1. Relative humidity is the amount of water vapor present in the air compared with the maximum amount of moisture that the air can hold at that temperature. It is expressed as "% relative humidity." Compete saturation equals 100% relative humidity (rain); the absence of moisture in the air equals 0% relative humidity.
  2. The higher the temperature, the greater amount of water the air can hold.
  3. Humidity is caused by evaporation of water.
  4. A hygrometer is an instrument that measures humidity. There are three types of hygrometer:


  • 2 lab thermometers, graduated in Fahrenheit and/or Celsius scale
  • empty milk container or box
  • piece of cotton or cotton shoelace
  • kite string or sewing thread
  • field journal




  1. Attach the cotton ball or piece of the shoelace to the bulb of one thermometer. The wet bulb is the thermometer with the cotton, the dry bulb is the thermometer without cotton.
  2. Attach both thermometers to the side of the milk carton or box by securing it with the thread or string.
  3. Wet the cotton with clean room temperature water.
  4. Immediately take temperature readings of both thermometers.
  5. Record the temperatures in journal.
  6. Take temperature readings of each thermometer after one minute.
  7. Using the attached chart, determine the relative humidity.


  • It may be necessary to fan the thermometers for the one minute period to allow for a greater volume of air to reach the thermometer bulbs and achieve a more accurate reading. If your thermometers give Celsius readings only, the conversion from Celsius to Fahrenheit is:

    °F= (1.8 x °C) + 32


Assessment Ideas

  • Help the students practice using the Relative Humidity chart (provided below).
  • Students should keep a log of readings in journals or on spreadsheet files.
  • Create graphs using paper and pencil or computer spreadsheets can be created.


  • Mathematics: Data can be entered into a spreadsheet and relative humidity readings can be taken at various times in the day and during an extended period.
  • Social Science: Students should consult local newspapers for weather forecasts and compare/contrast the relative humidity readings they have computed and those that have been published.


  • evaporation: the conversion of liquid to vapor due to heat.
  • hygrometer: one of many instruments used to measure relative humidity.
  • relative humidity: the amount of water vapor present in the air compared with the maximum amount of moisture that the air can hold at that temperature.
  • sling psychrometer: a device used to determine the dry and wet bulb temperature of the air. It consists of two thermometers mounted on a piece of metal that swings rapidly in the air for several minutes to achieve the required degree of wet bulb cooling.


  • Adapted from "Winds of Change" educational CD-ROM, Copyright Caltech and NASA/Jet Propulsion Laboratory
Relative humidity chart