WMOD00393

• Title: Weather Modification Glossary
• Prepared By: A. S. Dennis
• Date: 1/1/1987
• Weather Modification - Doc Type: Report

IV. SUPPRESSION OF WEATHER HAZARDS 613

dropping of dry ice pellets from airplanes. A typ-
ical seeding rate is I kg/km of flight path. The
dry ice pellets usually are dropped about 15 to 30
min of wind motion upwind of the runway to be
affected. The curtain of ice crystals produced by
the dry ice pellets advects to the target zone
while broadening at perhaps 1 to 2 m/sec by tur-
bulent diffusion. Typically, a very light shower
of snowflakes falls near the target zone and
enough water is extracted from the fog to im-
prove the visibility to above Instrument Flight
Rules (lFR) minimums. Clearing of supercooled
fog is most rapid at temperatures of -10 to
-ISOC, but success has been reported in cases
where the temperature at the fog top was as high
as -1 to -20C.
In cases where an ice fog forms from local
moisture sources, seeding cannot be effective.
Such situations can only be improved by re-
stricting the amount of moisture released into
the lower atmosphere.

B. MODIFICATION OF WARM FOG

The modification of fog at temperatures above
DoC has proven much more difficult than the
modification of supercooled fogs. The only in-
stability that can be exploited is the colloidal
instability of the small water droplets. The most
direct way to modify warm fog is to evaporate it
by heating, as in the FIDO system used in En-
gland during World War II. The Turboclair sys-
tems at Orly and Charles de Gaulle airports in
Paris, France, work on this principle. They con-
sist of jet engines located underground, which
blow their exhaust through gratings into the air
alongside the runways to be affected. The mix-
ing of the hot exhaust with the fog-filled ambient
air produces a partial evaporation of the fog
droplets and some improvement in visibility.
These systems have been in use for several
years, but the method can only be economically
effective at major airports.
Many other methods have been used to mod-
ify fog, including the initiation of downward mo-
tions in the air by hovering helicopters and liter-
ally sweeping fog droplets from the air through
massive suction devices. None of these appears
to have reached the point of being economically
feasible.
The second major approach to modification of
warm fog is the release of hygroscopic solutions
or powders. As each artificial giant CCN grows,
for example, to 100 /-tm in diameter, neighboring
fog droplets evaporate to maintain water satura-

tion and the visibility improves. Because of the
shallow structure of the fog and the absence of
updrafts, the artificial droplets reach the ground
as fine drizzle without capturing significant
quantities of water by coalescence. Turbulence
affects the operation in two ways. If turbulence
is too light, the seeding materials are not distrib-
uted widely throughout the mass of fog, requir-
ing seeding lines separated by only 200 to 300 m.
In contrast, strong turbulence, possibly supple-
mented by wind shear, will distort the seeded
volume and possibly destroy it by mixing with
ambient fog-filled air as it advects toward the
area of interest.
Numerical simulations show that seeding par-
ticles about 30 /-tm in diameter are most efficient
in a typical warm fog. As much as 15 g/m3 of
commercially available (nonuniform size) urea
may be required to obtain the optimum result.
The stronger the wind shear and turbulence, the
larger the particles that must be released to ex-
tract a significant fraction of the fog in the time
available before the seeded volume is destroyed.
Use of the larger particles, of course, greatly
increases the amounts of material to be distrib-
uted. The area that must be treated to ensure
clearing a landing area increases with the wind
speed. Advection fogs with a significant wind
(e.g., 5 m/sec) and the associated wind shear and
turbulence cannot be handled satisfactorily.
Despite extensive work in many countries, no
satisfactory method of suppressing warm fog
has yet appeared. Instead, aviation authorities
have found it more cost effective to provide in-
strument landing systems and ground-controlled
approach systems based on radar capable of
supporting aircraft operations under all except
the most severe fog conditions.

C. HAIL SUPPRESSION

Hail suppression seeding usually has involved
the introduction of ice nuclei in an attempt to
increase the number of freezing centers, which
theoretically should result in smaller hailstones
at the ground. There are a number of conceptual
models or physical hypotheses that indicate that
glaciogenic seeding might slow hailstone growth
and reduce damaging hail. The simplest is that
seeding reduces supercooled cloud water con-
centrations and thereby slows the rate of hail-
stone growth. Another is that glaciogenic seed-
ing provides additional hail embryos, which
compete with one another and the natural em-
bryos for the available supercooled water, so