The Collision - Coalescence Process
The Ice Crystal or Bergeron-Findeisen Process
or specifically, ...

14. PRECIPITATION MECHANISM: CONDENSATION-DIFFUSION PROCESS
Based upon H20 vapor molecules condensing upon cloud droplets;
Competing Effects
Curvature Effect
Solute Effect
Condensation needs to overcome surface tension of droplet;
Process would take 24 to 48 hours;
Hence, process is too slow!

15. PRECIPITATION MECHANISM: COLLISION-COALESCENCE PROCESS
Requires droplets of various sizes, with some greater than 20 m diameter;
Large droplets have greater fall speed;
Figure 8.2 Moran & Morgan (1997)

16. COLLISION-COALESCENCE THEORY (con’t.)
Different sized droplets have different surface electrostatic charges;
Large droplet has a wake “low”;
Thus, large droplets collect smaller droplets and form a rain drop.

17. PRECIPITATION MECHANISM: ICE CRYSTAL (or Bergeron-Findeisen) PROCESS
Requires a mixture of ice crystals and supercooled liquid droplets;
supercooled droplet remains a liquid at -40oC (-40oF) < T < 0oC (32oF);
smaller droplets are more frequently supercooled.

18. Saturation Vapor Pressure as a Function of Temperature See Fig. 6
Saturation Vapor Pressure as a Function of Temperature See Fig. 6.3 Moran & Morgan, 1997

20. ICE CRYSTAL THEORY (con’t.)
Saturation vapor pressure over supercooled liquid >> over ice at same T;
Therefore, ice crystal grows at expense of supercooled water droplet.
Figure 8.3 Moran & Morgan (1997)

21. F. PRECIPITATION TYPES
Distinguishing Features of Precipitation Types
Liquid (Rain, Drizzle)
Frozen (Snow, Ice Pellets, Hail)
Freezing (Freezing Rain, Freezing Drizzle) where --

22. LIQUID PRECIPITATION National Weather Service

23. FROZEN PRECIPITATION National Weather Service

24. Snow

25. Recall PHYSICAL PHASES of H2O
Solid (Ice) -- Note Hexagonal (6 sided) Structure

26. Varieties of Snowflakes

27. Varieties of Snowflakes See Fig. 8.6, Moran & Morgan, 1997

28. FREEZING PRECIPITATION National Weather Service

29. Freezing Rain

30. Freezing Rain/Ice Storm

31. Hail Stones

32. Hail Stones

33. Hail Storm

34. Snow Event

35. Rain Event

36. Freezing Rain Event

37. Ice Pellet (Sleet) Formation

38. Summary of Precipitation Type Formation

39. Freezing Rain Climatology

40. G. PRECIPITATION CLIMATOLOGY - Annual

41. H. WEATHER MODIFICATION BACKGROUND
Statement of Problem
Why no precipitation?
Can humans cause rain?
For precipitation, need:
sufficient atmospheric humidity;
a cloud;
sufficiently large raindrops/snowflakes to fall & reach ground as rain or snow.

42. INTENTIONAL ARTIFICIAL WEATHER MODIFICATION
Goals
Stimulate Precipitation
Dissipate Clouds & Fog
Suppress Hail
Modify Hurricanes

43. C. WEATHER MODIFICATION (con’t.)
Precipitation Stimulation Techniques - "Cloud seeding"
Early Weather Modification Techniques
Modern Precipitation Stimulation Strategies
Problems
Scientific
Legal

44. TYPICAL STRATEGIES
WARM CLOUDS
Add large hygroscopic nuclei.
COLD CLOUDS
Freeze all supercooled droplets with Dry Ice (Solid CO2);
Add freezing nuclei with ice-like crystal structure with Silver Iodide (AgI).

45. ATM OCN 100 - Summer 2002 LECTURE 16
ATMOSPHERIC OPTICAL PHENOMENA
A. INTRODUCTION
Definition & Scope
Historical Background

46. B. NATURE OF VISIBLE LIGHT
The Electromagnetic Spectrum
A review

47. B. NATURE OF VISIBLE LIGHT (con’t.)
Visible Light, Sunlight & the Electromagnetic Spectrum
I. Newton, sunlight & polychromatic light
Visible Light, Color & Human Visual Perception

48. C. OPTICAL PROCESSES Scattering Reflection Refraction Dispersion
Optical Phenomena depends upon Processes affecting sunlight
Review of Optical Processes
Scattering
Reflection
Refraction
Dispersion
Diffraction where ...

49. Rayleigh Scatter [small particles] Mie Scatter [large particles]
SCATTERING
Redirection of light ray due to suspended particles.
New direction may be in forward or backward direction.
Depends upon relative size of scatterer:
Rayleigh Scatter [small particles]
Mie Scatter [large particles]

51. REFLECTION Specular Diffuse
Redirection of light ray in backward direction by large surface.
Depends upon reflecting surface:
Specular
Diffuse

52. Reflection Relationship Angle of reflection = angle of incidence

53. Specular and Diffuse Reflection

54. Comparisons

55. REFRACTION
Redirection of light ray within medium due to density differences.

56. density of medium wavelength of light incident angle
REFRACTION (con’t.)
Depends upon:
density of medium
wavelength of light
incident angle
Relationship (Snell’s Law)
Angle of refraction  angle of incidence

57. Refraction (con’t.)

58. Refraction (con’t.)

59. DISPERSION
Separation of polychromatic light ray into component colors during passage through a medium.

60. DISPERSION (con’t.) density of medium wavelength of light
Requires concurrent refraction.
Depends upon:
density of medium
wavelength of light

61. Dispersion

62. DIFFRACTION
Constructive & destructive interference patterns of light waves due to slight bending of light ray moving around an object.

63. DIFFRACTION (con’t.) Requires small openings or objects.
Produces light and dark bands.
Depends upon wavelength.
Polychromatic light ray may be broken into component colors.