why does due from on the ground during clear, calm summer nights?

Answer 1

Dew is formed when the temperature reaches it's dew point. At the dew point, water vapor in the air condenses onto the ground in the form of dew. The dew point is different depending on the temperature, humidity, pressure, etc... so it can form any night.

Answer 2

Why does dew form?

it's a matter of temperature and humidity a complex formula that results in something called the dew point.

To simplify, when the air is warm it can hold more water.

Above 10 degrees celcius approx, the air can hold it's moisture but when the temp drops to 10 moisture shrinks and gets heavier specificcally and the air cannot contain it any longer so dew forms.

as it warms again the air can hold it again and it evaporates.

10C is an arbitrary number depending on humidty but it;s close too 100% humidity at that specific temperature generally heavier dew fog or rain are most common at or near that temperature which is probably close to the average ocean temperature on the surface.

Ob1

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Answer 3

Dew point is the answer, but, the ground cools more quickly because of radiation than does the air, so the air temperature does not need to fall exactly to the dew point for dew to form. Notice that on a summer's night condensation may even form on the metal parts of a car, even when the air temperature remains several degrees above the dew point...metal radiates heat even more quickly than most other objects.