Curiosity Corner: Calculating Yearly Rainfall


Curiosity Corner


Dr. Jerry D. Wilson

Emeritus Professor of Physics

Lander University

QUESTION: Where do they get average yearly rainfalls? (Asked by Russ Cook, of Greenwood, S.C.)

REPLY: Average annual (yearly) rainfalls are monitored by several sources. This is done chiefly by state and federal agencies. The main question is how it is done – the averaging, that is. The simplest method is the old arithmetic averaging. With a rain gauge, you might determine the annual rainfall for your location. Measure the rainfall for a month and you’ll have the monthly rainfall. Do this for 12 months and you’ll have one yearly rainfall for your location.

A single collection may be OK for a particular location – for example, an airport. But what about a large city? Here, several collection locations would be needed for averaging. If the rain gauges are uniformly distributed and the rainfall varies in a regular manner, the results will be satisfactory. This could be expanded to a countywide collection, and then be used in a statewide annual average rainfall. As you can see, the results vary with the number of collection locations and the size of the area.

So, one year gives an annual rainfall, but how many years are needed to get a good average? Generally, this is taken to be 30 years. For example, the precipitation averages based on data collected by weather stations throughout South Carolina and West Virginia from 1971 to 2000, as provided by NOAA National Climatic Data Center, are:

  • South Carolina: 49.8 inches
  • West Virginia: 45.2 inches

(Note: “Precipitation” averages include water inches of snowfalls.)

As stated at the beginning, this is a simple method and is only informative if the precipitation is fairly uniform. For example, the average annual precipitation for California is 22.2 inches. This is pretty high for southern California and pretty low for the northern region. Precipitation data is often wanted for a particular geographical area, like a river basin, so as to monitor and predicate river flow. There are advanced methods for this that use weighted means (or averages). Different models yield different results. They use statistical and mathematical modeling … sort of rocket science. So I’ll leave it at that.

C.P.S. (Curious Postscript): “Duct tape is like the force. It has a light side, a dark side, and it holds the world together.” —Oprah Winfrey

Curious about something? Send your questions to Dr. Jerry D. Wilson, College of Science and Mathematics, Lander University, Greenwood, SC 29649, or email Selected questions will appear in the Curiosity Corner. For Curiosity Corner background, go to

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