How is Precipitation Measured?

written by: R. Elizabeth C. Kitchen•edited by: Laurie Patsalides•updated: 7/8/2011

Understanding how precipitation is measured can go a long way in understanding our weather as a whole. This article will focus on how precipitation is measured and the tools used by meteorologists.

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Determining and recording the average annual precipitation is very important and for a meteorologist it is a vital piece of climatic data. There are several methods used by meteorologists to measure precipitation. Precipitation is generally rainfall, but is also includes snow, sleet, hail, and other types of water that falls to the ground. It is measured over a given period of time in units.

Precipitation in the United States is typically represented per 24-hour period in inches. This means that if an inch of rain fell within a 24-hour period of time and the ground didn't absorb the water, or the water didn't flow down a hill, after a storm has occurred there would be a one-inch layer of water covering the ground.

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Low-Tech Measuring Method

To measure precipitation using a low-tech method, one would use a flat-bottom container that has straight sides, such as a coffee can that is cylindrical in shape. With this method small amounts of precipitation are difficult to measure, but it can help to determine if a storm lead to one to two inches of precipitation being dropped. This method to measure precipitation is typically only used to measure rainfall.

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Rain Gauges

Rain gauges are an instrument used to measure precipitation that have wide openings at the top. The rain that falls will be funneled into a narrow tube, that is one-tenth the diameter of the gauges top. Since the funnel is less narrow than the tube, the measurement units are further apart than on a ruler making it possible for exact measuring to the one-hundredth of an inch. It is known as a trace of rain, when less than .01 inch of rain drops to the ground.

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Tipping Bucket

This instrument used to measure precipitation records precipitation electronically or on a rotating drum. Like a simple rain gauge, it has a funnel, but on a tipping bucket, two tiny buckets are what the funnel leads to. The two tiny buckets each hold .01 inch of water and they are balanced, similar to how a sea-saw balances when there is a person on each end. A tipping bucket tips down, when one bucket fills, and then it is emptied while the other bucket fills up with rain water. Every time the buckets tip, this precipitation measurement tool records a .01 inch increase in rain.

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Measuring Snow

There are two different precipitation measurement methods used to measure snow. The first instrument is similar to a yardstick and it is marked with the measurement units. It is used to measure snow that has already fallen to the ground. The second tool to measure snow is used to measure how much water a unit of snow contains. The snow has to be collected and then melted into water in order to obtain this ratio. In most cases, one inch of water will be produced by ten inches of snow. However, if the snow is fluffy and lose it can take approximately thirty inches of snow to produce the same amount of water as two to four inches of snow that is compact and wet.

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Factors That Modify Precipitation Amounts

Certain factors can modify precipitation amounts, such as buildings, topography, wind, and trees. Because of this, precipitation, such as snowfall and rainfall, are measured in areas that are free of obstructions. To determine the annual precipitation for a specific area, a thirty-year annual precipitation average is used.

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Resources

University of Illinois. (2010). Precipitation. Retrieved on April 20, 2011 from the University of Illinois: http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/prcp/home.rxml

University of Wisconsin Stevens Point. (2009). Forms of Precipitation. Retrieved on April 20, 2011 from the University of Wisconsin Stevens Point: http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/atmospheric_moisture/forms_of_precipitation.html

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Image Credits

Rain: sxc.hu - michaelaw