Humidity

1
Humidity

• Organisms need to remain in highly hydrated state
• Why?
• Most (all?) biochemical reaction require water to
  occur
• Water budget for organism
• liquid water uptake
• - water vapor loss
• breathing, sweating, evapotranspiration
• Water loss generally viewed as bad
• However, many cases where vapor loss is
  beneficial or essential for survival

2
Organism Survival through Vapor Transport

• Vapor loss or latent heat flux cools organism
• Can allow survival of organism in harsh
  environment
• Texas rice plants
• Provides 44 kJ/mol of energy loss
• Called latent heat of vaporization
• 580 x energy required to raise 1 mol water 1oC
• Amount of cooling defined by
• Difference in water vapor concentrations
• Conductance to water vapor from the surface to
  the air
• Remember Flux conductance (Conc, surface
  Conc, air)

3
Gas Concentrations

• We will specify gas concentrations in terms of
  mol fraction
• Mol of gas per mol of air
• Advantage later we will express fluxes in mol
  m-2 s-1

4
Water Vapor Saturated Conditions

• Equilibrium between liquid water and water vapor
  in a closed system is known as saturation vapor
  pressure
• It is temperature dependent
• Increase temperature, increase airs ability to
  hold water vapor

• Consider container of pure water
• Uncover in evacuated, closed chamber
• Water molecules in gas phase increase
• Finally, equilibrium reached
• Equilibrium those escaping those recaptured by
  liquid

5
Vapor pressure-Temperature-Relative Humidity
Diagram
6
TABLE A.3
7
Dew Point Temperature

• The temperature at which air, when cooled without
  changing pressure or water content, reaches
  saturation
• Air is seldom saturated
• Typically only find saturation at night (low
  temperature)
• Air typically does not cool below the dewpoint
• Takes significant amount of energy
• Latent heat of cooling

8
Wet Bulb Temperature

• The temperature reached by a thermometer when it
  is evaporatively cooled in ambient air
• Must be adiabatic (no heat exchange)
• Change in heat content of air must equal latent
  heat of evaporation
• Derived from energy balance analysis of wet bulb
• Measure both wet and dry bulb temperature
• Psychrometer

9
Psychrometer Constant (g)

• Important part of the wet bulb analysis
• Constant of 0.666 x 10-3 C-1 assumes adiabatic
  conditions
• This does not occur in reality
• Heat and water are exchanged during cooling
• Developed an apparent psychrometer constant (g)
• Well ventilated and shielded psychrometer will
  come close to real psychrometer constant

10
Diurnal Variation in Relative Humidity