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Hail suppression
Hail causes substantial economic loss to crops and property.
Many hypotheses have been proposed to suppress hail and
operational seeding activities have been undertaken in
many countries. Physical hypotheses include the concepts
of beneficial competition (creating many additional hail
embryos that effectively compete for the supercooled
water), trajectory lowering (intended to reduce the size of
hailstones) and premature rainout. Following these
concepts, seeding methods concentrate on the peripheral
regions of large storm systems, rather than on the main
updraught.
Our understanding of storms is not yet sufficient to allow
confident prediction of the effects of seeding on hail. The
possibilities of increasing or decreasing hail and rain in
some circumstances have been discussed in the scientific
literature. Supercell storms have been recognized as a
particular problem. Numerical cloud model simulations
have provided insights into the complexity of the hail
process, but the simulations are not yet accurate enough
to provide final answers. Scientists in operational and
research programmes are working to delineate favourable
times, locations and seeding amounts for effective modifi-
cation treatments.
A few randomized trials have been conducted for hail
suppression using such measures as hail mass, kinetic
energy, hailstone number and area of hailfall. However,
most attempts at evaluation have involved non-randomized
operational programmes. In the latter, historical trends in
crop hail damage have often been used, sometimes with
target and upwind control areas, but such methods can be
unreliable. Large reductions have been claimed by many
groups. The weight of scientific evidence to date is incon-
clusive, neither affirming nor denying the efficacy of hail
suppression activities. This situation is motivation for oper-
ational programmes to strengthen the physical and
evaluation components of their efforts.
In recent years, anti-hail activities using cannons to
produce loud noises have re-emerged. There is neither a
scientific basis nor a credible hypothesis to support such
activities. Significant advances in technology during the last
decade have opened new avenues to document and better
understand the evolution of severe thunderstorms and hail.
New experiments on storm organization and the evolution
of precipitation including hail are needed.
Other severe weather moderation
Tropical cyclones contribute significantly to the annual rain-
fall of many areas, but they are also responsible for
considerable damage to property and for a large loss of life.
Therefore, the aims of any modification procedure should
be to reduce the wind, storm surge and rain damage, but
not necessarily the total rainfall. Hurricane modification
experiments were conducted in the 1960s and early 1970s.
However, there is no generally accepted conceptual model
suggesting that hurricanes can be modified.
While modification of tornadoes or of damaging winds is
desirable for safety and economical reasons, there is
presently no accepted physical hypothesis to accomplish
such a goal.
There has been some interest in the suppression of light-
ning. Motivation includes reducing occurrences of forest
fires ignited by lightning and diminishing this hazard
during the launching of space vehicles. The concept
usually proposed involves reducing the electric fields
within thunderstorms so that they do not become strong
enough for lightning discharges to occur. To do this, chaff
(metallized plastic fibres) or silver iodide have been intro-
duced into thunderstorms. The chaff is postulated to
provide points for corona discharge which reduces the elec-
tric field to values below those required for lightning,
whereas augmenting the ice-crystal concentration is postu-
lated to change the rate of charge build up and the charge
distribution within the clouds. Field experiments have
used these concepts and limited numerical modelling
results have supported them. The results have no statisti-
cal significance.
Inadvertent weather modification
There is ample evidence that biomass burning, and agri-
cultural and industrial activities modify local and
sometimes regional weather conditions. Land-use changes
(e.g. urbanization and deforestation) also modify local and
regional weather. Air quality, visibility, surface and low-level
wind, humidity and temperature, and cloud and precipi-
tation processes are all affected by large urban areas. As
environmental monitoring and atmospheric modelling
capabilities are improved, it is increasingly evident that
human activities have significant impacts on meteorologi-
cal parameters and climatological mechanisms that
influence our health, productivity and societal infrastruc-
ture. Inadvertent effects need to be considered in the design
and analyses of weather modification experiments and oper-
ations (e.g. changes in background aerosol distributions
affect the cloud structure and may affect precipitation
processes).
Economic, social and environmental aspects of
weather modification
Weather modification is sometimes considered by coun-
tries when there is a need to improve the economy in a
particular branch of activity (for example, increase in water
supply for agriculture or power generation) or to reduce
the risks that may be associated with dangerous events
(frosts, fogs, hail, lightning, thunderstorms, etc.). Besides
the present uncertainties associated with the capability to
reach such goals, it is necessary to consider the impacts on
other activities or population groups. Economic, social,
ecological and legal aspects should be taken into account.
Thus, it is important to consider all the important complex-
ity and recognize the variety of possible impacts, during
the design stage of an operation.