METEOROLOGIST JEFF HABY
From experience, there is one important factor that will often distinguish between radiation fog and dew development and that factor is the
wind speed. First, a definition of each term will be given. Radiation fog is a cloud on the ground produced when the air is saturated and
the cooling that produces the fog occurs due to clear skies that have contributed to the temperature cooling off at night. Clear skies will
produce much more cooling than cloudy skies, all else being equal, due to longwave energy escaping to space. Dew is condensation that forms
on the ground when the temperature falls to the dewpoint. As the air tries to continue to cool, condensation occurs since the relative
humidity will not increase above 100%. Cooler air has less capacity to contain moisture thus cooling air will condense out some of the
moisture once the air tries to cool when the relative humidity is 100%.
Radiation fog and dew occur primarily from the same process: clear skies at night that allows enough cooling for the temperature to drop to
the dewpoint. What determines which one will dominate depends primarily on the wind speed. A calm wind will promote dew since the layer
of cool saturated air will stay very close to the ground surface. A gentle breeze will help mix the cool saturated air with air above
the ground surface. This will help promote a layer of fog at and near the surface. If the wind is too strong, then fog and dew formation
will tend to not be as significant. This is because the strong wind allows for a significant mixing with air even higher aloft that tends
to be much drier. This makes it more difficult for the temperature to drop to the dewpoint at the surface.
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