Flight Training - December 2014 - 53

The FARs say the times of civil twilight
are found in the Air Almanac, which you
aren't too likely to ﬁnd at most airports.
You can buy the CD version from the U.S.
Naval Observatory.
Unless you are a keen amateur astronomer, or want to use a sextant for celestial
navigation-as pilots who ﬂew overseas
did in the pre-GPS days-it's easier to
look up the times of civil twilight, sunrise,
and sunset on the U.S. Naval Observatory's "Complete Sun and Moon Data for
One Day" on the Internet (http://aa.usno.
navy.mil/data/docs/RS_OneDay.php).
SEASONAL DIFFERENCES. Places along
the 40 degrees north latitude line, which
runs across the United States roughly
from Philadelphia to Indianapolis to
Denver and on to Cape Mendocino on
California's northern coast, offer a good
example of the differences between summer and winter daylight.
On June 21, places along the 40th Parallel have 16 hours, ﬁve minutes of daylight
between the beginning of morning civil
twilight and the end of evening civil twilight. On December 21, these places have
only 10 hours, 22 minutes of daylight.
A student pilot trying to squeeze in
ﬂight instruction after work in a 9-to-5
job who can get in an hour or two of ﬂying after work on weekdays in summer is
limited to ﬂying only on days off during
the winter.
NORTH-SOUTH TEMPERATURE CONTRASTS CREATE STORMS. The big

differences in winter's daylight around
the country means weather patterns vary.
While the northern part of the country
is rapidly cooling off in the fall, the south
cools much more slowly. In Galveston,
Texas, for example, the June high and low
averages are 87 and 78. In January, Galveston's average high and low are 62 and 50.
With the shorter winter days in the
north, temperatures rapidly fall, which
increases north-south temperature
contrast. Between Des Moines and
Galveston, for example, the June difference in average high temperatures is only
5 degrees. By January, the average high in

Des Moines is 33 degrees cooler than in
Galveston.
Such temperature contrasts supply
almost all of the energy for the extratropical cyclones (storms) that follow various
routes across the country from late in the
fall until early spring with snow, sleet,
freezing rain, and ordinary rain.
Winter's clouds and storms that reduce
the time available for visual ﬂight rules
(VFR) are one of the big reasons why military ﬂight training has been concentrated
in the southern United States since before
World War II.
Shorter days offer winter's ﬁrst challenge to student pilots. The darkest days of
winter around December 21 bring roughly
two hours less sunshine on clear days in
the northernmost states compared with
those in the south.
Clear days are harder to come by in the
north, especially east of the Mississippi
River. For example, in Cleveland, Ohio, the
sky is cloudy (nine-tenths or more is covered by clouds) an average of 69 percent of
the time in December but only 28 percent
of the time in July. To the south, DallasFort Worth averages cloudy skies only 38
percent of the time in December and 13
percent of the time in July.
You can escape many cloudy days, but
not long nights, by heading west, farther
away from oceans and large lakes that add
moisture to the sky that makes clouds. In
Denver, the sky is cloudy only 26 percent
of the time in December, compared with
16 percent of the time in July.
If nothing else, learning to ﬂy during
a northern winter gives a student pilot
plenty of practical experience in obtaining weather brieﬁngs during "interesting"
weather. (The last thing a pilot wants to
hear is a meteorologist saying, "We're
going to have some interesting weather.")
Learning about weather begins with
learning the basic science behind it. Learning more about meteorology is a good way
to spend the time when darkness and winter weather ground a planned ﬂight.
Jack Williams is an instrument-rated private pilot.
His latest book is The AMS Weather Book: The
Ultimate Guide to America's Weather.

DECEMBER 2014 FLIGHT TRAINING

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Flight Training - December 2014

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