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Let’s Look at Rainfall Data for the Last 94 Years

Florida’s Southwest Water Management District has the rainfall data for 16 counties in this area for the previous 94 years. I graphed three counties, two adjacent and the northern county in the district: Hillsborough, Polk and Levy. While there has been major fluctuation in all the counties, the oscillation has been between 40 inches and 80 inches in all three counties.

I recently read a paper by Naziha Azli and Norhafizah Ramli (University of Malaysia) regarding trends from climate data that oscillates. There are those who want to perform a regression analysis of the data to plot a straight line. In every case the straight line will be ascending which people translate to mean that the average temperature/rainfall, etc. is steadily rising. However, Azli and Ramli found that if you plot a random series of numbers that oscillate up and down as rainfall and temperature do, the regression analyses will always yield an ascending line. It is not appropriate to do this analysis on this type of data. When reviewing the rainfall data for this region, there is clear oscillation but no apparent change in the range. We cannot say from this data that rainfall is dropping or dramatically rising. It also depends on how you pull the data. If you look at the first 30 years of data for Hillsborough County, the average rainfall is 52 inches, which is considered the average for the area. If you pull 32 years different but consecutive years, the average is 62 inches. So what cycle are we in at any given time? Only God knows.