Transcript 922 Lecture, Clouds, Precipitation

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
http://vortex.plymouth.edu/clouds.html
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 es
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
<-------------- O --------------->
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.
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.