How Washington's crossing of the Delaware presaged a changing world

Spend some time staring at the famous painting "Washington Crossing the Delaware," and you can't miss the ice. It's everywhere. Cold weather became part of Washington's military strategy, says Alex Robb, an educator at Washington Crossing Historic Park outside Philadelphia. "It does a lot to impede the crossing and endanger the whole operation," he said, "but it actually becomes our shield." In:

Spend some time staring at the famous painting "Washington Crossing the Delaware," and you can't miss the ice. It's everywhere. Cold weather became part of Washington's military strategy, says Alex Robb, an educator at Washington Crossing Historic Park outside Philadelphia. "It does a lot to impede the crossing and endanger the whole operation," he said, "but it actually becomes our shield."

At the end of 1776, after a string of losses, Washington's army was on the verge of collapse. But Robb says that on Christmas, with ice forming in the Delaware River, the enemy assumed it was too dangerous for the Americans to cross.

They were wrong … and the cold weather handed Washington the element of surprise. His victory at Trenton was a sign that the war could still be won.

Robb said, "Looking back, had the weather proven more mild, they most definitely would've encountered resistance outside Trenton."  Just a few degrees made the difference between winning and losing a battle.

At that time, Americans were used to colder winters. We know that from Thomas Jefferson's meticulous, handwritten weather records. But since then, winter has gotten warmer. "Ever since Washington was here, there has been a steady increase," said Jen Brady, a data analyst at the science non-profit Climate Central. Their research shows that average winter temperatures in the Philadelphia area have gone up and down over the years. But overall, they are now 5.5 degrees warmer than they were in 1970.