Flight Training - December 2013 - 49

encounter and the codes used to designate
them in aviation routine meteorological
reports (METARs).
Frost: Ice crystals that form when water
vapor in the air deposits directly onto
exposed objects. Deposition means the
vapor turns directly into ice without first
condensing into water. No METAR code.
Snow: Precipitation of ice crystals,
which can be branched dendrites such
as the snowflakes you see in photos or
drawings; broken pieces of dendrites;
or simple, six-sided plates or columns.
METAR Code: SN.
Freezing rain: Precipitation of liquid
water drops more than 0.02 inches in
diameter that freeze upon impact with the
ground or other exposed objects. METAR
code: FZRA
Freezing drizzle: Fairly uniform precipitation composed exclusively of fine
drops with diameters less than 0.02 inch
that fall very close together and freeze on
impact. METAR code: FZDZ
Ice pellets: Small, translucent balls of
ice less than 0.2 inch in diameter (about
the size of a pencil point). Ice pellets often
bounce when they hit a hard surface. The
National Weather Service calls ice pellets
"sleet," and this is the common name used
in the United States. METAR code: PL
Freezing fog: A suspension of numerous minute water droplets that freeze
upon contact with the ground or other
objects, such as an aircraft. Reduces visibility. METAR code: FZFG.
Hail: Balls or irregular lumps of ice
(called hailstones) from roughly 0.2 to six
inches in diameter. Hail falls from strong
thunderstorms. Unlike other kinds of icy
precipitation, hail is rare in the winter.
METAR code: GR.
Snow pellets: Precipitation of white,
opaque grains of ice that are spherical or
sometimes cone shaped; also called small
hail. METAR code: GS.
Snow grains: Very small white, opaque
grains of ice. METAR code: SG.
Precipitation and fog with the word
"freezing" in the name are the kind you
need to worry about the most, because
the drops are supercooled and will freeze
when they hit your airplane. You also

need to worry about frost. Several years
ago researchers found that just over a
hundredth of an inch of frost on an airplane's wings can reduce lift by 25 percent
while also increasing drag. Frost on wings,
which can be almost invisible, has been
blamed for a few takeoff crashes.
SLEET AND FREEZING RAIN. Sleet and
freezing rain form when snow falls from
cold, high clouds into a layer of abovefreezing air and then back into frigid air
near the surface. As they fall through
the warmer air, snowflakes melt into
raindrops. When they reach the belowfreezing air close to the ground, these
raindrops become supercooled. If the
layer of cold air is deep enough, the drops
will turn back into ice and become ice
pellets (sleet).
The lesson: If ice pellets are falling on
an airport you plan to take off from, or are
in the METAR for your destination or any
en route airport, you need to worry about
ice forming on your airplane while aloft.
Sleet hitting the ground means supercooled water drops in the air somewhere
above the airport. In other words, there
is freezing rain aloft, which can coat your
airplane with ice.
SNOW: NOT ALWAYS INNOCENT. Light
snow, which the National Weather Service defines as falling snow that doesn't
reduce the visibility below a half-mile,
might seem harmless. Nevertheless, in the
mid-1990s scientists from the National
Center for Atmospheric Research (NCAR)
studied 10 airline takeoff crashes caused
by ice and found light snow was at least
partly responsible.

They discovered that light snow, which
falls when temperatures are between
roughly 25 and 31 degrees F, can have
drops of supercooled liquid water stuck
to the snow crystals. When the snow and
attached supercooled water drops hit the
airplane, the water formed ice-just as
supercooled drops in a cloud would.
Snow crystals in the cases studied
were small, so visibility was better than
it would have been with large crystals.
Before the accidents, both the flight and
ground crews assumed that what looked
like a little snow on the wings would blow
off early in the takeoff run. Instead of just
snow on the wings, however, a thin layer
of ice was attached.
One result of the NCAR study was
that airports and air carriers began using
more sophisticated systems that measure
snow's liquid content, as well as how
much snow falls and the visibility. These
systems enable airlines to make better
decisions about deicing airplanes so that
none take off with ice on them, while
avoiding the expense of unnecessary
deicing.
Since most general aviation pilots don't
have the option of deicing their airplanes
before takeoff, ensuring that no ice is
stuck to the airplane should be a regular
part of all winter preflight inspections.
If temperatures have been below freezing, you should brush off any snow that's
on the airplane and carefully check for
frost, which might be hard to see on white
wings.
Jack Williams is an instrument-rated private pilot.
His latest book is The AMS Weather Book: The Ultimate
Guide to America's Weather.

PRECIPITATION AND FOG WITH THE WORD "FREEZING"
IN THE NAME ARE THE KIND YOU NEED TO WORRY ABOUT
THE MOST.

DECEMBER 2013 FLIGHT TRAINING

/ 49
Flight Training - December 2013

Table of Contents for the Digital Edition of Flight Training - December 2013
