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

Clouds Precipitation Read Anthes 2 and begin 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

Cloud Photos Plymouth State University Meteorology Program Cloud Boutique

Precipitation and Energy

Mark’s Photos

Precipitation and Energy

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.

Fog, 8:00 am Sept. 14th

No fog, 8:45 am Sept. 14th

Fog, 6:30 am Sept. 17th

No fog, 8:30 am Sept. 17th

Divergence and Convergence: Bays and Headlands

Clouds Are Formed by Lifting

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

Comparison of Droplet Sizes

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

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 e s Snow forms most of our precipitation (even in summer!)

The Ice Crystal Process Wegener-Bergeron-Findeisen Theory 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.

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

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

Hydrologic Cycle

“Clouds in a glass of beer” Clouds = liquid droplets suspended in gas. Beer = gas droplets suspended in liquid. CO 2 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.