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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