Atmospheric Moisture by 6xNrDX


									Atmospheric Moisture

    Henry Robinson
         Water is an Odd Molecule
• Boiling point much higher than one would
  expect from molecular weight. (Lighter
 molecular weight moving faster for given
 temperature so have colder boiling temperature)
 Subtance         Molecule weight Boiling Temp

 Oxygen           32               -183 oC

 Nitrogen         28               -196 oC

 Water            18               100 oC

 Carbon Dioxide 44                 -79 oC
       Water is a Polar Molecule

Net positive charge on one side and a negative side on the other.
              Polar Molecule
• Electrical negative will attract positive charge.
• The electrical attraction of the polar molecules is
  quite strong.
• The attraction makes water molecules want to join
  together, raising the temperature of the boiling and
  freezing point. It is often referred to as a
  Hydrogen Bond.
• Polar attraction also makes water a good liquid
  solvent. Other molecules bind to water in
               Ice Crystal
• 105 degree separation of hydrogen causes
  six sided figure.

                          Snow crystals six sided
Reason why water is most dense at
 +4 degrees Celsius and ice floats
      Temperatures Colder than
        Boiling Temperature
• At temperatures colder than boiling temperature,
  water can exist as a liquid (or solid if below the
  melting temperature). Molecules in liquid attract
  each other strongly. Held together by Van der
  Wals forces.
• Some water molecules in the air are far enough
  apart that they don’t feel the attraction of other
  water molecules so they can remain a gas even
  though the temperature is below the boiling point.
               Water Vapor
• As the number of water vapor molecules in the air
  increase in number, they will have more chance of
  being close to another molecule and being
  attracted to each other.
• After there are a certain number of molecules in
  the air, any more molecules will cause the
  molecules to clump together forming a liquid.
• This point is the saturation point.
           Saturation Point
• Saturation point dependent upon:
• temperature (higher temperature, faster
  molecules get away from one another)
• pressure (number of molecules present)
• Liquid-gas interface (surface of water) will
  exchange molecules.
• Some molecules go from liquid to gas,
  some go from gas to liquid. When air is at
  saturation the number of molecules going
  equal the number of molecules coming.
            Water Vapor Pressure

• Water vapor molecules are only a fraction of the molecules
  in the air.
• Pressure is the sum of the weights of all the molecules
  (Dalton’s Law of partial pressure)
• The water vapor pressure is the pressure caused by the
  water vapor molecules in the column above you.
     Saturation Vapor Pressure
• Saturation vapor pressure is the pressure
  when air is at saturation for water
  molecules. (temperature dependent)
           Moisture Indices
• A number of indices have been developed
  to describe the amount of water vapor in the
• Different indices are useful for different
         Water Vapor Variables
• Absolute
  –   Vapor Density
  –   Specific Humidity
  –   Mixing Ratio
  –   Dew Point Temperature
  –   Vapor Pressure
• Relative
  – Relative Humidity
  – Heat Index or Effective Temperature
            Vapor Density
• Vapor density r
• Mass of water vapor per unit volume
    r = mass of water vapor
          unit volume
         Specific Humidity

• Specific humidity= mass of water vapor
                      total mass of air
• Only depends on the amount of water vapor
• Not dependent on pressure or temperature
• Frequently used in weather forecast model
              Mixing Ratio
• Mixing ratio =mass of water vapor
                  mass of dry air
• Also conservative in that it only depends on
  amount of moisture. Does not depend on
  temperature or pressure.
• Frequently used in calculations of transport
  of air parcels, such as in thunderstorms,
  storms, etc.
             Relative Humidity
• Relative Humidity=actual vapor pressure * 100%
                         saturation vapor pressure
• Is the ratio of the air’s content to its capacity.
• When relative humidity reaches 100%
  condensation will occur.
• Saturation vapor pressure is dependent upon
  temperature, so RH is dependent on both the
  amount of moisture in the air and the temperature
  of the air.
• Frequently used by public.
Relative Humidity Calculation
Relative Humidity will vary during the day as
 temperature changes even if the amount of
         moisture remains constant
      Dew Point Temperature
• Dew Point Temperature is the temperature
  where saturation occurs.
• Dependent only on moisture content.
• Used by the NWS for observations of
• When dew point temp=air temperature,
  100% RH
       Wet Bulb Temperature
• Lowest temperature that can be reached by
  evaporating moisture into the air.
• Related to air temperature, moisture
• Table in Appendix D for conversions.
• Used to measure cooling potential of
  evaporation. Used to measure moisture
  content with sling psychrometer instrument.
                      Heat Index
     • The “feels like” temperature.
     • Function of air temperature and relative humidity.
     • Used to measure potential for heat related health

Table in Appendix G
   Moisture Index Conversions
• All of the moisture indices are related to
  one another through mathematical formulas
  or tables. (see Appendix D and G, Table 1
  and figure 5.10 in Chapter 5)
• Application determines which index is used.
        RH =
          A=
                8 mb
          A=         = 67%
                12 mb
          B=
                24 mb
     B    B=         = 57%
                42 mb

      Td = dew point

       “The temperature to
       which air must be
       cooled in order for
       condensation to occur”
                      Dew point and RH
Dew Point (DP) is temperature where the RH is 100% for a given
amount of moisture in the air.
RH is the actual vapor pressure divided by the saturation vapor pressure
for the current temperature.

Example 1: Air temperature 70F = 21C
           Dew Point 10F =-12C
SVP=25mb (from graph 70F go up to curve
             and over to 25mb)
Actual vapor pressure=2.4mb (go up from 10F
       to curve and over to 2.4mb)

Example 2: Air T=70F; DP=50F; SVP=25mb
    actual VP=12mb.     RH=12/25=48%
      Moisture Measurements
        Hair Hygrometer
• Human hair gets curly (changes length)
  with humidity. Levers amplify the change in
• Measures Relative Humidity directly.
• Relatively cheap instrument.
      Moisture Measurements
       Sling Psychrometer
• Two thermometers; one wet, one dry. Move
  through the air (either fan or swing around).
• Wet thermometer measures wet bulb
      Moisture measurements
• Cool mirror until dew (or frost) forms on
• Used recently by NWS at all automated
  surface observing stations.
New NWS Humidity Sensor
      Moisture Measurements
      Electrical Hygrometer
• Uses a chemical film that absorbs moisture
  which changes the electrical resistance.
• Used in radiosonde balloon measurements.
• Water is a unique molecule. It is strongly polar
  with + and – sides that attract other molecules.
• Can have free water vapor molecules in air at
  temperatures colder than boiling point of water.
• When number of water vapor molecules exceed a
  certain number, water will start to condense out of
  the air. This is the saturation point.
• The vapor pressure of water vapor is the weight of
  all the water vapor molecule above the point.
           Summary (cont.)
• Saturation vapor pressure
• Moisture indices:
• Specific Humidity, Mixing Ratio, Relative
  Humidity, Dew Point, Wet Bulb
  temperature, Heat Index.
• Moisture measurements: Hair hygrometer,
  sling psychrometer, chilled mirror,
  electrical hygrometer.

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