"Normal" Rainfall Changes

By: Stephen Bowers Email
By: Stephen Bowers Email

What we call "normal" rainfall is actually an average based on 30 years worth of rainfall totals. Each decade, that average is recalculated based on the most recent 30 years. That means that each decade, our "normal" rainfall amount changes, and it has gone through major changes since the 1920s.

Here are the 30-year annual rainfall averages for Tallahassee since 1920:

  • 1920: 55.72"
  • 1930: 55.87"
  • 1940: 55.01"
  • 1950: 57.02"
  • 1960: 55.60"
  • 1970: 60.68"
  • 1980: 64.01"
  • 1990: 66.39"
  • 2000: 64.12"

Throughout the 1900's, it's easy to see how the rainfall that is "normal" for a year has increased. The rainfall total for 2007, 44.47" was nearly 20 inches below our 30-year average, but the deficit would have been significantly less just 50 years ago - even with the same amount of rain in the year. In 2010 when the 30-year average is recalculated, it may decrease significantly from the average calculated in 2000.

As WCTV Chief Meteorologist Mike McCall pointed out, averages are calculated using a lot of high extremes and a lot of low extremes. In the case of rain, a drought and a flood average out to normal. Neither are especially good, but it's important to remember that what is "normal" is based on perspective - and that perspective changes. What is normal now was not normal 100 years ago, and it will not be normal 100 years from now.

Why do we see more rain on average now than we did in the 1920s? Climate, or "average weather," is cyclic. These cycles can be affected by El Niño and La Niña and other similar global weather patterns. The cycles extend to everything from rainfall and temperature trends to the number of hurricanes we experience in a year and even where those hurricanes go seems to be a cycle. So although we may be lacking in rainfall for a year - or even several years - remember, we'll make up for it eventually.

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