For Release September 17, 2002

Does A Dry Summer Lead to A Dry Fall?

AGRI-VIEWS
by Chuck Otte, Geary County Extension Agent

I am not a weatherman. I do not like to predict what the weather is going to be, although I will make unfortunate prognostications from time to time. But I am a statistician and I enjoy reviewing what has been and then trying to put some analysis of significance on that data.

This year has been dry. People have talked about 2002 being drier than the 1930s, which here in Junction City werenít as dry as you might think. And trying to compare this year with the 1930s or the 1950s is very difficult because of changes in cropping practices, changes in land use practices, even changes in how we live. So I wonít venture down that slippery slope of saying if this is as dry as the 30ís, because I wasnít here. All I have is the data to look at.

We have monthly rainfall data back to 1931 and good daily rainfall data from the late 1940s. This year, May through August, rainfall was 59% of normal. So does that mean anything for possible September to December precipitation? I went back and looked at how many years, of those now 71, we had May to August rainfall below normal and that means anything less than 100%. If it was 99% of normal then, even though it might seem inconsequential, it is still below normal.

What I found was that in 35 years of 71 we had May to August rainfall below 100% of the long term average. You know what? With that many years of data, that is exactly what I would expect to see. Averages are made up of all the extremes and if you get enough data points then half should be above normal and half below.

Does the past four months dictate what the next four months will be, or in other words does the past predict the future? I took those 35 years with below normal rainfall and looked at the September to December period as a percent of normal. 15 of those 35 years we had more than 100% of normal, in the next four months and in the other 20 years we had less than 100% of normal. Probably not statistically significant, BUT a slight trend to indicate a dry summer may lead to a dry fall, or at least it did in 20 years out of 35.

Letís take it one step further. The last four months that weíve experienced just werenít dry, they were real dry. So how many of those years did we have 59% or less of normal rainfall? Weíve only had summers that were this dry or drier in 6 years. In those 6 years, the following four month period was wetter than normal 3 times, and drier than normal 3 times. If we were to apply full analysis to this we would probably find that statistically it doesnít matter.

We know that weather patterns get in place and can last for a long time. We can just as easily have an 8 month drought as a 4 month drought. But I think that we can also see that it is just as likely that we can be at the end of one pattern and moving into a different, hopefully wetter pattern.

So is this as dry as the 1930ís? The year isnít over with yet. We are one half inch below the driest year recorded since 1931 and that happened to be 1966. The 1930ís were not that dry, averaging just a couple inches below the long term average. The driest period, in the last 70 years was 1952 - 1956 which was about ten inches below normal. So yes, based on rainfall alone, this is drier than the 1930ís or any year in the 1930ís. So far anyway. We still have over three months to go, and anything can happen. And thatís what makes the weather so much fun!

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