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Legal aspects may be particularly important when

weather modification activities are performed in the prox-

imity of borders between different countries. However, any

legal system aimed at promoting or regulating weather

modification must recognize that scientific knowledge is

still incomplete.

The implications of any projected long-term weather

modification operation on ecosystems need to be assessed.

Such studies could reveal changes that need to be taken

into account. During the operational period, monitoring of

possible environmental effects should be undertaken as a

check against anticipated impacts.

Summary statement and recommendations

To answer the need for more water and less hail in many

regions of the world, some progress has been made during

the past 10 years in the science and technology of weather

modification. Large numbers of programmes in fog disper-

sion, rain, snow enhancement and hail suppression are in

operation. Several research experimental programmes are

supported in some countries and include randomized

statistical evaluations. Improved observational facilities,

computer capabilities, numerical models and understand-

ing now permit more detailed examination of clouds and

precipitation processes than ever before, and significant

advances are consequently possible. New technologies and

methods are starting to be applied and will help to lead to

further understanding and development in this field.

In the light of this review of the status of weather modi-

fication, the following recommendations are made to

interested Members of WMO:

(a) Cloud, fog and precipitation climatologies should be

established in all countries as vital information for

weather modification and water resource studies and

operations

(b) Operational cloud-seeding projects should be strength-

ened by allowing an independent evaluation of the

results of seeding. This should include measurements

of physical response variables and a randomized statis-

tical component

istry, and other associated sciences should be an

essential component of weather modification projects.

Where the necessary capacity does not exist, advantage

should be taken of facilities of other Members

(d) It is essential that basic measurements to support and

evaluate the seeding material and seeding hypothesis

proposed for any weather modification experiments be

conducted before and during the project

(e) Weather modification programmes are encouraged to

utilize new observational tools and numerical modelling

capabilities in the design, guidance and evaluations of

field projects.

While some Members may not have access or resources to

implement these technologies, collaboration between

Member States (e.g. multinational field programmes, inde-

pendent expert evaluations, education, etc.) are encouraged

that could provide the necessary resources for implement-

ing these technologies.

Guidelines for advice and assistance related to the

planning of weather modification activities

1. These guidelines are addressed to Members requesting

advice or assistance on weather modification activities.

They include recommendations that are based on

present knowledge gained through the results of world-

wide theoretical studies as well as laboratory and field

experiments. A synthesis of the main basic concepts

and main results obtained in the weather modification

programmes is given in the WMO Statement on the

Status of Weather Modification. This Statement was

revised during the twentieth session of the Executive

Council Panel of Experts/CAS Working Group on

Physics and Chemistry of Clouds and Weather

Modification Research and was approved by the fifty-

third session of the Executive Council in June 2001

2. Members wishing to develop activities in the field of

weather modification should be aware that research

and operational applications are still under develop-

ment

It should not be ignored that under certain condi-

tions, seeding may be ineffective or may even enhance

an undesirable effect (increase of hail, reduction in

rain). However, properly designed and conducted

projects seek to detect and minimize such adverse

effects. It is recognized that scientific evaluation may

be a difficult task, but this is the only way presently

known to avoid negative results, quantify positive

economic effects and allow improvements in the under-

standing and methodology that is used. The revised

WMO Statement on the Status of Weather Modification

referred to in paragraph 1 distinguishes the various

types of weather modification and the degree of confi-

dence necessary to obtain the desired effect from cloud

seeding. The confidence level is very high for opera-

tional dissipation of supercooled fog and moderate for

increasing snowfall from orographic clouds. The confi-

dence level is not high for suppressing hail.

3. WMO recommends that operational cloud seeding

projects for precipitation modification be designed to

allow evaluation of the results of seeding through phys-

ical measurements and statistical controls associated

with some randomization of the seeding events. The

physical measurements should include characterization

of the seeding material. Care should be taken to engage

qualified operators. The objective evaluation should

be performed by a group independent of the opera-

tional one. Such programmes should be planned on a

longduration basis because precipitation variability is

generally much greater than the increases or decreases

claimed for artificial weather modification. The use of