Atmospheric moisture

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Atmospheric moisture

• Chapter 5 GEOG 31062

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Hydrologic Cycle
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Evaporation / Condensation
Saturation when evaporation equals condensation
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How we measure moisture in the atmosphere

• Absolute values
• Specific humidity
• Dew point
• Relative values
• Relative humidity

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Saturation specific humidity

• Specific humidity is grams of water per kilogram
  of air
• Far more water vapor can exist in warmer air than
  colder air.

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Relative humidity

• Given as a percentage
• Relates the total amount of water vapor present
  relative to the total amount possible
• Depends on air temperature and total water vapor
  present
• Why is this not an absolute measure of water in
  the atmosphere?

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Diurnal cycle of RH
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Dew point

• Temperature to which the air must be cooled to
  reach saturation
• (i.e. dew will form)

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Dew point/temperature relationships in
unsaturated air (a) and saturated air (b c)
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January Dew Points
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July Dew Points
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Precipitable water climatology (Pitt.)
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How do we reach saturation?

• Addition of water vapor
• Mixing cold air with warm, moist air
• Cooling air to the dew point

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Also

• Effect of Curvature
• Small drops exhibit greater curvature more
  evaporation takes place
• Supersaturation may occur
• Effect of nuclei
• Droplets are not clean
• Haze can form at RH80
• End result
• Fairly complex saturation values in the
  atmosphere

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Ice nuclei

• Ice Nuclei
• Water only freezes at 0oC (32oF) if it has
  something to freeze onto
• Supercooled droplets can exist down to
  -40C
• Spontaneous nucleation occurs at -40C

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Measuring humidity
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How do we get air to dew point?

• Diabatic processes
• Adding or removing heat energy
• Adiabatic Processes
• No net exchange of energy (through the expansion
  of rising air)

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Lapse rates

• Change of temperature with height
• Dry adiabatic lapse rate
• -1oC/100m (-5.5oF/1000ft)
• Sinking parcels experience compression warming
• Saturated adiabatic lapse rate
• Occurs in saturated air
• -0.5oC/100m (-3.3oF/1000ft)
• Environmental lapse rate
• What is actually happening in the atmosphere

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A comparison of adiabatic and environmental
cooling rates
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Condensation

• Dew or frost
• Liquid or solid condensation on surface objects
• Diabatic processes
• Fog
• Condensation near the ground of water vapor on
  nuclei in the air

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Advection fog / Radiation fog
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Upslope fog and Valley fog
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Precipitation or evaporation fog
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Heavy fog days