Changing Chesapeake: Frost Days

For my first post here on the SciencePensieve website, I thought I would take a crack at showing off some of the results of the wonderful work that Kari has been doing over the last several months.  As readers familiar with this page are aware, Kari has been working to analyze long-term climate data from a few different sources.  By using various metrics or indices of climate condition such as Frost Days per year, Growing Season Length, and Tropical Nights per year (just to name a few) we can detect long-term changes in the climate around Chesapeake Bay.

Today we’ll focus on the Frost Days metric and then discuss others in later posts. Frost Days is a Cold Event Index, in other words it’s an indication of how cold one year is relative to other years. Specifically, it is the number of days in calendar year when daily minimum temperature is less than 0 degrees C (32 degrees F). There is a higher probability of frost occurring on these days. A year with a higher than average amount of frost days would be more likely to be characterized as cold compared to years with an average or less than average number of frost days.

Figure 1 - Annual Frost Days Index calculated from Northern and Southern NCDC stations located around Chesapeake Bay.

Figure 1 – Annual Frost Days Index calculated from Northern and Southern NCDC stations located around Chesapeake Bay.

Looking at the Frost Days analysis of 18 National Climatic Data Center (NCDC) stations located around Chesapeake Bay (Figure 1), it is apparent that there has been a decrease in the number Frost Days each year over the period 1900 to 2014. The data were separated into Northern stations and Southern stations and then averaged (Figure 2) as shown by the blue and red lines on the graph. The thicker lines represent the rolling 21 year averages of the same data, which basically gives us a smoother line to look at while maintaining the up or down direction of the trend.

The annual data (the thin red and blue lines) show that the data is highly variable from year to year. Despite this variability, the general trend is a decrease in the number of frost days over time. Comparing the early 1900s to the early 2000s, the Chesapeake experienced a decrease of 33 Frost Days per year in the northern portion and 28 days in the south. Simply put, the Chesapeake region is significantly less cold than it was 100 years ago. The number of Frost Days is directly related to the length of the growing season for plants in that with fewer Frost Days, you get a longer growing season. We’ll discuss that in a later post.

Figure 2: Map showing the Northern (yellow diamonds) and Southern (blue diamonds) NCDC stations used in this analysis. Also shown are the NERRS locations (triangles).

Figure 2: Map showing the Northern (yellow diamonds) and Southern (blue diamonds) NCDC stations used in this analysis. Also shown are the NERRS locations (triangles).

So, why do we care about fewer frost days? Most importantly, it is an indication that climate around Chesapeake Bay is changing and becoming more like what can be found at more southerly latitudes. An increase in growing season could potentially be beneficial for local plants and crops. However, many agricultural diseases and pests are limited in the northern extant of their range by winter temperature minimums. If the Chesapeake is less cold in the winter, we may see new plant pests and diseases in our region. Warmer winters could mean savings on heating costs for Chesapeake residents. Unfortunately, as we will discuss in a later post, this region is experiencing more hot summer nights, which could increase air conditioning costs.

Stay tuned!

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