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Livestock Wx for Nov. 11, 2019:Average Precipitation vs. Precipitation Frequency: The Difference Might Surprise You

A longer version of this article first appeared on Livestock Wx (click here for original article) and was written by Becky Bolinger. Dr. Bolinger is the Colorado assistant state climatologist and is based at Colorado State University. She is a frequent contributor to Livestock Wx, the U.S. Drought Monitor and tracks all things climate.

Precipitation: It’s Complicated

It’s a pretty common practice to refer to climatological variables, such as temperature and precipitation, by their averages. 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 average temperature, and then really cold temperatures and really warm temperatures trail off toward extremes pretty equally on both sides.

Things get a little more complicated for precipitation. 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. For most then, 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.

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 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.

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 is important, however, 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 three maps showing the number of times precipitation has been below 90% of average threshold (we use 90% because this could be thought of in the context of Pasture, Rangeland, and Forage insurance) for 2-month time periods throughout the Contiguous United States from 1990-2019.

Over this 30-year time period, any region shaded with orange, red, or maroon experience less than 90% of average precipitation over half the time. We chose to show these time periods because they are historically the wettest times of the year for many parts of Texas and Oklahoma, yet for at least two of these timeframes, the majority of the years out of the last 30, have been below average. So, while average is interesting to understand a potential magnitude for precipitation it really doesn’t mean anything about the likelihood of it actually happening. That’s not very reassuring but it might explain why we are often surprised when a drought hits.

For those interested in seeing all the maps that were produced for this article please take a look at the original article on Livestock Wx (click here for original article).