100 AOS / 101 Lecture summary: 9/28/07
Reading: Ahrens 7th Ed. Ch. 6, pp. 133-142
Ahrens 8th Ed. Ch. 5, pp. 106-123
Cloud Condensation Nuclei
Water does not readily condense onto drops on its own
Aerosols in ATM: small particles (dust, soot, sulpheric acid
droplets, salt) suspended in atmosphere
Water condenses onto these aerosols in the atmosphere. Evidence:
Ship tracks, dust storms, etc
Source of Aerosols:
Continental Aerosols
Crustal aerosols - erosion of earth's surface -> dust storms,
deserts
Combustion and secondary aerosols - anthropogenic activities ->
N. American and European sources
Carbonaceous Aerosols: soot, biomass burning, pollen, spores ->
tropical sources
Anthropogenic sources account for ~30% of all aerosol production;
~60% over urban areas
Marine aerosols: mainly sea salt
Big source: bubbles (foam) popping at ocean surface
As bubble pops, 4-5 large (~50 micrometer) drops are ejected into
the air
Also, surface film of popping bubble ejects 15-20 small (~1-10
micrometer) droplets into the air
Droplets evaporate, leaving salt aerosol in ATM
Haze around ocean regions in afternoon
Aerosol concentrations:
Continental regions: > 100,000 per cubic cm (cm^3)
Marine regions: ~1000 per cm^3
==> for the same amount of water mass in a cloud, continental
clouds have smaller, and a lot more cloud droplets than marine
clouds
Hygroscopic (water seeking) aerosols: very efficient CCN -> sulfate
aeroslols, salt
Hygrophobic: not efficient -> CCN oils, parafin waxes, gasoline
Sulfate particles -> dominant form of CCN in continental and marine
(DMS) environments
CCN allow clouds to form at or below 100% RH
** In class example: Cloud in a Jar **
Fog
Fog: visibility < 1km, cloud at surface
Four main types: radiation, advection, upslope, steam (evaporation)
fog
Radiation fog:
Radiation cools ground and air
Heavier, cold air flows toward lower elevations
Air cools to dew point, water vapor condenses
Sun rises, warms surface. "Burns" fog off
Radiation fog forms at night, burns off in the morning. In mountain
valleys, it can persist for days in winter
Common in Midwest. River valleys in Driftless area
Advection fog:
Advection: horizontal transport
Warm moist air is advected (moved horizontally) over a cloder
surface
Fog forms as air (in contact with ground) cools to dew point
Advection fog: common in W...