Time and Temperature; Philadelphia, July 4, 1776

7, [1] pages. Notes. From information found on-line: "Writers have always watched Philadelphia's weather during the summer of 1787 and have, with few exceptions, described it as hot and oppressive, apparently because the heroic stature of the framers seems to require them to have conquered the elements as well as the political crisis in the nation. Farrand did not publish meteorological records for the summer of 1787. David M. Ludlum of the American Weather History Center compiled statistics on Philadelphia's weather during the summer of 1787. Ludlum calculated "normal" temperature and rainfall by using the Pennsylvania Hospital Records, 1825-71. He concluded that "in general terms Philadelphia enjoyed a cool summer in 1787." It is, perhaps, not without interest to compare these findings with a recent discovery that, despite the heat in which the imaginations of generations of writers have enveloped Philadelphia on July 4, 1776, Independence Day was also cool.' It is obvious, however, from letters that the impact of temperature was subjective; what to one person was hot was to another pleasant. (See. Paul H. Smith, "Time and Temperature: Philadelphia, July 4, 1776," Quarterly Journal of the Library of Congress 33 (October 1976): 297-98.) " Using observations from Jefferson, Pemberton and Marshall, we can piece together a very good picture of that day's weather. Jefferson didn't record an observation between 3 and 4 o'clock but he did at 1 p.m. After all, he had other, more pressing matters on that day. Overall, the Fourth of July was a pretty good day in Philadelphia. The high temperature was in the mid-70s and there was no rain. These days, Philly's average July 4 high is 87 degrees. Even if the average high back then was a little cooler than it is now, that temperature is still mild. July 4 began with clear skies and temperatures in the upper 60s. The wind was light out of the north. By midday, there were a few clouds and the wind switched to a southeasterly direction. In the afternoon, the temperature rose to the mid-70s as winds switched to the southwest, clouds increased and humidity levels rose. Marshall wrote that the wind switched from the southeast to the southwest during the afternoon and it became more humid, while other reports indicated that the barometric pressure fell .25 inches of mercury, or 7.5 millibars, during the day. The exact barometric pressure reading, however, is unclear. There were no weather maps in 1776, but people like Jefferson kept enough weather records to formulate opinions (and pretty good ones) about general weather patterns. As a meteorologist, and just for fun, I like to gather weather information and speculate about what the general surface weather pattern could have looked like. Starting on July 1 in Philadelphia, it was very warm and humid with afternoon thunderstorms. There was likely a moist flow of southwesterly winds with a cold front approaching. Since there were lots of clouds and showers on July 2, there was either a secondary front that passed through, or perhaps the original cold front passed through slowly. July 3 was sunny and mild, so a high-pressure system was likely moving toward the Philadelphia area. The high was probably on the weak side, since high and low temperatures were moderate. The clear sky and light north wind on the morning of July 4 was indicative of the high-pressure system approaching the area. Dry and mild conditions probably prevailed across much of the Northeast and mid-Atlantic regions on that day. In Philadelphia, the wind shifted to the southeast and then to the southwest in the afternoon, and the barometric pressure fell. That means the high-pressure system likely moved eastward into the Atlantic, and since winds around high-pressure systems flow clockwise, the return wind flow would be out of the southwest. There could have been another cold front west of the area, but there was no mention of rain by Jefferson until July 8, so perhaps the next front. …