Flight Training - March/April 2021 - 22

PREFLIGHT WEATHER

BY THOMAS A. HORNE

FOG TALK
WHY COOLING IS BAD

what happens to water vapor when the temperature-dew point
ever heard the term " dew point temperature, " or " dew point " for spread closes. And if water vapor condenses into tiny, suspended
short. But the more you fly, the more you'll realize the imporfog droplets when temperatures cool, they evaporate as temtance of this term. Think of the dew point as a kind of threshold
peratures climb and the temperature-dew point spread widens.
for fog formation. When the air temperature drops to within 5
Nightfall is one way of cooling surface temperatures to the
or so degrees Fahrenheit (or about 2 degrees Celsius) of the dew
dew point. After a night's worth of cooling, temperatures at
point, fog is likely. So, when you hear someone say that there's
dawn can be low enough for a layer of fog to form. Clear nights
a close temperature-dew point " spread, " you'll know what they
further aid cooling by allowing plenty of surface heat to dissimean. And when you hear that the temperature and dew point
pate into the atmosphere.
are " on top of each other, " you'll know that the temperature and
Cold air is denser than warm, so it can easily sink into valleys
dew point are the same. At times like that, it's almost guaranteed and riverbeds, other great spots where fog is prevalent. You'll
that fog-or some form of precipitation-is present.
Fog is basically a cloud on the ground, and it's
one of the biggest adverse weather events you'll face
as your flying career evolves. It can make surface
Here's a METAR for the Frederick, Maryland, Municipal Airport (FDK)-AOPA's
visibilities low enough to prevent safe takeoffs and
home base-taken 15 minutes before I wrote this:
landings, and it can close even the largest, most
KFDK 161847Z 02010KT 1/2SM SN OVC006 M02/M03 A3009
well-equipped airports if visibilities go to zero. The
National Weather Service defines fog in terms of
And here's the translation: On December 16 at 1847 Zulu time (161847Z, or
yielding surface visibilities at or below 5/8 statute
1:47 p.m.), wind was 020 at 10 knots (02010KT), visibility one-half mile in snow
mile. Under visual flight rules (VFR)-the rules
(SN), sky overcast at 600 feet (OVC006), temperature minus 2 degrees Celsius/
you'll fly under as a student pilot-you can't legally
dew point minus 3 Celsius (M02/M03), altimeter setting 30.09 inches of mercury
take off if the visibility drops to three statute miles
(A3009)
or less.
There you have it-a one-degree temperature-dew point spread, with both
Cooling is the main driver in fog creation. That's
below freezing. Visibility was greatly diminished by the snowfall, but no fog. Yet.
because cooling-whether caused by a frontal pasFog and mist are forecast in two hours, lasting until 1 a.m. By the next day at 7 a.m.
sage, rain, snow, or simply the presence of a cooler
the forecast calls for winds to come from the northwest, with seven-mile visibilities
air mass-promotes condensation. Which is exactly
in what could be the last phase of the snowstorm. -TAH
UNTIL YOU BECAME INVOLVED IN FLYING, YOU MAY NOT HAVE

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22 FLIGHT TRAINING MARCH/APRIL 2021

Flight Training - March/April 2021

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