Flight Training - November/December 2022 - 24

PREFLIGHT WEATHER
BY THOMAS A. HORNE
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FOG FACTS
THE WHYS AND WHERES OF FOG
FOG, OFTEN DESCRIBED AS A CLOUD ON
the ground, is a low-lying air mass so
saturated with water vapor that surface
visibility is reduced to less than 5/8 statute
miles. That's the official definition; if
the visibility is 5/8 sm to 6 sm, then the
conditions are described as mist. In meteorological
terminal air reports (METARs)
and other weather reports and forecasts,
fog is abbreviated as FG, and mist as BR.
With visibilities that low, pilots
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restricted to flight in visual meteorological
conditions can't safely-or
legally-take off or land. Pilots who are
instrument-rated and current can legally
fly in these conditions, but they will have
to follow strictly defined procedures to
take off and land. Even then, if visibilities
are lower than the limits published on
instrument approach charts, you may not
see the runway at all-and have to execute
a missed approach, climb away, and seek
out a destination with better weather. For
these reasons and more it's important to
know some fog basics.
One main point to remember is that
fog is associated with cooling temperatures,
but cooling alone won't do. The
moisture content of the air mass is also
important. Here we get into the relationship
between an air mass's temperature
and its dew point-with dew point
being the temperature to which an air
mass must be cooled in order to create
coalescence of its water vapor into water
24 FLIGHT TRAINING NOVEMBER/DECEMBER 2022
droplets. As a rule of thumb, fog forms
when temperatures drop to within 5
degrees Fahrenheit (2 degrees Celsius)
of the air mass's dew point. By the way,
dew points correspond to humidity. The
higher the dew point, the more water
vapor it can hold.
The more the temperature-dew point
" spread " narrows, and especially when
the two values are the same or " on top
of each other " as the saying goes, fog is
virtually guaranteed. Another means
conducive to fog formation is the addition
of water vapor to an air mass.
So how and where is fog most likely to
form? Nighttime cooling is one common
way. As the sun goes down, any daytime
heat radiates away from the surface,
which is why this type of fog is called
radiation fog. Temperatures drop to the
dew point, and fog forms at the surface
during the coldest hours of the night.
Clear nights and calm winds help assure
its formation. For this reason, this type of
fog is called radiation fog. With sunrise
and the heat of the day, the temperature-dew
point spread widens, and the
fog evaporates.
Advection fog happens when moist
air moves over a colder surface. (Advection
means horizontal movement.) Here,
the cold surface lowers the moist air to
its dew point. This is common along the
West Coast with its adjoining cold waters,
and the southeastern and central states,
MIKE FIZER
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Flight Training - November/December 2022

Table of Contents for the Digital Edition of Flight Training - November/December 2022
