Coastal Climate Extension

So far for the month of December, according to the National Weather Service the average high temperature in Charleston through Dec. 21 is 66.0 degrees Farenheit – usually, the average high for the entire month is 61 degrees. The average daily temperature – the average of all the high temperatures and all the low temperatures for the month – is 55.8 degrees, which is about 4.3 degrees warmer than normal. The difference isn’t quite so dramatic in North Carolina: for Cape Hatteras, the average high temperature so far this month is 60.8 degrees, compared to a normal average of 57.3. The average daily temperature is 53.6, which is 2.7 degrees above normal. Of course, there’s still an awful lot of December to go, so a few days in the deep freeze could put us back to normal. However, forecast lows in the 50s on the Carolina coasts for the latter part of this week, with highs in the upper-60s in Charleston and the lower- to mid-60s in Hatteras, December could still shake out a hair on the warm side.

November was a different story. In November, the National Weather Service calculated the average high temperature as 59.9 degrees at Cape Hatteras – which is 4.9 degrees lower than the average 64.8 degrees for the month. Numbers were similar further down the coast – about 4.3 degrees below the normal average high of 69.6 degrees in Charleston. As you can see by the NOAA map above, statewide, November was the 10th coldest on record for SC – and the 19th coldest in NC. However, NOAA reports that for the entire U.S., November 2008 was warmer than average – even though temperatures were cooler in the Southeast.

One may be tempted to look at November and say, “What global warming? It was colder than normal in the Carolinas.” This is a pretty common misperception about the way climate and climatology work. Data from one place, or one month, or one year don’t provide evidence either for or against climate change. First, what happens locally is not what happens globally – as evidenced by the November 2008 temperatures in the US. Second, it’s a matter of how averages work. Since it’s the holiday season, let’s do a holiday example.

You and I are wrapping gifts for your December holiday of choice. Say that in one hour, you wrap 7 gifts and I wrap 5. That hour, we wrap an average of 6 gifts each. In the second hour, you speed up and wrap 9 gifts, but I slow down and only wrap 3. We still wrap an average 6 gifts each in the hour, but we’ve gotten there by much more extreme means – i.e., we keep the same average, but you wrap faster and I wrap slower. Climatological averages are like that, too. Yes, it was colder in the Carolinas last month – but looking at the nation overall, November was warmer than usual. Similarly, one place can actually get colder as the climate changes. If those changes are outweighed by big increases elsewhere, the overall global average temperature still goes up. In our example, if you wrapped gifts even faster in the third hour – say, 11 gifts wrapped to my 3 – our average gifts-wrapped-per-person speeds up to 7 gifts per hour… even though I slowed down. Make sense?

Source: NOAA

So what’s in store for the rest of the winter? NOAA’s winter outlook gives the Carolinas equal chances of being warmer or colder than average. That means that it’s just as likely to be cold as it is to be warm, more or less – there’s no strong indication either way. However, there is a good chance (greater than 40%) that it will be drier than normal. That’s bad news, especially for the northern Outer Banks which are just beginning to recover from the drought but still abnormally dry (according to the U.S. Drought Monitor). We’ll all have to stay alert to the situation.

That’s likely it for this year – your friendly regional climate extension specialist is headed out of town for the holiday and to take some time to finish up that pesky dissertation. Best wishes to all, and see you in 2009!

Oh, about that day length calculation: I cheated and used the US Naval Observatory calculator for sunset and sunrise times, then did a little quick subtraction (remember to correct for those pesky minutes).