cal properties related to icing. The length of supercooled
back along its original route to avoid icing. Most icing
liquid-water content patches was measured. A patch was
areas may be scanned completely through with a detec-
defined as having supercooled liquid-water content of
tion range of 80 km or more, as suggested by Curry
at least 0.025 g m3 for at least 0.5 km of flight. Patch-
and Liu (1992).
es terminated when supercooled liquid-water content
Relatively little is known about the horizontal extent
was less than 0.025 g m3 for 0.5 km. Average patch
of icing conditions, though more is known today because
length was 4.3 km, with a mean liquid-water content of
of modern research flights than was known in 1979
0.13 g m3, and the median patch length was 1.7 km.
when Milton Beheim of NASA Lewis Research Center
About 90% of patches were less than 7 km in length,
indicated that the horizontal extent of the icing cloud
with less than 2% longer than 50 km. Flights were made
had not been adequately defined (Beheim 1979). He
in low-level stratus clouds and within low-pressure areas
also indicated that the fine-grain structure of the icing
and through fronts.
cloud had not been well defined.
Politovich (1982) describes flights through super-
FAR 25, Appendix C, tries to address icing spatial
cooled stratiform clouds over the Great Lakes and the
scale by providing tables for continuous conditions
Great Plains in 1981. A cloud extent started when the
within stratiform clouds and intermittent conditions
aircraft was within supercooled liquid cloud for at least
within cumulus clouds (FAA 1991). Jeck (1983) indi-
1 km, and cloud elements "separated by less than the
cates, however, that the horizontal extent of icing speci-
element length were combined unless the gap was
fied in Appendix C has no specification for the discon-
greater than 6 km." The average icing encounter in Great
tinuity in icing and the size and frequency of any cloud
Lakes stratiform clouds was 9 km long, and within Great
gaps. He concludes that "horizontal extent," as indicated
Plains stratiform clouds the average encounter was 24
in Appendix C, does not imply the overall dimensions
km long. Embedded cells of supercooled liquid water
of icing cloud systems.
within bands of frozen clouds averaged 6 km in length.
Jeck (1983) provides two methods of expressing the
The larger extents were a result of large-scale lifting of
horizontal extent of icing. For engineering-design pur-
air masses. Though isolated pockets of higher liquid-
poses, he indicates that the horizontal extent of icing
water content occurred, the clouds were generally fairly
encounters, consistent with Appendix C, is the "dis-
uniform at a given flight level.
tance flown during a given icing encounter until a cloud
Cooper et al. (1982) characterized distances of
gap of some specified duration signals the end" of the
liquid-water content encountered greater than specified
encounter. Of more use in determining the utility of
thresholds, and the frequency and size of gaps between
remote sensing is the horizontal extent of individual
icing encounters (where evaporation or sublimation of
icing events, where an icing event is the actual distance
ice accreted on an aircraft could occur). Data from 1083
of icing, which ceases at a cloud gap of any length.
flight hours in California, Montana, Utah, Florida, Kan-
Jeck reanalyzed NACA data by the horizontal extent
sas, Illinois, Michigan, and the Great Lakes in summer
of the icing event and presented modern data in the same
and winter conditions were used to compile the infor-
way. The analyses indicate, as is consistent with Appen-
mation. In all seasons, flights were in icing conditions,
dix C, that there is an inverse relationship between
but at higher altitudes in summer than in winter. The
liquid-water content and event horizontal extent. Hori-
flights deliberately sought the most severe icing condi-
zontal extent is about 33 km at a liquid-water content
of about 0.01 g m3 and is about 5.5 km for the largest
tions. Cooper and his colleagues present information
observed liquid-water contents, about 1.5 g m3. This
indicating exposure distance in two ways:
should not imply, however, that liquid-water content is
Probability of exceeding a given liquid-water con-
constant for these distances. These are average values,
tent in a given distance
and individual patches of larger or smaller liquid-water
Probability of exceeding a liquid-water content of
contents can occur within these extents. An aircraft with
0.1 g m3 for each region.
ice protection may be able to tolerate liquid-water con-
tent to a given magnitude but may have to avoid larger
As examples, when averaged over 1 km, liquid-water
content exceeding 0.1 g m3 occurred about 5% of the
liquid-water contents. Thus, it may also be helpful to
time and exceeded 0.5 g m3 about 1% of the time.
know the size of icing "patches" with larger than speci-
fied liquid-water contents.
When averaged over a distance of 10 km, liquid-water
content exceeded 0.5 g m3 about 0.5% of the time.
The size of liquid-water content patches may be
ascertained from measurements during research flights.
Viewed regionally, there are large differences in the
extent of liquid-water content greater than 0.1 g m3.
In 1992, Cober et al. (1995) flew 31 missions, as part
of the Canadian Atlantic Storms Project (CASP), into
The Great Lakes area, which has the lowest overall
East Coast winter storms to measure cloud microphysi-
liquid-water content, has the longest continuous icing
22
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