Flight Training - February 2014 - 45

basics of today's instrument flying-for
what was called "blind" flying in the 1920s.
While Doolittle demonstrated that with
the proper instruments and training, pilots
could fly through clouds, he did not "overcome the fog peril." Thick fog produces
zero ceiling and visibility. That is, a pilot trying to land in such conditions can see little
outside the airplane except fog. Ordinary
flight instruments don't make this possible.
Until late in the twentieth century
pilots had no safe way to land with zero
or near-zero ceiling and visibility. Today,
to legally and safely land in thick fog you
have to be qualified to make Category
III C instrument landings, be flying an
airplane that meets stringent equipment
requirements, and land at an airport
equipped and approved for such landings.
THE AMOUNT OF water vapor the air can
hold before the vapor begins condensing
into water depends on the air's temperature. The colder the air, the less water it
can hold. Ordinary reports of weather conditions include the dew point temperature,
which is the air temperature at which the
water vapor in the air at the observation's
time and place will begin condensing.
When the air chills to the dew point
temperature, water begins condensing onto objects near the ground, such
as grass, to create the tiny water drops
called dew-thus the term "dew point."
Meteorologists say that air is "saturated"
when it reaches the dew point temperature. When air becomes saturated above
the ground, water vapor begins condensing on tiny, invisible aerosols in the air
such as dust to form water drops floating

in the air. High above the ground these
drops create clouds. At ground level, tiny
water drops floating in the air are called
fog. In other words, fog is a stratus cloud
that's touching the ground.
The most common kind of fog forms
when humid air cools to the dew point.
This occurs in various ways, but one of
the most common is when heat radiates
away from the ground overnight, which
cools the air right above the ground to
the dew point. This forms radiation fog,
which is also called ground fog.
Radiation fog is most likely to form
when the sky is clear, which allows the
Earth's heat to freely radiate away to
space. For fog to form, winds have to be
relatively calm. Brisk winds stir up the
air, mixing cold air next to the ground
with warmer air higher up and preventing the air from cooling to the dew point.
If the sky stays clear and the winds
light, radiation fog continues forming
until a little after sunrise when sunlight
begins warming the ground. This is why
the fog is often thickest around sunrise,
before sunlight begins burning it off.
Advection fog forms when light wind
pushes humid air across cold or snowcovered ground, which chills the air to
the dew point. Widespread, dense advection fog can form when warm air moves
north following a storm that has covered
the ground with snow. (Meteorologists
use "advection" to describe the horizontal
movement of air or meteorological properties such as temperature or humidity.)
Upslope fog forms as air that's moving uphill cools to the dew point as the
air's pressure decreases to match lower

pressure aloft. You sometimes see streaks
of upslope fog on hills or mountains.
Thicker, heavier upslope fog forms when
humid Gulf of Mexico air gains elevation and cools as it flows across the Great
Plains from close to sea level to roughly
5,000 feet elevation on the eastern side of
the Rocky Mountains.
Water that evaporates from falling rain
or drizzle can increase the air's dew point
to match the temperature of the surrounding air to form precipitation fog.
Forecasting when and where fog will
form and how long it will last requires
detailed information about the atmosphere-especially how much water vapor
is where. You should be ready for fog
when the temperature and dew point are
close together, the air is cooling off-or
having more water vapor added, such as
from evaporating drizzle-and the winds
are calm or nearly calm. If the expected
fog doesn't form, take it as another
example of how tricky forecasting can
be. This is the reason for the general rule
pilots often hear: "If the temperature and
dew point are within 5 degrees Fahrenheit
of each other, and it's getting colder, such
as it usually does overnight into the early
morning, watch out for fog."
You're flying at night to land around
dawn. The temperature at your arrival airport is now 59 degrees F and the dew point
there is 55. You should be ready to find fog,
possibly reducing the visibility to less than
a quarter-mile when you arrive.
Jack Williams is an instrument-rated private pilot.
His latest book is The AMS Weather Book: The Ultimate
Guide to America's Weather.

NATIONAL WEATHER SERVICE: Fog Definitions
Mist is a visible aggregate of
minute water particles suspended in the atmosphere that
reduces visibility to less than
seven statute miles but greater
than or equal to five-eighths of a
statute mile. METAR code: BR

Fog is a visible aggregate of minute water particles that are based
at the Earth's surface, which
reduces horizontal visibility to
less than five-eighths of a statute
mile. METAR code: FG

Freezing fog consists of supercooled water drops that
instantly turn to ice when they
hit something, such as an
airplane. METAR code: FZFG

Dense fog reduces visibility to
one-eighth mile or less over a
widespread area. This term is used
in public forecasts but not in
METARs and TAFs, which characterize all obstructions to visibility
such as fog in terms of how far
you can see-"visibility."

FEBRUARY 2014 FLIGHT TRAINING

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Flight Training - February 2014

Table of Contents for the Digital Edition of Flight Training - February 2014