In 1980, winters in Evanston, Illinois felt more stable. When Arctic air arrived in December, the cold usually stayed for many days or even weeks. A snowstorm often marked the beginning of a long cold period. Temperatures stayed below freezing, and snow on the ground turned hard and icy. The small harbors on Lake Michigan froze, and the ice stayed for most of the season. For many residents, cold air and snow were clearly connected. If it was very cold, people expected that snow would arrive soon and that winter would continue in a predictable way.
By 2020, the pattern had changed. The climate had begun to decouple, which means that old relationships in the weather were weaker. Evanston could have one short burst of very cold air and then jump back to mild temperatures after just a day or two. A heavy snowstorm might be followed by rain and melting the next afternoon. The lake ice was thinner, sometimes breaking up in the middle of winter. People noticed that cold air and snow did not always travel together anymore. A week of freezing temperatures did not guarantee deep snow, and a big snowstorm did not guarantee a long winter.
In 1980, winter in Evanston still followed familiar rules. In 2020, winter felt more like a series of surprises. This weakening of the connection between cold air, snow, and season is one example of climate decoupling.