Why Coastal Cities Have Milder Winters

Boston and Chicago sit at roughly the same latitude. In January, Boston's average low is around 23°F. Chicago's is about 18°F. The difference is the Atlantic.

Large bodies of water change temperature slowly. In summer they stay cooler than the surrounding land; in winter they stay warmer. Prevailing winds carry that moderating effect onto the nearest shore, giving coastal cities milder extremes in both directions.

Seattle, despite sitting at the same latitude as Montreal, almost never sees sub-zero winter lows. The Pacific is a powerful thermostat, and the marine influence is one reason Cascadia feels so different from the Midwest despite comparable solar angles.