Slide 1 - Atmospheric and Oceanic Sciences

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					Lab 3: Moisture in the Atmosphere

                                    Alice DuVivier

                                         Alice DuVivier

Why do we care about water in the atmosphere?

What terms would you use to describe how much
       water there is in the atmosphere?
      Water in the Atmosphere

Amount of Water in Atmosphere is Regulated by:
sun, ocean currents, aerosols, rain, land use, etc.
Describing water in the                  Dragonfly with Dew

Absolute measures
• Mixing ratio (kg water
  vapor / kg dry air)
• Specific humidity (kg
  water vapor / kg air)
• Vapor pressure
• Dewpoint temperature
Non-absolute measures
• Relative Humidity        From:
                Relative Humidity Equations
                                                      water vapor in the atmosphere x100
                                                                water vapor capacity

                                                                    mixing ratio x100
                                                                 saturation mixing ratio

                                                               actual vapor pressure x100
                                                                saturation vapor pressure

                                                              You MUST have units in your
                                                                calculations for them to be
From:               correct!
Observing Humidity
       Sling Psychrometer
       Measures dry bulb and wet
       bulb temperatures
           • Dry bulb is the
           atmospheric temp
           • Wet bulb is the temp
           after the water
       IF it is below freezing outside
       this method has problems.
Observing Humidity
      How to use it:
      1. Wet cotton wick with water.
      2. Shade with body while measuring to
         avoid direct solar heating.
      3. Read initial wet temp then swing for
         10 sec. Swinging helps liquid
      4. Read wet temp again and then
          swing again for 10 sec.
      5. Repeat until the wet bulb temp stays
          constant. Note: the wick must
          remain wet during this time.
      6. When the wet bulb temp is constant
          record it, then turn over the
          thermometer and read the dry temp
          and record that as well.
                              Evaporative Cooling
                                                         Evaporative cooling:
                                                         • Water requires heat to
                                                         evaporate (ex. boiling water)

                                                         • Evaporated water takes away
                                                         heat and the surroundings
                                                         become colder.

                                                         • Difference between wet bulb
                                                         and dry bulb temperatures show
                                                         the potential for evaporative
From:   cooling.
              Evaporative Cooling

         Dry Air                        Wet Air
Dry air has “room” in the   Wet air doesn’t have “room” for
air for more water to       much water.
                            Saturated air is “full” of water
                            and can’t take any more air in
                            without pushing liquid water out
                            (no evaporation).  like sponge

  Dry air: Large difference between the bulb temps because
there is evaporation and the wet bulb temperature decreases.
 Wet air: Small difference between the bulb temps because
  there is less evaporation and the wet bulb temperature
                      doesn’t decrease.
Standard Deviation (σ - sigma):
1.σ is a measure of the variability of a data set
2.Low σ means that the data points tend to be very close
to the mean.
3.High σ means that the data are spread out.
4.One σ is ~68% of the data. Two σ are ~95% of the data.
(images from wikipedia)
Things to consider about measurement error:
• Errors accumulate
• Error of measurement (thermometer, human) vs. true
   variability (actual temperature changes)

Instruments can have internal error of two kinds:
                      Accuracy vs. Precision
High accuracy, low precision     High precision, low accuracy
   Time to make measurements!

       Inside: Psychrometer
           and Pressure

     Online: Temp, Dew Point
       (Convert from F to C)

Outside: Psychrometer (only if above
  freezing) and Cloud observation

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