Chapter 7 Water and Atmospheric Moisture

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Chapter 7Water and Atmospheric Moisture

• Geosystems 6e
• An Introduction to Physical Geography

Robert W. Christopherson Charles E. Thomsen
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Water and Atmospheric Moisture

• TOPICS
• Water on Earth
• Unique Properties of Water  
• Humidity  
• Atmospheric Stability  
• Clouds and Fog  

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Water on Earth  

• Earths water originated from
• Icy comets
• Hydrogen and oxygen
• Outgassing
• Example geysers 
• Eustasy, Glacio-eustasy, Isostasy

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Ocean and Freshwater Distribution
Figure 7.3
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Unique Properties of Water  

• The water molecule H2O
• Polarity to and to
• hydrogen bonding
• Surface tension
• Capillarity

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Unique Properties of Water  

• Heat properties
• Change of state requires energy be absorbed or
  released!
• Phase changes
• Freezing/melting (solid to liquid to solid)
• Condensation/vaporization (gas to liquid to gas)
• Sublimation/deposition (gas to solid to gas)

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Evaporation Condensation

• The process by which molecules break free of a
  liquid volume is evaporation
• When water vapor molecules randomly collide with
  the water surface and bond with adjacent
  molecules is condensation

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Three States of Water
Figure 7.5
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Humidity

• Humidity refers to water vapor in the air
• Relative Humidity is the ratio (expressed as )
  of the amount of water vapor that is actually in
  the air compared to the maximum water vapor
  possible in the air at a given temperature
• Air becomes saturated when the rate of
  evaporation and the rate of condensation reach
  equilibrium 100 relative humidity

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Humidity

• Equation for Relative Humidity 
• actual water vapor
• (specific humidity)
• RH ------------------------------ X 100
  max amount that can be
  held at that temp
• (saturation specific humidity)

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Relative Humidity
Actualwater vapor
Figure 7.8
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Relative Humidity

• Relative humidity is the indication of how close
  the air is to saturation and when condensation
  will begin
• Dew-point temperature not really a temperature,
  but a measure of moisture content
• When air temperature tries to decrease below the
  dew point, surplus water vapor is removed from
  the air by condensation

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Dew Point Temperature
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Distribution of Water Vapor
January
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Distribution of Water Vapor
July
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Humidity Patterns
Figure 7.11
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Vapor Pressure

• One of 2 measures of relative humidity
• As water molecules evaporate and become part of
  the air, they become water-vapor molecules. In
  this state, they exert a portion of the air
  pressure called vapor pressure
• Saturation vapor pressure
• Increasing temperature means an increase in
  saturation vapor pressure kinetic energy

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Specific Humidity

• One of 2 measures of relative humidity
• Specific humidity is a measure of humidity that
  remains constant as temperature and pressure
  change
• Specific humidity is the mass of water vapor (g)
  per mass of air (kg) at any specified temperature
• Maximum specific humidity

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Condensation Nuclei

• Pure water droplets are uncommon
• Homogeneous nucleation
• Hygroscopic aerosols
• Heterogeneous nucleation
• Condensation nuclei

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Potential Test Question

• Relative humidity has an inverse
• relationship with
• A) dew-point temperature
• B) temperature
• C) air pressure
• D) eustasy

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Potential Test Question

• Which of the following has the highest water
  vapor content in the air?
• A) southern Alabama
• B) Oklahoma
• C) eastern Montana
• D) southern Missouri

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Adiabatic Processes  

• Adiabatic processes
• Begins with a parcel of air
• Bouyancy caused initially by differences in
  (near) surface temperature
• Less dense, warmer air rises, more dense, colder
  air sinks, after which
• Ascending or descending air will undergo changes
  in temperature with no exchange of heat. This is
  an adiabatic process.

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Adiabatic Processes  

• Adiabatic processes
• Ascending or descending air will undergo changes
  in temperature with no exchange of heat. This is
  an adiabatic process.
• Changes in temperature will be due solely to
  changes in pressure! (Example?)
• Note difference with parcel of air undergoing
  changes in temperature when we discussed
  humidity! No motion was implied!
• Heat WAS exchanged diabatic process.

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Adiabatic Processes  

• Adiabatic processes
• Note also differences in the volumes of the
  parcels of air between diabatic (heat exchange)
  and adiabatic (no-heat exchange)

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Adiabatic Process!
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Buoyancy
Figure 7.15
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Adiabatic Processes
Figure 7.17