MADISON’S CURRENT WEATHER Madison Weather at 1000 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 (1015.2 mb) http://www.ssec.wisc.edu/localweather/

                                                                                                                                                                                                                                                                                                                                          Last 24 hrs in Madison FOG http://weather.uwyo.edu/cities/

Current Surface Weather Map with Isobars (“iso” = equal & “bar” = weight), Fronts and Radar http://maps.weather.com/images/maps/current/curwx_720x486.jpg

Current Temperatures (°F) & Isotherms (“iso” = equal +”therm” = temperature) http://maps.weather.com/images/maps/current/acttemp_720x486.jpg

Current Dewpoints (oF) http://maps.weather.com/images/maps/current/actdew_720x486.jpg

Tomorrow AM Forecast Map http://maps.weather.com/images/maps/forecast/amfcst_720x486.jpg

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 ...

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).

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 103 = 1000 times larger.

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.

Virga

A Convective Rainshower

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, ...

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!

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)

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.

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.

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

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)

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

LIQUID PRECIPITATION National Weather Service

FROZEN PRECIPITATION National Weather Service

Snow

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

Varieties of Snowflakes

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

FREEZING PRECIPITATION National Weather Service

Freezing Rain

Freezing Rain/Ice Storm

Hail Stones

Hail Stones

Hail Storm

Snow Event

Rain Event

Freezing Rain Event

Ice Pellet (Sleet) Formation

Summary of Precipitation Type Formation

Freezing Rain Climatology

G. PRECIPITATION CLIMATOLOGY - Annual http://metart.fao.org/~~/gbr/E-CLimGL.htm

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.

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

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

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).

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

B. NATURE OF VISIBLE LIGHT The Electromagnetic Spectrum A review

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

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

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]

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

Reflection Relationship Angle of reflection = angle of incidence

Specular and Diffuse Reflection

Comparisons

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

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

Refraction (con’t.)

Refraction (con’t.)

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

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

Dispersion

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

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.