Building Weather Instruments

A hygrometer is an instrument used to measure the amount of moisture, or water vapor, in the air. There are many different parameters used by meteorologists and other scientists to measure the amount of water vapor in the air. Examples include the relative humidity, dew-point temperature, specific humidity, and mixing ratio. You can go to the "Mesonet Meteograms" page for more information. Using mathematical formulas, or tables, you can convert from one parameter to another. Below, you will measure the dew-point temperature and then convert it to relative humidity using a table.


  1. A container with a shiny surface outside. Metal is best. The one shown below is a sugar container purchased at Wal-Mart.
  2. A thermometer
  3. Some finely crushed ice in a separate container.
  4. Set up the equipment outside or leave it in a room, depending on where you want to take your humidity measurement.


  1. Using the thermometer, read the temperature of the air and record on a piece of paper. This is your AIR TEMPERATURE.
  2. Fill the shiny container about two-thirds full of tap water and place the thermometer inside. Read the temperature and record the value on a piece of paper.
  3. Add a small amount of crushed ice. Using the thermometer swirl it around the container until it melts. Then read the temperature again. Did it drop?
  4. Check the outside of the container for condensation (or dew). If none formed, repeat the previous step. It is important that you swirl the ice and allow it to melt, before checking for dew. When the ice touches the side of the container, dew will form, but this does not provide an accurate measurement. The temperature of all the water in the container has to be cold enough for dew to form on the outside.
    As soon as the first hint of dew appears on the outside of the container go to the next step
  5. Once you can see dew on the outside of the container, the water in the container has dropped all the way to the dew point temperature of the air around it. Record the temperature when the first hint of dew appears on the outside of the container. This is your DEW POINT TEMPERATURE. It is important that you get as close as possible to the first sign of dew on the container. This will provide the most accurate dew-point temperature measurement.
  6. Using the chart below and your AIR and DEW POINT temperatures, you can determine the relative humidity value in the room.

 Relative Humidity Conversion Chart 
Locate the dew point temperature in the leftmost column, and the air temperature in the top most row of each table section

Dew-point to RH calculator

Enter Tair and Tdew in , then click in the RH box to display the relative humidity 

Air Temperature (Tair °):

Dew Point (Tdew °)          :

Relative Humidity (RH %):