Under what conditions would dew not form?
Posted October 8, 2008
MIKE MOSS SAYS: Joe, Dew will not form if the temperature of the ground, grass, tops of cars, etc does not fall to equal the dew point. This can occur if the air is very dry so that it simply does not have enough water vapor to become saturated, which will occasionally happen in the wake of a cold frontal passage during the cooler parts of the year. In addition to situations where the air is very dry (and thus the dew point is lower than the temperature can become) dew can also fail to form because the temperature of the surfaces I mentioned above remains too warm and does not fall to match the dew point. Common reasons for this include the presence of substantial cloud cover at night, in which case enough downard radiation from the cloud bases is present to offset outgoing radiation from the surface and prevent temperatures from falling dramatically. In addition, a windy night will often end up free of dew, because the resulting mixing of the lower atmosphere prevents the surfaces from cooling sufficiently by constantly transporting a fresh supply of warmer air to the surface from a few hundred feet up.
To summarize, dew may be absent in the presence of very dry air, substantial overnight cloud cover, significant overnight wind, or some combination of those three.