What was the weather on July 4, 1776?
In the summer of 1776 when Thomas Jefferson, an avid weather observer, arrived in Philadelphia, he promptly bought a thermometer. Apparently, he felt watching the weather was almost on par with the important meetings that would take place regarding our freedom from Great Britain.
His records indicate that the first few days of July, 1776 were marked by changes in the weather. On July 2, the day Congress voted to declare America's independence from Great Britain, clouds and heavy thunderstorms occurred for several hours.
On July 3, 1776, the day the delegates met to hash out language issues in the Declaration, the unsettled weather pattern broke, and sunshine and pleasant conditions ensued.
On July 4, 1776, when the Declaration of Independence was adopted, the comfort of the Philadelphia weather was noteworthy. Despite the world-changing event on that Thursday, Jefferson was able to make four temperature observations:
The long-term average low temperature in July for Philadelphia is 69 degrees and the normal high is 87 degrees. Thus, the morning low would have been a bit below average and the high would have been considerably lower. In short, it was a comfortable, tranquil day. An observant Philadelphia resident, Phineas Pemberton, also noted an increase in clouds on the afternoon of the Fourth after a sunny start to the day, falling air pressure, and a wind shift from the north to the southwest.
The weather sequence described in the records of these keen weather watchers indicates a disturbance aloft in the atmosphere and an attendant cold front at the surface swept through Philly on July 2nd and triggered the storms. Passage of the upper-level feature and the cold front was followed by much calmer weather. The remarks by Mr. Pemberton about the clouds returning during the afternoon of the Fourth and the wind shift would imply surface high pressure moved away to the east and another weather system was located to the west of Philadelphia.
No one knew it at the time, but the events that changed world history in early July, 1776 were mirrored by the changeable Philadelphia weather.
And what did Jefferson do shortly afterwards? Holding true to his great enthusiasm about the weather, he bought a barometer on July 8th!