By definition, dew is the moisture that gathers on objects when temperature falls to the dew point. We have already talked about the concept of the dew point (Lesson 5) in regard to forming clouds. Cooling air at a specific moisture content (no change in moisture) will at some temperature produce dew as the air will reach the point where it can no longer hold as much water vapor. This is the temperature where relative humidity equals 100% or the air is saturated, holding all the water it can.

Dew Point = Temperature when RH = 100%

Because air holds less water when cooler, something will happen when relative humidity is at 100%. Fog will form, dew will form, or a "super saturated" condition will exist. Sometimes the air temperature does not reach a humidity of 100%, yet dew is observed. Air temperature does not need to drop to the dewpoint because objects may become cooler than the air around them. This is important to the concept of dew formation. It may be assumed that dew is "usually" present at night when the relative humidity exceeds 95% in the atmosphere.

The atmosphere at 95% relative humidity does not produce fog (Fig. 7.1). But dew will form if temperatures of items are enough colder than the air surrounding them that they are at the dew point.

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Fig. 7.1 Wetness (dew) will form on leaves at 95% relative humidity in the air.

On clear, cool, and still nights radiative frost (if temperatures drop below freezing) or dew is possible. Under mid-level to low cloud cover, dew is not likely. When wind blows, dew is not likely. With light wind, however, fog is possible. In addition, some source of moisture is needed. When relative humidity levels are high, on a clear, still, cool night fog is likely. If the relative humidity is not quite as high, but the ground provides moisture from a wet soil, growing plants or moist vegetation for some reason, heavy dew is probable.

The concept of dew formation is rather old. Dew is mentioned as an ecological factor in the Old Testament. It may have been necessary for occurrence of manna (Exodus 13:16). The factors causing dew have been observed well back into history. The logic and conditions for dew formation became apparent to people. Mathematical models followed after a hundred years of observation. There are some rather interesting aspects to relative humidity and soil conditions. Several factors necessary for dew formation are:

Lack of Cloud Cover

The night sky cover (cloud amount) must be less than 50% and preferably 100% clear. If more than half the sky is obscured with clouds, dew will not likely form. The clouds must be mid- or low-level clouds. High clouds do not re-radiate enough energy back toward the surface to keep the surface warmer than dew point (Fig. 7.2).

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Fig. 7.2 Dew formation occurs on clear windless nights, trees and cloud cover emit longwave radiation and keep temperatures above the dew point while cloudless skies will allow sufficient heat loss for the temperature to fall to the dew point.

Air Flow

A weak pressure gradient must exist. The dew forecaster looks at the weather maps to determine pressure differences identifying a front coming in, if the air mass is going to change, or if there will be substantial winds overnight. A weak pressure gradient will likely not have substantial winds. If there is a wind of over 7 mph, the assumption is that dew will not form. Air moving by an object that is warmer (or cooler) than the air gains (or loses) heat from the object while lowering (or raising) the temperature of the object.

Moist Conditions

Enough moisture near the surface must exist to create relatively humid conditions. The relative humidity by midnight needs to be 85% or greater. If the relative humidity is at 85% by midnight, it is likely that dew will develop.