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Apr 15 2010

How cold was our island winter?

cold winter chart-2largeBy Ian Millar

Charleston City has some temperature data that go back a long way. Those data can help bring a long term perspective to our recent winter experience here on Seabrook/Kiawah. The reason is that the temperature relationship between locations as close as Seabrook/Kiawah and Charleston’s downtown seems to be relatively consistent over time. We say “seems to be” because at this time our local data has been gathered from just three weather stations (two on Seabrook and one on Kiawah) and those stations have data which overlap for only 2007, 2008, 2009 and 2010 year to date. However, the daily data across the three stations match very closely and we have averaged them to create a Seabrook-Kiawah temperature record.

For those last 3+ years, the consistent relationship between Charleston City (CC) and Seabrook-Kiawah (S-K) can be summarized as follows. On Seabrook, the annual mean temperature (the average of average daily temperatures over a given year) tracks 1 ºF lower than that of Charleston’s downtown. The annual mean maximum temperature (the average of daily high temperatures over a given year) tracks 0.5 ºF higher and the annual mean minimum temperature (the average of daily low temperatures over a given year) tracks 1.5 ºF lower than those of Charleston’s downtown. Note that the temperature differences have been rounded to the nearest 0.5 ºF.

This relationship is a little different if the focus is seasonal rather than annual. For example, Seabrook’s December-January-February winter temperatures relate as follows. Relative to Charleston’s downtown, our winter mean temperature tracks 1.5 ºF lower and our winter mean maximum and mean minimum temperatures track 1 ºF and 2 ºF lower respectively.

For the moment (until we can lay our hands on additional and longer temperature records on Seabrook and/or Kiawah), let us assume that this relationship between CC and S-K winter temperatures was also consistent before 2007. By applying this assumption to Charleston City data, we can venture the following about Seabrook’s 2009/2010 winter: The coldest winter in the last 139 on Seabrook was 1977/1978 when the mean temperature would have been 44.2 ºF (see chart). Seabrook’s mean temperature for winter 2009/2010 was actually 47 ºF. That made it the 18th coldest winter in the Seabrook-Kiawah data record we have been able to create (1871 to the present). By breaking winter down into its components, we see that the mean temperature for December 2009 was a relatively balmy 51.7 ºF (the 92nd coldest December), but that the mean temperatures of January 2010 (44 ºF) and February 2010 (45.4 ºF) made them the 15th coldest January and the 14th coldest February respectively.

We don’t know what future winters hold, but what a contrast it would be to experience a rerun of one of our unusually warm winters such as 1889/1890 or 1931/1932 (see chart).

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