Wednesday, July 03, 2013

Fourth Of July Meteorological Lies Your Teacher May Have Told You

I believe I am not the only one who remembers learning about the Continental Congress and the drafting of the Declaration of Independence in elementary school, and specifically, just how swelteringly hot it was in Philadelphia in June and July of 1776.

But was it really that hot? Thomas Jefferson’s own records say it probably was not.

Meteorologists Jon Nese and Glenn Schwartz authored The Philadelphia Area Weather Book in 2002 (which, even 11 years later, is still a good read, in my opinion) and in it they say that Jefferson “recorded temperatures of 68 F at 6 A.M., 76 F at 1 P.M., and 73.5 F at 9 P.M.” on July 4, 1776.

By comparison, the average high temperature in Philadelphia these days for July 4 is 85 degrees, and the average low is 68.

Of course, there is much more to account for than just those three temperature readings. If Jefferson had taken those readings in the shade, they could actually be a few degrees too low. It is also likely that there was an afternoon high temperature a few degrees warmer between 1 p.m. and 9 p.m.

And then of course there’s the one thing every Philadelphian knows about in summer: humidity. Unfortunately, Colonial-era thermometers and barometers were not accompanied by early sling psychrometers, so there is no way to know for sure just how humid it was.

But there are a few clues.

In 2011, another Philadelphia-based publication, Weatherwise Magazine, presented a more in-depth account of the first four days of July, 1776, based on Jefferson’s records as well as some other scattered diary entries.

"A page from Thomas Jefferson's Weather Memorandum Book showing his temperature observations for the first two weeks in July 1776. He recorded a temperature of 76 degrees at 1:00 p.m. on July 4, the day the Declaration of Independence was adopted."   (Image and caption courtesy of Weatherwise Magazine.)

According to the magazine’s write-up, July 1 featured an afternoon thunderstorm. The next morning there was heavy rain for about four hours between 10 a.m. and 4 p.m., and the temperature remained nearly steady at 78 degrees throughout the morning. Although 78 degrees is by no metric “sweltering” heat, the accompanying humidity following four hours of heavy rain (and thunderstorms the prior evening as well) could certainly make for a very sticky-feeling air in Old City. Weather diaries also mention southeasterly winds, which would (naturally) transport more moist air from the Atlantic into the city.

The next morning, the morning low was at least 71.5 degrees, and by the 4th, the low was at least 68 degrees, which indicates that by the 4th, the dew point temperature had to be at or below 68. And while 68 is still rather humid, it typically takes dew points over 70 to make the air feel truly “oppressive.”

So where does this leave us? It seems that July 4, 1776, really wasn’t the swelteringly hot day your fifth grade teacher said it was. (That said, it probably still was not particularly comfortable to be dressed in traditional Colonial-era garb while sitting in a poorly-ventilated brick Independence Hall all day.)

But how does that first famous Independence Day compare to others that Philadelphia has seen since?

Given that it was 76 at 1 p.m. on July 4, 1776, I used 80 as a rough guesstimate for what the high actually was that day if Jefferson’s readings are accurate. In an admittedly non-scientific gathering of data, I found that of the 139 years since the Weather Bureau (the eventual National Weather Service) began keeping daily records in 1874, 107 (77.0%) Independence Days were warmer than July 4, 1776. Only 28 (20.1%) were cooler, and four July 4ths were exactly 80 degrees.

The last July 4th in Philadelphia that was cooler than it probably was in 1776 was in 1996, a day in which the afternoon high was just 74 degrees, although as recently as 2007, the afternoon high was just 80 degrees. And looking at the forecast, it doesn’t seem that the streak of warmer-than-1776-weather will break in 2013.

Now if I was an excellent data analyst, I would have also counted for how many July 4ths the afternoon high was between the known value of 76 and my assumed value of 80. But I’m not an excellent data analyst, and I want to enjoy the holiday just as much as you do. So enjoy your July 4th, and give your fifth grade history teacher a call too if you have time.