No Fooling--It’s Really Fall

As the mercury climbed above the 90-degree mark again Monday, colorful fall foliage provided one of few clues that it actually is autumn.

But the heat does not delay the annual color change and the fall of the leaves. That turn of leaf color is triggered not by cold but by the change in the hours of sunlight, as the autumn days grow shorter and the nights longer, said John Sherwin, meteorologist with WeatherData Inc., which provides forecasts for The Times.

Trees rely on sunlight to manufacture chlorophyll, the substance that gives them their green color. Weather conditions such as an extended dry period or an early autumn freeze can cause the leaves to turn earlier in the season and dull their colors, Sherwin said.