Adapted with permission by National Oceanic and Aeronautic Administration.
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 moisture inland.
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 your neighborhood.
- 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 (see picture)
- Measuring spoons: 1 teaspoon and 1/4 teaspoon
- Hammer and nails to secure the rack
- Felt tip marker
- Rain gauges are used to gather and measure how many inches of precipitation have fallen over a unit area during a set period of time. 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!