Everybody knows that rainy days tend to be cooler than sunny days. It's a simple matter of clouds keeping sunshine out. But rain can also have a lingering cooling affect on temperatures in the days that follow the rain, even if those days are sunny. Here's how.

When the earth's surface absorbs sunlight, some of the energy heats the ground. But if the ground is wet - as it might be after a recent rain - some of the sun's energy is used to evaporate water. That means less energy goes toward heating the ground. Since the ground is really what heats the air, it follows that air temperatures on a sunny day when the ground is wet usually won't get as high as they would if the ground was dry.

Forecasters need to be especially aware of situations when this cooling effect from past rains might come into play, because the computer models that we use to help in our forecasting don't adequately account for it.