METEOROLOGIST JEFF HABY
Two ingredients are critical for fog which are saturated air and cooling. It is the same
ingredients that develop a cloud except the fog is a cloud on the ground. The cooling of
saturated air causes the moisture to condense out into liquid cloud droplets. Once fog
actually forms, how does it dissipate? The term "mix out" means that the fog dissipates.
1. When fog forms it tends to be thicker in some areas than areas. It is where the fog is
thinner that is dissipates quicker. The edges are often areas in which the form is thinner. On
satellite images when watching fog dissipate it will often dissipate at the edges first. Fog in the
valleys can take longer to dissipate since it is thick.
2. Warming the air can dissipate fog. It is no surprise that early morning fog will often
dissipate as the sun climbs higher in the sky. Some of the solar radiation is able to
penetrate the cloud and warm the surface. As the surface warms, the relative humidity
will decrease and cloud droplet moisture will evaporate. When fog dissipates it will often
dissipate at the surface first leaving a layer of stratus just above the surface. As daytime
heating progresses even the stratus aloft will mix out to a partly cloudy or clear day.
3. When fog "mixes out" the term mixing comes from drier air entraining into the fog to
evaporate the moisture away. As the surface warms, rising thermals will begin to mix
the air since warmer air under cooler air is an unstable situation. The fog layer at the surface
can mix with drier air aloft as the convective process begins with daytime heating. Since air
higher aloft is further from the cooling and moisture at the surface it will have a
relative humidity less than 100%. When this drier air mixes with saturated air it will
evaporate the fog moisture away (mixing out).
4. Fog generally does not like moderate to strong wind. It is more likely fog will mix out
when winds are stronger since there will be more mixing with air higher aloft. After sunrise
the winds will tend to increase in the morning since the rising thermal are set in motion.
Next time you see fog, think about the processes that can mix out the fog and watch it