Average annual precipitation is a vital piece of climatic data - one that is recorded through a variety of methods. Precipitation (which is most commonly rainfall but also includes snow, hail, sleet, and other forms of water falling to the ground) is measured in units over a given time period. In the United States, precipitation is commonly represented in inches per 24-hour period. This means that if one inch of rain fell in a 24-hour period and water wasn't absorbed by the ground nor did it flow downhill, after the storm there would be a layer of one inch of water covering the ground.
The low-tech method of measuring rainfall is to use a container with a flat bottom and straight sides (such as a cylinder coffee can). While a coffee can will help you determine whether a storm dropped one or two inches of rain, it's difficult to measure small amounts of precipitation.
Weather observers use more sophisticated instruments, known as rain gauges and tipping buckets to more precisely measure precipitation. Rain gauges have wide openings at the top for rainfall. The rain falls and is funneled into a narrow tube, one-tenth the diameter of the top of the gauge. Since the tube is thinner than the top of the funnel, the units of measurement are further apart than they would be on a ruler and precise measuring to the one-hundredth (1/100 or .01) of an inch is possible. When less than .01 inch of rain falls, that amount is known as a "trace" of rain.
A tipping bucket electronically records precipitation on a rotating drum or electronically. It has a funnel, like a simple rain gauge, but the funnel leads to two tiny "buckets." The two buckets are balanced (somewhat like a sea-saw) and each holds .01 inch of water. When one bucket fills, it tips down and is emptied while the other bucket fills with rain water. Each tip of the buckets causes the device to record an increase of .01 inch of rain.
Snowfall is measured in two ways. The first is a simple measurement of the snow on the ground with a stick marked with units of measurement (like a yardstick). The second measurement determines the equivalent amount of water in a unit of snow. To obtain this ratio, the snow must be collected and melted into water. Generally, 10 inches of snow produces one inch of water. However, it can take up to 30 inches of loose, fluffy snow though as little as 2-4 inches of wet, compact snow can produce an inch of water.
Wind, buildings, trees, topography, and other factors can modify the amount of precipitation that falls so rainfall and snowfall tend to be measured away from obstructions. A thirty-year average of annual precipitation is used to determine the average annual precipitation for a specific place.