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How Clouds Work
What links oceans, atmospheres, and biology?
Where is atmospheric water?
• Vapors (Humidity) • Liquids (Clouds!) • Solid (Ice)
visible infrared (water vapor window)
infrared (water vapor)
Then 31% of the Sun’s energy will be reflected back to space
Incoming solar radiation
Outgoing reflected energy Fs (αp~31%)
Outgoing absorbed energy FI (infrared)
Adapted from K.N.Liou, 1992; Aerosol effects from IPCC 2001;
CO2+ The
Greenhouse Effect
The White House Effect
+ Aerosols + Land
S0 = 346 W m-2
FS = S0*(1-αp) = 346*(1-0.31) = 239 W m-2
Fghg
Inter-Tropical Convergence Zone (ITCZ) Warm Pool
South Pacific Convergence Zone (SPCZ)
storm track storm track
storm track
Reflected Solar Radiation by Clouds
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Cumulus Clouds Swelling Cumulus
Active heaped-up cloud with flat bottom and growing cauliflower top. [http://www.fox8wghp.com/spacious.htm]
10.2
Cumuliform Clouds
Cumulonimbus Clouds
Cumulonimbus Massive cloud system producing heavy showers, sometimes with hail. Most active clouds may have lightning and thunder. A few spawn tornadoes. [http://www.fox8wghp.com/spacious.htm]
10.2
Stratus Clouds Stratus
Low lying layer of cloud (called fog if on the ground) with no structure. [http://www.fox8wghp.com/spacious.htm]
10.2
Cirrus Clouds
Cirrus
An ice crystal cloud, wispy in appearance. May produce ice crystal snow in winter or in mountains. [http://www.fox8wghp.com/spacious.htm]
Altostratus Clouds Altostratus
Thickly layered water droplet cloud. Sun seen as through ground glass. [http://www.fox8wghp.com/spacious.htm]
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Nimbostratus Clouds
Nimbostratus Thick layered cloud - usually dark gray. Produces continuous rain or snow over large area. [http://www.fox8wghp.com/spacious.htm]
Fog
http://www.tqnyc.org/2009/00767/fog.jpg
Clouds in the Atmosphere
From http://www.realclimate.org/index.php?p=245
Saturation
• Saturation units are vapor pressure or specific humidity (SH) (grams of water per kg air)
• Relative humidity (RH) is the amount of water vapor as a percentage of saturation
• RH < 100% water evaporates • RH > 100% water condenses • RH = 100% equilibrium at saturation
Saturation and Temperature
Relative Humidity = Actual Amount of Water in Air Specific Humidity at Current Temperature
= 100% if Actual Water exceeds Specific Humidity
Spec
ific
Hum
idity
SH
Cloud formation is like boiling water.
• “Saturation” (driving force) – Like heating water to 100°C
(“boiling point”)
• “Nucleation” (trigger) – Like starting the first bubbles
Liquid >>>>>>>>>>>>>>>>>>>> Vapor Heat til Saturation then scratches start Nucleation
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Other Kinds of Nucleation: Rock Candy
Liquid >>>>>>>>>>>>>>>>>>>> Solid Add sugar til Saturation then add stick for
Nucleation
Other Kinds of Nucleation: Dew
Gas >>>>>>>>>>>>>>>>>>>> Liquid Humid air cools to Saturation then Nucleation on
surface
What is needed to form a cloud? • “Saturation” (driving
force) – You need enough
water to exceed the amount that can be in the gas phase
– 100% “relative humidity” (also called the “dew point”)
• “Nucleation” (trigger) – You need a starting
point to allow the formation of a new phase (liquid water) in air (gas mixture)
– Aerosol particles
Vapor >>>>>>>>>>>>>>>>>>>> Liquid Cool til Saturation then particles start Nucleation
• Liquid (H2O/EtOH) supersaturated with vapor (CO2) nucleates on salt to form bubbles
Bubbles • Vapor (air)
supersaturated with liquid (H2O) nucleates on particles to form droplets
Clouds