Build a Rain Gauge
Grade Level: 5-9
For precipitation to form, particularly over a large area, several ingredients
are necessary. First there must be a source of moisture. The primary moisture
sources in the United States are the Atlantic and Pacific Oceans as well as
the Gulf of Mexico. Winds around high- and low-pressure systems transport this
Once the moisture is in place, clouds still need to form. The most effective
way for this to occur is for the air to be lifted. This is accomplished by forcing
the air up and over mountains or, more commonly, by forcing air to rise near
fronts and low-pressure areas.
Cloud droplets and ice crystals are too small and too light to fall to the
ground as precipitation. So there must be processes through which cloud water,
or ice, can grow large enough to fall as precipitation. One process is called
the collision-and-coalescence or warm-rain process. In this process, collisions
occur between cloud droplets of varying size, with their different fall speeds,
sticking together or coalescing, forming larger drops.
How much precipitation falls in your area? To find out, use the instructions
below to create your own rain gauge. Then head outside to measure rainfall in
- Straight-sided glass or plastic container, with a diameter of about two
inches or less (such as an olive jar)
- Coat hanger or wire bent to make a holding rack
- Measuring spoons: 1 teaspoon and 1/4 teaspoon
- Hammer and nails to secure the rack
- Felt tip marker
- Rain gauges measure the amount of rainfall in cubic inches. So your first
task is to make a scale for your container that shows how many cubic inches
of water are in the container. One cubic inch of water is about 3 1/4 teaspoons,
so you can draw the scale on your container by pouring 3 1/4 teaspoons of
water into your container, then drawing a short line at the level of the water.
If you look closely, the top of the water will seem to be slightly curved
and thickened. Draw your line so that it matches the bottom of the curved
surface (which is called a meniscus). This line corresponds to a rainfall
of one inch.
- Add another 3 1/4 teaspoons of water to the container and draw another line.
The second line corresponds to a rainfall of two inches.
- Repeat Step 2 until you have at least five marks on the container. This
will be enough for most rain events, but you may want to add another line
or two - just in case!
- Find a location for your rain gauge where there is nothing overhead (such
as trees or a building roof) that could direct water into or away from your
gauge. The edge of a fence away from buildings is often a good spot. Another
possibility is to attach your rain gauge to a broomstick driven into the ground
in an open area. Be sure to record rainfall soon after a rain event to avoid
false readings caused by evaporation.
- Get outside and empty your gauge after each reading, and you are ready for
the next rain event!