EES 108 Class 06

EES 108 Class 06 - Moisture&Humidity EES108 Class#6...

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EES 108 Class #6 Tuesday September 13 2011
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Very local variations:  Urban heat island
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Urban Heat Islands
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What causes heat islands? Albedo: § Dark pavement Heat capacity § Concrete, asphalt store more heat than vegetation, soil § Release this heat at night, keeping temperatures high Water: § Less moisture in ground. § Less evaporative cooling Trees, plants: § Evapotranspiration through leaves cools air § Greater albedo Waste heat from buildings, vehicles, etc. Local greenhouse effect
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Downtown LA temperatures I r i g a t o n ,   O c h d s u e U b z P v m B l k f
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New ideas: Cool pavements
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Hydrologic Cycle Evaporation/Condensation: Moves 3 times more heat around atmosphere than convection 1/6 of all heat flow into/out of atmosphere
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Condensation of Water Vapor Creates Fog
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Deposition of Water Vapor Creates  Frost
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Latent heat Latent heat  of phase change much greater than heat  of  temperature change . Latent heat: Temperature remains fairly constant  when phase is changing
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Phase Transitions & Latent Heat
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Partial pressure, mixing ratio,  and humidity Partial Pressure : § Total pressure  = sum of partial pressures § Partial pressure  of  X: Proportional to fraction of  in mixture § Example: Air at sea level: 1013 millibar total pressure P total 1013 =  P N2 +  P O2 +  P Ar +  P H2O + … Oxygen is 21% of the molecules P O2 = 21% of 1013 = 213 millibar
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Mixing Ratio Mixing ratio  = ratio of masses: § Mixing ratio  of water vapor =  Partial pressure compares # molecules,  Mixing ratio compares mass air   dry   of   Mass vapor water    of   Mass
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Humidity Absolute Humidity: Specific Humidity: Mixing Ratio: air   of   volume vapor water    of   Mass air   of   mass    Total vapor water    of   Mass air   dry   of   Mass vapor water    of   Mass
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Humidity Absolute Humidity: Specific Humidity: Mixing Ratio: Absolute humidity vs. specific humidity  § Absolute humidity changes if volume changes § if material is added or removed air   of   volume vapor water    of   Mass air   of   mass    Total vapor water    of   Mass air   dry   of   Mass vapor water    of   Mass
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Evaporation, Sublimation Water vapor off liquid or solid surface § Depends mostly on temperature
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This note was uploaded on 12/15/2011 for the course EES 108 taught by Professor Giligan during the Spring '11 term at Vanderbilt.

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EES 108 Class 06 - Moisture&Humidity EES108 Class#6...

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