The seasons clock

How Is global warming affecting the Four seasons?

This is a complex question—partly because there’s no universally agreed-upon definition of a “season.” The simplest approach is to fix specific calendar dates (e.g., December–January–February for winter), which makes statistical analysis straightforward. However, this method doesn’t allow to assess how the actual length of seasons may be changing over time.

To measure those changes, we need a more dynamic definition—one that marks the start and end of each season based on environmental indicators. A common approach is to use temperature thresholds, determined from long-term averages, to define when a season begins or ends. For example, summer and winter can be defined by consistently high or low temperatures, while spring and autumn lie between those extremes.

An example of this method is shown in the animation for Prague (Czech Republic). I like to call it a “season clock”: the angle represents the day of the year, and each concentric ring corresponds to a different year. The four seasons are marked in distinct colors, making it easy to see how their timing and duration shift over time, which advances like a clock following the red line.

Can you spot the long-term trend?
Summer is creeping earlier into May and later into September, while winter is compressing into a narrower window—mostly December to February. This pattern is exactly what we’d expect in a warming climate, where not only do average temperatures rise, but the shape of the temperature distribution also changes. In other words, where summer typically began in June before 1980, it now often starts by mid-May.

This analysis can be applied to many locations across Europe, and the results are consistent: winters are shrinking, summers are expanding, and the transitional seasons—spring and autumn—are becoming harder to define. The boundaries of the seasons are blurring as the climate warms.