What causes dew?

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Dear Tom,
What causes dew? In general, is dew a good thing or a bad thing?
— Stephen Verhaeren
Dear Stephen,
Dew is water in the form of droplets that appear on exposed objects at night. As exposed surfaces cool by radiating their heat, moisture in the air condenses at a rate greater than it can evaporate, resulting in the formation of water droplets on the surfaces.
In arid climates or in climates with seasonally dry weather, dew is an important source of water for vegetation (and even for animals and insects) because of its great frequency. However, in moist climates such as Chicago’s, this is not important.
The primary negative aspect of dew is its ability to induce plant diseases. The moisture, and its frequency, can lead to molds and rusts on plants.
When water droplets form on objects while they’re cooling, it’s called dew. Though dew isn’t vital in Chicago’s climate, it can make a difference for plant diseases.


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