Dr. Michael MacCracken, The Climate Institute, Washington, DC

1. Family Homecoming Special Event"Can Climate Engineering Serve as a Complementary Step to Aggressive Mitigation?" • Dr. Michael MacCracken, The Climate Institute, Washington, DC • Friday, Sept. 25 at 4:00 pm in Olin 1, with cookies

2. CloudsPrecipitationRead Anthes 2 and begin 3

3. Other special types • Orographic - clouds that form via interaction between wind and mountainous terrain features • Artificial clouds - contrails • Instability waves generated by wind shear (waves are always there, but are visible when there is moisture condensing in them

4. Cloud Photos • Plymouth State University • Meteorology Program • Cloud Boutique • http://vortex.plymouth.edu/clouds.html

5. Precipitation and Energy

6. Mark’s Photos

7. Precipitation and Energy

9. Fog is a Cloud on the Ground • How Fog is Formed • Radiation fog (local)-- Radiational cooling of a shallow moist layer with dry layer above it. Dissipates with morning sun. • Evaporation fog (local) -- Cold air in contact with a warmer water surface (e.g. lakes in autumn). • Upslope fog (mountains) -- gentle lifting of a moist layer. • Advection fog (regional) -- warm moist air moves over a cold surface. E.g. Pacific coast cold ocean surface. • Precipitation fog (regional) -- warm rain falls through a layer of cold air or over a snowfield.

10. Fog, 8:00 am Sept. 14th

11. No fog, 8:45 am Sept. 14th

12. Fog, 6:30 am Sept. 17th

13. No fog, 8:30 am Sept. 17th

14. Divergence and Convergence:Bays and Headlands

15. Clouds Are Formed by Lifting

16. Precipitation Terminal velocity > updraft velocity Drops must be large enough to fall to ground and not evaporate. Otherwise we call them “fall streaks.”

17. Comparison of Droplet Sizes

18. How do cloud droplets grow? • Curvature effect (-) • The greater the curvature of a droplet, the greater the rate of evaporation. So small droplets tend to disappear unless the air is supersaturated. • Solute effect (+) • Hygroscopic salt particles in a droplet slow the rate of evaporation, allowing small droplets to grow larger. • Collision-Coalescence Process • Drops of different sizes fall at different rates. Big drops sweep up little drops. • Droplets collide and grow. • 1 raindrop = 1 million cloud droplets

19. Ice Crystal Process • Cold cloud process (ice and supercooled water droplets) • Water evaporates and deposits onto ice crystals • Ice crystals grow at expense of water droplets because of es • Snow forms most of our precipitation (even in summer!)

20. The Ice Crystal ProcessWegener-Bergeron-FindeisenTheory • Since ice crystals have a lower saturation vapor pressure than water droplets, molecules migrate from the droplets to the ice. The ice grows at the expense of the water droplets. • Clouds can be “seeded” by chemicals like Silver Iodide which have hexagonal structure. They can be supercooled by substances like Dry Ice.

21. Saturation vapor pressure as a function of T for ice and water surfaces

22. Important terms • Divergence • <-------------- O ---------------> • Convergence • --------------> O <-------------- • Supersaturation • RH > 100% due to curvature of droplets • Supercooled • Cloud droplets can remain liquid at T < 0

23. Hydrologic Cycle

24. “Clouds in a glass of beer” • Clouds = liquid droplets suspended in gas. • Beer = gas droplets suspended in liquid. • CO2 at 2 atm. in the bottle = supersaturated. • Rapid expansion: T goes from 5 C to -36 C • Water vapor condenses in neck of bottle. • Bubbles in glass do not form randomly, but at nucleation sites. • Bubbles (rain) reach an upward terminal velocity (buoyancy vs. drag) • Beer clouds can be “seeded” with explosive results. • White foam is thin -- scattering at all wavelenghts. • Yellow liquid is dense -- short, blue wavelenghts absorbed.