Intensity of Rain

The intensity of rain can be measured by placing a microphone in a metal can and then recording the impacts of rain on it. A cassette tape recorder can be inserted totally into a large coffee can with plastic lid or you can use a microphone with cable to a recorder indoors. In the latter case, the microphone cable should be carefully grounded, especially when lightning is possible. It is best if the can is fitted into foam plastic to muffle extraneous sounds but most importantly so that the drops strike it only at the flat end. This is also a convenient place to place a rain gauge. A possible setup is shown in the diagram.

Measure the area of the can which the drops hit and record the number of hits for a minute. The ratio of drops per unit area per unit time gives the rainfall intensity. If it is suspected that two drops have hit almost simultaneously, use a computer with the appropriate software (usually comes with the sound board) to make a .wav file and the editor program to look at the wave form of the sound. The patterns on the screen will usually help determine if there was one or two drops at a time.

If enough equipment is available, groups of students can analyze different segments of the tape plot the values of intensity and determine the total and average numbers of drops per unit area over the life of the shower. If you also set up a rain gauge next to the can, you can determine the average mass or weight of a drop for the entire shower.