The Difference between Average Precipitation and Precipitation Frequency

Note from the Editor

A growing number of livestock producers are utilizing Pasture, Rangeland, and Forage insurance (PRF) to protect their operations from the effects of drought, or more accurately, when precipitation is 90% of average, or less, during 2-month intervals that are significant to their operation. Given the gaining popularity of this insurance product, we thought it would be interesting to look at the number of times precipitation has been 90% of average, or less, over the last 30 years in these two-month time periods for the Contiguous United States.  I would like to thank Dr. Becky Bolinger for assembling and plotting the data and for adding her perspective on how to “climatologically” think about temperature and precipitation.

Precipitation: It's complicated

It’s a pretty common practice to refer to climatological variables, such as temperature and precipitation, by their averages. The difference between weather and climate is that weather is constantly changing, while climate remains more consistent. One common way to refer to the differences between weather and climate is: the climate dictates the clothes in your closet, while the weather dictates what you will wear that day. In order to determine the climate, we take an average of all the weather events for a specific time, extended across 30 years or more.

For a variable like temperature, which is evenly distributed, calculating the average makes a lot of sense. Similar to the “bell curve” distribution, temperature follows what we call a “normal” distribution. Most of the temperatures for a given time period fall close to the mean (average) temperature, and then colder temperatures and warmer temperatures trail off toward extremes pretty equally on both sides.

Things get a little more complicated for precipitation. Take a look at Fort Morgan CO precipitation for example. You can calculate the average of all the values to estimate the climatology of precipitation, but for many locations, the average does not represent the most commonly occurring precipitation events. In fact, unless you live in a tropical rainforest near the equator, the average is really not a good indicator of how your location experiences precipitation. In Fort Morgan, most of the days in a year have zero precipitation. But the average is .03”. That means, for most days of the year, precipitation is less than average and for some days of the year, it’s a lot greater than average. Very rarely is the daily precipitation actually equal to the average.

Average vs. Frequency

The example for daily precipitation is pretty extreme. It’s not quite as extreme when considering monthly (or 2-month) precipitation. But it’s still something that you should know – the average doesn’t actually happen very often. And something else to keep in mind? During the dry season, it’s pretty easy to have a below average precipitation month or two. In fact, for many locations and times of year, having below average precipitation is more common than having near or above average precipitation.

Let’s look at February-March precipitation for Corson County, South Dakota. This graph shows what the % of average precipitation they’ve had for all February-March time periods from 1895-2019 (data from NOAA’s Climate at a Glance tool). Their average precipitation for February-March is 1.45”. It’s only been close to that average in 21 out of 125 years. In 60 years, it’s been below 90% of average. In fact, the most common occurring total is between 25% and 75% of average. In other words, it’s more common for the precipitation to be between 0.36” and 1.08” than to be near average.

What does this mean? Well, it means for many times of year, depending on where you live, you should not depend on the average precipitation to tell you what may actually happen. It’s important to find out the difference between what’s average, and what occurs most frequently.

Frequency of precipitation deficit at key times can be important for understanding your operational risk as a livestock producer. Below are maps showing the number of times precipitation has been below the 90% of average threshold for 2-month time periods throughout the Contiguous United States from 1990-2019. Over this 30-year time period, any region shaded with yellow, orange, or red experience less than 90% of average precipitation over half the time.

Frequency of Precipitation Deficits 1990-2019

Notice that for much of the Great Plains, the frequency of drier than average conditions is lower when the wet season starts (specifically around April-May). For much of the northeast, regardless of time of year, drier than average time periods are less frequent. Areas with a very strong seasonal cycle, like California, see a very strong pattern in the frequency as well. During the dry season (July-August is a very dry season!), getting above average precipitation in California almost never happens and drier than “average” precipitation happens almost all the time!

Getting a better understanding for how precipitation acts in your location is very important. Having that knowledge can help you better prepare for what is more common for your area and can reduce the element of surprise when you find out it was dry again.