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prolonged period of time, as well as to search for the exis-
tence of any extra-area effects. Existing methods should
be improved in the identification of seeding opportuni-
ties and the times and situations in which it is not
advisable to seed, thus optimizing the technique and
quantifying the result.
Also, it should be recognized that the successful conduct
of an experiment or operation is a difficult task that requires
qualified scientists and operational personnel. It is difficult
and expensive to fly aircraft safely in supercooled regions
of clouds. It is also difficult to target the seeding agent from
ground generators or from broad-scale seeding by aircraft
upwind of an orographic cloud system.
Stratiform clouds
The seeding of cold stratiform clouds began the modern
era of weather modification. Shallow stratiform clouds can
be under certain conditions made to precipitate, often
resulting in clearing skies in the region of seeding. Deep
stratiform cloud systems (but still with cloud tops warmer
than –20°C) associated with cyclones and fronts produce
significant amounts of precipitation. A number of field
experiments and numerical simulations have shown the
presence of supercooled water in some regions of these
clouds and there is some evidence that precipitation can
be increased.
Cumuliform clouds
In many regions of the world, cumuliform clouds are the
main precipitation producers. These clouds (from small fair
weather cumulus to giant thunderclouds) are characterized
by strong vertical velocities with high condensation rates.
They can hold the largest condensed water contents of all
cloud types and can yield the highest precipitation rates.
Seeding experiments continue to suggest that precipitation
from single cell and multicell convective clouds have
produced variable results. The response variability is not
fully understood.
Precipitation enhancement techniques by glaciogenic
seeding are utilized to affect ice phase processes while
hygroscopic seeding techniques are used to affect warm
rain processes. Methods to assess these techniques vary
from direct measurements with surface precipitation
gauges to indirect radar-derived precipitation estimates.
Both methods have inherent advantages and disadvan-
tages.
During the last 10 years there has been a thorough
scrutiny of past experiments using glaciogenic seeding. The
responses to seeding seem to vary depending on changes
in natural cloud characteristics and in some experiments
they appear to be inconsistent with the original seeding
hypothesis.
Experiments involving heavy glaciogenic seeding of
warm-based convective clouds (bases about +10°C or
warmer) have produced mixed results. They were intended
to stimulate updraughts through added latent heat release
which, in turn, was postulated to lead to an increase in
precipitation. Some experiments have suggested a positive
effect on individual convective cells but conclusive evidence
that such seeding can increase rainfall from multicell
convective storms has yet to be established. Many steps in
the postulated physical chain of events have not been suffi-
ciently documented with observations or simulated in
numerical modelling experiments.
In recent years, the seeding of warm and cold convective
clouds with hygroscopic chemicals to augment rainfall by
enhancing warm rain processes (condensation/collision-
coalescence/break-up mechanisms) has received renewed
attention through model simulations and field experiments.
Two methods of enhancing the warm rain process have been
investigated: first, seeding with small particles (artificial CCN
with mean sizes about 0.5 to 1.0 micrometres in diameter)
is used to accelerate precipitation initiation by stimulating
the condensation-coalescence process by favourably modi-
fying the initial droplet spectrum at cloud base; and second,
seeding with larger hygroscopic particles (artificial precipi-
tation embryos about 30 micrometres in diameter) to
accelerate precipitation development by stimulating the
collision-coalescence processes. A recent experiment utiliz-
ing the latter technique indicated statistical evidence of radar
estimated precipitation increases. However, the increases
were not as contemplated in the conceptual model but seem
to occur at later times (one to four hours after seeding), the
cause of this effect is not known.
Recent randomized seeding experiments with flares that
produce small hygroscopic particles in the updraught
regions of continental, mixed-phase convective clouds have
provided statistical evidence of increases in radar-estimated
rainfall. The experiments were conducted in different parts
of the world and the important aspect of the results was
the replication of the statistical results in a different
geographical region. In addition, physical measurements
were obtained suggesting that the seeding produced a
broader droplet spectrum near cloud base that enhances
the formation of large drops early in the lifetime of the
cloud. These measurements were supported by numerical
modelling studies.
Although the results are encouraging and intriguing, the
reasons for the duration of the observed effects obtained
with the hygroscopic particle seeding are not understood
and some fundamental questions remain. Measurements
of the key steps in the chain of physical events associated
with hygroscopic particle seeding are needed to confirm
the seeding conceptual models and the range of effective-
ness of these techniques in increasing precipitation from
warm and mixed-phase convective clouds.
Despite the statistical evidence of radar estimated precip-
itation changes in individual cloud systems in both
glaciogenic and hygroscopic techniques, there is no
evidence that such seeding can increase rainfall over signif-
icant areas economically. There is no evidence of any
extra-area effects.