Atmospheric Moisture and Precipitation

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Atmospheric Moisture and Precipitation
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Keys to understanding atmospheric moisture

• The parcel concept.
• Saturation and dewpoint
• How can a parcel of air reach dewpoint?
• More general question how can a parcel change
  temperature?
• diabatically (adding/removing sensible heat)
• adiabatically (changes in density, latent heat)

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The Parcel Concept
Figure 7-14
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Air Parcels

• How will the size of an air parcel change as it
  rises?
• it expands
• What happens to the temperature of a parcel as it
  expands
• it cools

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Air Parcels

• How will the size of an air parcel change as it
  descends?
• it compresses
• What happens to the temperature of a parcel as it
  compresses
• it heats up.

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Whats this have to do with moisture and
precipitation?

• Air parcels contain moisturethree forms
• solid (ice crystals)
• liquid (droplets)
• gas (vapor).
• The form of moisture depends on its temperature
• Saturation and dewpoint

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Saturation and Dewpoint

• Water content of air (humidity) can be measured
  in two ways, absolute or relative.
• What is absolute humidity?
• the actual amount of moisture in air
• Vapor Pressure (mb)
• Specific humidity (g/kg)
• Precipitable water vapor (g/cm2)

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Saturation and Dewpoint

• What is relative humidity?
• the amount of moisture in air relative to
  saturation.
• What is saturation?
• How much moisture air can hold in vapor form
• Actually, its the point at which evaporation
  from and condensation on a surface are balanced.

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evaporation
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Saturation

• What happens when the rate of condensation
  exceeds evaporation?
• condensation occursdew, fog
• we call this the dew-point
• What happens when the evaporation exceeds
  condensation?
• evaporation occurs

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Dewpoint and Relative Humidity

• What sets the dewpoint of a parcel of air?
• Temperature
• Example
• at 40? C
• saturation is 47 g/kg

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Dewpoint and Relative Humidity

• Whats the relative humidity of a 40? C air
  parcel with 23.5 g/kg of water vapor?
• 23.5 ? 47
• 0.5
• 50 RH

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What happens to RH when we cool a parcel of air
without adding or removing any water?

• 40? C
• 10 g/kg H20
• Cool to 15? C

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• At 40? C
• RH 10/47
• 21
• At 30? C
• RH 10/27
• 27
• At 15? C
• RH 10/10
• 100

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How can we get a parcel to dewpoint?

• Diabatically Removing heat
• Radiative heat loss
• e.g. ground surface at night
• Adiabatically
• due to expansion and compression
• and release of latent heat.