minute water droplets light enough to remain suspended in the air. If the condensed water
particles form in sufficient amounts near the surface, the resulting condition is fog. For fog to
form, three conditions must be satisfied:
Condensation nuclei must be present in the air,
the air must have a high water content (a low dew point spread), and
light surface winds must be present.
Recall from Chapter Two, when the air temperature is equal or nearly equal to the dew point
temperature, there is a low dew point spread, and the air is close to saturation. Once saturation is
achieved, either through the cooling of the air or through the evaporation of water into the
atmosphere, water will condense from the vapor state into water droplets or ice crystal.
Wind velocity is an important consideration in the formation of fog. As will be discussed
shortly, the radiational cooling of the Earth's surface is one of the main causes of fog formation.
When light surface winds are present, on the order of one to ten knots, the speed differential
resulting from friction slowing the air directly next to the surface causes the air to tumble in a
mild eddy current (Figure 5-17). This brings more air in contact with the surface, enabling more
air to be cooled, producing a thicker layer of condensed moisture. If the winds become too fast,
however, this layer lifts away from the ground, lifting the bases higher with increasing speeds.
Figure 5-17 Wind Causing Eddy Currents, Cooling Air to Saturation
Weather Hazards of Turbulence, Icing, Ceilings, Visibility, and Ash Clouds