Hot air, and independence

Today marks the 236th anniversary of the day that the freshly written Declaration of Independence was distributed to the commanders of the Continental Army.

On a day like this, it's natural to admire the Founding Fathers for forging that historic document on the heat island of Philadelphia in the dead of summer.

But have you noticed that we never see images of Franklin, Jefferson or any of their colleagues in shorts and T-shirts?

Thanks to Jefferson, a Renaissance man to the bone, we know that those first two weeks of July 1776 actually were quite comfortable.

Jefferson had just purchased a thermometer, and his log begins on July 1. From the 1st through the 14th, the temperature never got higher than the low 80s, he reported.

Jefferson's readings are corroborated by those of Phineas Pemberton, who was keeping a log out his way, what we now call Roxborough. Natuarally, Pemberton's were a shade cooler.

The high Jefferson recorded on the fateful 4th was a mere 76. Granted, it's likely that Philadelphia's temperatures were cooler in the summer of 1776 than they are these days.

The city was less developed, and the world still was passing through the so-called "Little Ice Age," a prolonged period of global cooling that lasted from the Renaissance to the beginning of the Industrial Revolution.

Even the low 80s, however, would have been considered warm to a Londoner. Typically, July temperatures in London top out in the 70s.

Philadelphia's daily highs are 12 to 15 degrees above the typical London maximums in July.

The other thing to consider is that the solar energy in Philadelphia would have been significantly stronger than it was in London.

The difference in latitude between London and Philadelphia is similar to the difference between Philly and Hilton Head.