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Why late summer, early fall are ‘dew season’

LOVE_LIFE/Getty Images grass with dew drops
WET MORNINGS: There is a reason why the grass is often damp when you first go outside on a late summer or early fall morning. These are the mornings when you wait for it to dry before harvesting.
Weather Wise: Heavy dew in the morning becomes more common in early autumn.

It seems like there are often heavy dews in the mornings especially in later September and into October. What causes dew? What time of year is it most likely to occur?

Let’s take a closer look. Dew is caused by a very simple process. Dew forms when air near the surface cools to the dew point and the air becomes saturated. Water condenses out and onto the surface.

Dew usually forms when the sky is clear; the surface radiates off heat and cools without clouds trapping the heat. This surface radiation allows the air to cool more rapidly overnight, and it is more likely to fall below the dew point. Dew forms similarly to frost. It just occurs above the freezing point. 

For dew to form, the air doesn’t have to be exceptionally moist. But more water in the air comes with the warmer temperatures and higher dew points that are seen commonly in late summer and autumn months. Higher dew points are also observed in the Midwest when plants and crops are in full force and evapotranspiration is at a high. 

Evapotranspiration means water evaporates out of leaves and plants. This can lead to dew points as high as 80 degrees F in extreme cases. These are dew points not commonly seen outside of the tropics.

Dew in some parts of the world is being studied as a potential source of fresh water. Hundreds of liters of water can be collected some nights during peak dew season.

Autumn is the peak dew season because the air is generally cool enough to fall below the dew point, but not cold enough to create frost. Autumn also lends itself to clearer, calmer nights, which are critical to the cooling and surface radiation that allow the surface temperature to fall below the dew point.

Eggert is a Purdue University graduate who works for the Indiana State Climate Office. He writes from West Lafayette, Ind.

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