Discovering Dew Point Temperature --
Measuring Moisture in the Atmosphere

INTRODUCTION

Tropical rainforests are typically warm and humid. If the relative humidity in Washington, D.C. is 90%, and 60% in Belem, Brazil, then is the air more humid in Washington, D.C. than in Brazil? As you will discover, not necessarily! But first, we must develop some fundamental understandings about atmospheric moisture (humidity).

PROCEDURE

  1. Outdoor versus Indoor Relative Humidity
    1. Use a barometer, NOAA Weather Radio, or the Internet to obtain the current air pressure (in either millibars or inches of mercury) for KIAD (Dulles International Airport, Washington, D.C.). Record this data in the following table.

      Current Air Pressure
      Date Time Air Pressure



    2. Following your instructor's directions, use a sling psychrometer to collect dry bulb and wet bulb temperature data outdoors and indoors. Record these data in the following data tables.

      Outdoors
      Dry Bulb Temperature
      (Air Temperature)      		 oF
      Wet Bulb Temperature oF
      Dew Point Temperature oF
      Relative Humidity %

      Indoors
      Dry Bulb Temperature
      (Air Temperature)      		 oF
      Wet Bulb Temperature oF
      Dew Point Temperature oF
      Relative Humidity %

    3. Use the air temperature, wet bulb temperature, and air pressure to determine the dew point temperature and relative humidity for the air outdoors and indoors (courtesy The Weather Calculator, National Weather Service, El Paso, TX Office). Record these data in the preceding data tables.

  2. Interrelationship Among Air Temperature, Dew Point Temperature, and Relative Humidity
    1. Use the Internet to obtain a 24-hour meteogram for Washington, D.C. (courtesy The American Meteorological Society's DataStreme Project). From the meteogram, use the air temperature, dew point temperature, and air pressure to determine the relative humidity every three hours. Enter these data (time, air temperature, dew point temperature, and relative humidity) into a ClarisWorks spreadsheet (you may need to refer to step-by-step directions for creating a ClarisWorks spreadsheet).

    2. Use ClarisWorks to prepare a graph (Step #8) of air temperature (T), dew point temperature (DP), and relative humidity (RH) versus time. Observe the interrelationships among T, DP, and RH.

ACTIVITY EXTENSION

Use the Internet to visit CLIMVIS (The Climate Visualizer) to obtain Mean Dew Point data for October 1997 in Washington/Dulles, Virginia and Belem/Val de Caes, Brazil (Select the Graph Type Display One Parameter For Two Stations in Two Separate Regions, then follow the prompts). (You may also want to compare/contrast Mean Temperature.) Apply what you have learned about dew point temperature to explain why on average, the tropical rainforest is more humid than mid-latitude locations such as Washington, D.C.

Copyright (c) 1998 by Walter Sanford. All rights reserved.
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