The Science of a Brilliant Fall

Every autumn Vermonters ask the same question: will the color be good this year? The answer is written in the weeks beforehand, in a delicate interplay of sunlight, temperature, and moisture that determines whether the hillsides blaze or merely fade.

The reds are the prize and the most weather-dependent. Unlike the yellows, which are always present in the leaf and simply revealed as chlorophyll breaks down, the vivid reds of the sugar maples are made fresh each fall from trapped sugars. Bright, sunny days and cool but not freezing nights produce the most of them, which is why a run of clear autumn weather sets foliage watchers cheering.

Other factors nudge the display. A summer drought can dull the show or bring it early; a warm, cloudy stretch can mute the reds; an ill-timed windstorm can end it overnight. It's why no two Vermont autumns look quite alike, and why locals watch the forecast in September with the intensity others reserve for a big game. The mountains keep the outcome to themselves until the very last week.