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appropriate numerical models may help in reducing

the time required to evaluate the project

4. WMO recommends that a detailed examination of the

suitability of the site for cloud seeding should be

conducted similar to that done in the Precipitation

Enhancement Project, for which WMO reports are

available. To increase the chances of success in a

specific situation, it should be verified through prelim-

inary studies that:

(a) The climatology of clouds and precipitation at the site

indicates the possibility of favourable conditions for

weather modification

(b) Conditions are suitable for the available modification

techniques

fication hypothesis

(d) For the frequency with which suitable conditions occur,

the changes resulting from the modification technique

can be detected at an acceptable level of statistical

significance

(e) An operational activity can be carried out at a cost

acceptably lower than the socio-economic benefit that

is likely to result.

All prospective studies require expert judgement and the

results are expected to depend on the site chosen and on

the season.

5. There are no quantitative criteria for the acceptance of

the results of a weather modification experiment.

Acceptance will depend on the degree of the scien-

tific objectivity and the consistency with which the

experiment was carried out and the degree to which

this is demonstrated. Also important are the physical

plausibility of the experiment, the degree to which bias

is excluded from the conduct and analysis of the exper-

iment, and the degree of statistical significance

achieved. There have been few weather modification

experiments that have met the requirements of the

scientific community with respect to these general crite-

ria. However, there are exciting possibilities now for

making progress in our understanding of weather modi-

fication issues using modern research tools, including

advanced radar, new aircraft instruments and powerful

numerical models

6. Weather modification should be viewed as a part of an

integrated water resources management strategy.

Instant drought relief is difficult to achieve. In partic-

ular, if there are no clouds, precipitation cannot be

artificially stimulated. It is likely that the opportuni-

ties for precipitation enhancement will be greater

during periods of normal or above normal rainfall than

during dry periods

7. The Members should be aware that the scope of efforts

involved in the design, conduct or evaluation of a

weather modification programme precludes the WMO

Secretariat from giving detailed advice. However, if

requested, the Secretary-General may assist (by obtaining

advice from scientists on other weather modification

projects or with special expertise) on the understanding

that:

(a) Costs will be met by the requesting country

(b) The Organization can take no responsibility for the

consequences of the advice given by any invited scien-

tist or expert

dispute that may arise.