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direct proportion to our understanding of the phenome-

non. The search for physical predictors, therefore, holds a

high priority in weather modification research. Physical

predictors may consist of meteorological parameters (such

as stability, wind directions, pressure gradients) or cloud

quantities (such as liquid water content, updraught speeds,

concentrations of large drops, ice-crystal concentration or

radar reflectivity).

Objective measurement techniques of precipitation quan-

tities are to be preferred for testing weather modification

methods. These include both direct ground measurements

(e.g. raingauges and hail pads) and remote sensing tech-

niques (e.g. radar, satellite). Secondary sources, such as

insurance data (as have in the past been employed to show

changes in hail intensity) are, at least by themselves, not

held to be satisfactory in most situations.

Operational programmes should be conducted with

recognition of the risks inherent in a technology which is

not totally developed. For example, it should not be ignored

that, under certain conditions, seeding may cause more

hail or reduce precipitation. However, properly designed

and conducted operational projects seek to detect and mini-

mize such adverse effects. Therefore, weather modification

managers are encouraged to add scientifically-accepted eval-

uation methodologies to be undertaken by experts

independent of the operators.

Brief summaries of the current status of weather modifi-

cation are given in the following sections. These summaries

were restricted to weather modification activities that

appear to be based on acceptable physical principles and

which have been tested in the field.

Fog dispersal

Different techniques are being used to disperse warm (i.e.

at temperatures greater than 0°C) and cold fogs. The rela-

tive occurrence of warm and cold fogs is geographically and

seasonally dependent.

The thermal technique, which employs intense heat

sources (such as jet engines) to warm the air directly and

evaporate the fog, has been shown to be effective for short

periods for dispersal of some types of warm fogs. These

systems are expensive to install and to use.

Another technique that has been used is to promote

entrainment of dry air into the fog by the use of hovering

helicopters or ground-based engines. These techniques are

also expensive for routine use.

To clear warm fogs, seeding with hygroscopic materials

has also been attempted. An increase in visibility is some-

times observed in such experiments, but the manner and

location of the seeding and the size distribution of seeding

material are critical and difficult to specify. In practice, the

technique is seldom as effective as models suggest. Only

hygroscopic agents should be used that pose no environ-

mental and health problems.

Cold (supercooled) fog can be dissipated by growth and

sedimentation of ice crystals. This may be induced with high

reliability by seeding the fog with artificial ice nuclei from

ground-based or airborne systems. This technique is in oper-

ational use at several airports and highways where there is

a relatively high incidence of supercooled fog. Suitable tech-

niques are dependent upon wind, temperature and other

factors. Dry ice has commonly been used in airborne

systems. Other systems employ rapid expansion of

compressed gas to cool the air enough to form ice crystals.

For example, at a few airports and highway locations, liquid

nitrogen or carbon dioxide is being used in ground-based

systems. A new technique, which has been demonstrated

in limited trials, makes use of dry ice blasting to create ice

crystals and promote rapid mixing within the fog. Because

the effects of this type of seeding are easily measured and the

results are highly predictable, randomized statistical verifi-

cation generally has been considered unnecessary.

Precipitation (rain and snow) enhancement

This section deals with those precipitation enhancement

techniques that have a scientific basis and that have been

the subject of research. Other non-scientific and unproven

techniques that are presented from time to time should be

treated with the required suspicion and caution.

Orographic mixed-phase cloud systems

In our present state of knowledge, it is considered that the

glaciogenic seeding of clouds formed by air flowing over

mountains offers the best prospects for increasing precip-

itation in an economically-viable manner. These types of

clouds attracted great interest in their modification because

of their potential in terms of water management, i.e. the

possibility of storing water in reservoirs or in the snowpack

at higher elevations. There is statistical evidence that, under

certain conditions, precipitation from supercooled

orographic clouds can be increased with existing tech-

niques. Statistical analyses of surface precipitation records

from some long-term projects indicate that seasonal

increases have been realized.

Physical studies using new observational tools and

supported by numerical modelling indicate that super-

cooled liquid water exists in amounts sufficient to produce

the observed precipitation increases and could be tapped

if proper seeding technologies were applied. The processes

culminating in increased precipitation have also been

directly observed during seeding experiments conducted

over limited spatial and temporal domains. While such

observations further support the results of statistical analy-

ses, they have, to date, been of limited scope. The cause

and effect relationships have not been fully documented,

and thus the economic impact of the increases cannot be

assessed.

This does not imply that the problem of precipitation

enhancement in such situations is solved. Much work

remains to be done to strengthen the results and produce

stronger statistical and physical evidence that the

increases occurred over the target area and over a