different conditions experienced by light aircraft and
and Walker 1951). Large values were typically sought
helicopters below 3049 m. Supercooled liquid-water con-
during these flights for establishing engineering stan-
tents of up to 1.7 g m3 were found, but 99% of values
dards.
were less than 1.1 g m3, and 95% were less than 0.6 g
Perkins (1952) instrumented with icing rate meters
m3 for all cloud types. Larger liquid-water contents are
four United Airlines DC-4 aircraft that flew from New
theoretically possible below 3049 m, but values greater
York City to San Francisco from January through May
than 2.2 g m3 are not likely. Liquid-water values were
1951. Of a total of 1120 hours of flight time on typical
largest in cumuliform clouds and behind cold fronts in
commercial routes and altitudes, icing was encountered
maritime air.
1.5% of the time. Maximum liquid-water contents did
not exceed 1.0 g m3, and 80% of measurements were
Jeck (1982) also sorted the database described above
less than 0.4 g m3.
(Jeck 1983) according to synoptic situation and air-mass
In March 1979, Jeck (1980) made in-cloud liquid-
category. The resulting liquid-water contents represent
water measurements over Lake Michigan and in the
average values over uniform cloud intervals of more than
vicinity of Lake Erie. He compared instrumentation
1 km, so peak liquid-water values are not represented.
used in research flights from the 1940s and 1950s to
In general, modern data liquid-water contents were 1.1
to 1.2 g m3 in cumulus clouds within lake-effect areas,
measurement error when using multicylinders to meas-
modified continental air masses, high-pressure areas
ure liquid water. The most significant problem was run-
without fronts, and in maritime air masses. Liquid-water
contents were lowest, 0.2 to 0.4 g m3, in warm frontal
off due to incomplete freezing of water impinging upon
stratus clouds, occluded-front stratus clouds, cold-front
the cylinders. This occurred when the proper combina-
cumulus clouds, and upslope stratus clouds.
tions of air temperature and liquid-water content caused
the ice temperature to remain at 0C, the so-called
Jeck later expanded the database for icing conditions
below 3049 m to include all altitudes and presented
"Ludlum" limit. Jeck mounted a JohnsonWilliams hot-
seasonal analyses of liquid-water content for design
wire liquid-water probe that measured liquid water accu-
rately only when drop sizes were smaller than 30 m
purposes (Jeck 1989). Seasonal liquid-water magnitudes
(due to design limitations) on a Lockheed Super Con-
were isolated not by calendar date, but by grouping
stellation aircraft. Measured liquid-water contents were
measurements by the height of the freezing level, with
somewhat smaller than historical measurements, in part
lower freezing levels occurring during winter condi-
because his measurements were in the lower portions
tions. Freezing levels below 1524 m agl were used for
winter conditions and above 3049 m for summer con-
of stratus clouds, whereas earlier measurements sought
ditions. Nearly 85% of stratus cloud occurrences were
the largest values typically encountered, near cloud tops.
below 3049 m, with the largest liquid-water contents,
On the Jeck flights, icing generally did not occur on the
maximizing near 0.9 g m3, occurring near 1524 m for
aircraft when liquid-water contents were less than 0.08
to 0.10 g m3. In stratus clouds less than 1524 m agl,
warm and cold seasons. Liquid-water content was less
than 0.3 g m3 90% of the time in the stratus. More than
95% of all liquid-water measurements were less than
0.6 g m3.
50% of convective clouds occurred above 3049 m, and
In a comprehensive review, Cooper et al. (1982) and
cold-season convective clouds typically had liquid-
water contents of less than 2.0 g m3, with this maxi-
Sand et al. (1984) summarized five years of flights made
mum occurring near 3659 m. Summer convective
ming King Air. Over 98% of summer and winter and
clouds, however, had maximum supercooled liquid-
water contents approaching 5.0 g m3, but only above
continental and coastal cloud liquid-water measure-
6098 m.
ments--423,787 seconds of measurements with a
In a review of the state of knowledge of aircraft icing
JohnsonWilliams hot-wire liquid-water probe and a for-
conditions from around the globe, Hoffman (1984)
ward-scattering spectrometer probe (FSSP)--were less
than 1.0 g m3, and only 0.2% of samples exceeded 2.0 g
stated that icing occurs within liquid-water contents of
m3. Liquid-water contents nearly as high as 3.0 g m3
0.01g m3 to 6.0 g m3, though values larger than 2.5 g
were encountered, but in less than 0.01% of all measure-
ments.
At any given altitude, liquid water within stratus clouds
can vary between 0.01 to 1.0 g m3, and in cumulus
Jeck (1983) and Masters (1983) compiled a new
clouds it can vary between 0.01 and 1.7 g m3.
database of supercooled cloud properties up to 3049 m
from about 12,955 km of icing observations using a
Twenty-five flights measuring liquid water in strato-
mix of old and new measurement technology, from
cumulus clouds in Germany with a JohnsonWilliams
multicylinders to newer optical and hot-wire instrumen-
probe indicated that liquid-water contents varied from
tation. This database was constructed to address the
15
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