below cloud base and begin to evaporate, but lag times
level flight, so during approach and departure static
can be on the order of 10 s (Fletcher 1962).
outside air temperature (OAT) at the aircraft will not
Droplets may also be cooler than the surrounding air
be a reliable indicator of air temperature within the flight
within warm tongues of air advected over colder sur-
path ahead of the aircraft. Vertical temperatures can
face air and below colder air aloft. Snow falling into
vary, from nearly isothermal over large vertical dis-
these warm layers from above may partially or com-
tances to changing by tens of degrees over a few hun-
pletely melt. However, until fully melted their tempera-
dreds of meters, especially when transiting inversions.
ture remains at 0C, so they remain colder than the warm
As an example, Schroeder (1990) illustrates a winter
layer until all ice melts and the drops begin to heat
temperature inversion over the Denver area of about
22C within a vertical distance of less than 500 m. Such
through convection and radiation. These areas are often
identified on radar displays as "bright bands"--zones
rapid changes are not unusual during winter.
where falling ice crystals melt and coalesce into rain-
This evidence suggests that OAT measured at the
drops.
aircraft, though a general indicator, is insufficient for
Temperature within clouds may fluctuate several
determining if liquid water in the flight path is super-
degrees over distances of only a few meters. In addi-
cooled. Confidence in the representativeness of OAT
tion, within-cloud temperatures can be considerably
to predict temperature ahead of the aircraft varies with
different from outside-cloud temperatures. Rapid and
the meteorological conditions around the aircraft and
significant temperature fluctuation from cloud to cloud,
with the mode of flight: ascent, level, or descent.
and within clouds, makes determination of supercool-
Nonthermal parameters may be useful surrogates for
ing difficult. For example, data from NCAR Winter
indicating temperature. Detection of glaciation within
Icing Storms Project (WISP) flights indicate that tem-
a cloud suggests that any liquid water within the cloud
perature fluctuations from clear air to cloud can be as
is supercooled. However, if ice crystals are not present,
much as 6C (NCAR 1990). Flights in Poland with a
the method is not effective because there is no physical
rapid-response airborne thermometer show temperature
indication of supercooling.
fluctuations within clouds of 2C in distances of less
In addition to detecting temperature within the flight
than 150 m (Haman and Malinowski 1996). Time series
path, range-resolved temperature must also be sensed
of temperature through the core of a warm cumulus
above and below the aircraft to provide a potential route
cloud (Lawson and Cooper 1990) showed a 3 to 4C
of escape from icing into warm air. Since air tempera-
increase of temperature upon entering the cloud, with
ture varies more rapidly vertically than horizontally,
similar subsequent cooling upon exit. Temperature
especially within inversions, sensing temperature above
within the cloud was nearly constant. Penetrations of
and below aircraft may be useful.
supercooled stratiform clouds showed, depending upon
The accuracy of temperature measurement may also
the thermometer observed since several were being
be critical because of its effect, with liquid-water con-
tested, a 0 to 1C decrease in temperature when inside
tent and drop size, on ice type, density, and shape on
the cloud as compared with dry air around the cloud. In
leading edges (Wright 1995). Since very small changes
another case, but without identification of cloud genera,
in temperature may create large changes in ice accretion
cooling of 3C was observed within the cloud when
amount and shape, it may be useful to measure temper-
compared with surrounding dry air. Lawson and Rodi
ature ahead, above, and below the aircraft with high
(1992) penetrated warm cumulus humilis clouds with
accuracy.
fast-response thermometers and showed immediate 6C
cooling when entering the clouds and immediate 6C
4.3.5 Spatial structure
Spatial scales of icing conditions affect the utility of
warming when exiting.
remote-sensing systems. Icing conditions that are spa-
Temperature changes can also be large and rapid in
tially homogeneous over thousands of square kilometers
the horizontal when an aircraft transits fronts, though
offer less potential for avoidance without climbing or
not as rapid as upon entering or exiting clouds. For
descending. The size of icing cells and storm areas also
example, transiting a cold front in horizontal flight can
produce temperature changes of 0.2C per kilometer or
affects the needed sensing range of remote-sensing sys-
tems. Storms with small icing cells may be sensed by a
more (Berry et al. 1945). Smaller changes are observed
remote-sensing system sufficiently to allow an aircraft
when transiting warm fronts in level flight. This is ignor-
to progress iteratively through the system. Storms with
ing turbulent mixing in the shear zone along frontal
large icing cells may be too large to be sensed through,
surfaces, which can cause more rapid localized tem-
potentially trapping aviators (Fig. 2) (Kirkpatrick 1970).
perature changes.
The inability to sense completely through icing reduces
The most rapid temperature changes, however, are
avoidance options and may limit an aircraft to turning
experienced during ascent or descent rather than within
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