Saturday, January 31, 2015

Extreme Weather in Kansas

One of theorized consequences of climate change/global warming is that more extreme weather is supposed to occur. One possible measure of extreme weather is the number of days where the temperature meets or exceeds 90 degree fahrenheit. This is one of the monthly variables that NOAA happens to collect and report for each weather station in the country.

This post examines monthly weather data in Kansas from 1900 through 2013 to determine if the number of days where the temperature has met or exceed 90 degrees has increased over time. For this analysis, data from every weather station in Kansas was collected from the NOAA website. All the stations within a particular county were averaged together to produce a county number for each month and each variable.

As one might imagine, the number of weather stations has grown over time. Going back to 1900, the number of counties with an unbroken set of data for the variable DT90 (days above 90 degrees) is quite small. As long as a county had a record of the DT90 variable from spring through fall for all 114 years, it was kept in the analysis. Only the counties of Decatur, Franklin, Kingman, Montgomery, and Riley met this criteria. Monthly data for each of these counties was summed together to get the number of days in the year where the temperature was 90 or higher.

Figure 1 shows the plot of the number of days per year where the temperature was at least 90 degrees for the five counties being examined. As anyone who has lived in Kansas knows, the weather can be quite variable and this is also reflected in the line graphs of each county. The number of days were the temperature exceeds 90 degrees has ranged from over 100 days to less than 20 days. However, in no instance is there any evidence that weather is becoming more extreme. In fact, the trend lines for all 5 counties is actually slightly negative.

Figure 1. Number of Days With a Temperature of at Least 90 Degrees

Of the five counties examined, only Riley County has any type of population center. Riley county is the home of Manhattan Kansas and also Kansas State University. Manhattan, with a population of 56,000, is by no means a large city. To examine another hypothesis that urban influences are affecting weather station measurement, I included a county that does have a large city. Sedgwick county is the home to Wichita (population 387,000). Sedgwick county only has consistent weather data for the DT90 variable back to 1948. The second figure shows the original 5 counties and Sedgwick County from 1948 through 2013.
Figure 2.  Number of Days Above 90 Degrees Since 1948

The trend lines for the original 5 counties are nearly the same as before. Sedgwick County also has a nearly flat trend line for the number of days exceeding 90 degrees but it is increasing slightly (slope = 0.06 days per year). Thus, it would take over 16 years to get, on average, another day above 90 degrees 

There are probably many measures to use to represent extreme weather. I chose the number of days where the temperature met or exceeded 90 degrees fahrenheit as it was readily available from NOAA. Based on the the 6 counties examined here, it is clear that this measure has not changed very much over time.

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