Kansas climate basics: Pt. 4 -- Changes in frost dates from 1901 to 2014
(Editor’s note: The following article is one in a series of articles in the Agronomy eUpdate that examines the historical climate observations in Kansas. The methods used to do this analysis is explained in the introductory article in this series, from eUpdate No. 571, May 20, 2016. – Steve Watson)
Changes in frost dates and frost-free season length in Kansas
A change in the number of days when the minimum daily temperature falls below freezing (frost days, using 32 degrees F as a threshold) is an extreme signal in climate perspective. The length of the frost-free season and the changing trend in the length of that season, if any, have direct implications for crop production in Kansas. Figure 1 provides a comparison of the first frost dates in the fall, using two recent 30-year periods of time, 1951-1980 and 1981-2010. On average, changes in first fall frost dates between those two periods of time were greatest in eastern Kansas. In that region, the first frost dates of 1951-1980 (30 years) were earlier than in 1981-2010 (30 years). However, northwestern areas in Kansas showed an earlier first frost date in the 1981-2010 period.
Figure 1. Average first frost dates in Kansas from (a) 1951-1980 and (b) 1981-2010.
For the last frost date in spring in Kansas, the changing pattern across the state is consistent with the changing pattern of the first frost date due to climate change. That is, eastern Kansas had earlier last frost dates and southeast Kansas had later dates in the 1951-1980 period than in the 1981-2010 period (Fig. 2). Trends of frost dates (both first frost date and last frost date) are highly varied when different temporal scales were selected.
Figure 2. Average last frost dates in Kansas from (a) 1951-1980 and (b) 1981-2010.
Such a distinct spatial pattern of frost dates resulted in a significant change in the number of frost-free days (i.e., first frost date minus last frost date, or frost-free season length) over time. The frost-free season length in all three regions of Kansas showed a statistically significant increase from 1901 to 2014 at a rate of 0.99 ± 0.79 days per decade in western Kansas, 0.94 ± 0.90 days per decade in central Kansas, and 1.47 ± 0.74 days per decade in eastern Kansas (Fig. 3).
Figure 3. Time series of frost-free season length in Kansas from 1901 to 2014: (a) Western, (b) Central, and (c) Eastern Kansas (see Fig. 1 or 2 for these three regions of Kansas). The base indicates average frost-free days calculated from 1901 to 1960 (60-year period as a base period). The Ave 1991-2012 – Ave 1901-1960 is the difference of frost-free season length between 1991-2012 and 1901-1960. When trends are statistically significant the trend rates are displayed. The ± values are the trends’ confidence intervals at 95% confidence levels. All adjusted p values are shown.
Xiaomao Lin, State Climatologist, Department of Agronomy
John Harrington Jr., Department of Geography
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist