Weather Instruments to Make
Relative humidity can be measured by an instrument called a hygrometer.
The simplest hygrometer - a sling psychrometer - consists of two thermometers
mounted together with a handle attached on a chain. One thermometer is
ordinary. The other has a cloth wick over its bulb and is called a wet-bulb
When a reading is to be taken, the wick is first dipped in water and then
the instrument is whirled around. During the whirling, the water evaporates
from the wick, cooling the wet-bulb thermometer. Then the temperatures of both
thermometers are read.
If the surrounding air is dry, more moisture evaporates from the wick,
cooling the wet-bulb thermometer more so there is a greater difference between
the temperatures of the two thermometers. If the surrounding air is holding as
much moisture as possible - if the relative humidity is 100% - there is no
difference between the two temperatures. Meteorologists have worked out charts
of these differences for each degree of temperature so that the observer can
find relative humidity easily. A sample is shown below:
Partial Relative Humidity Chart for 30° C
Dry Bulb and Wet Bulb
You can make a sling psychrometer by using two commercial thermometers.
Wrap the bulb of one tightly with a piece of cloth. Attach the thermometers
to a narrow, thin board with wire or strong tape. Drill a hole in the top
of the board and attach a wooden handle to the board with a short piece
The Miami Museum of Science also has a description of Making a psychrometer
Source: Tannenbaum, Beulah and Harold E. Making and Using Your Own
Weather Station.1989:New York. Venture Books.
See also: a website