WMOD00399

• Title: WMO Training Workshop on Weather Modification for Meteorologists - Lecture Notes
• Date: 12/1/1979
• Weather Modification - Doc Type: Report

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All these WMO activities listed above essentially serve to inform and advise its
Members. They are somewhat passive in nature since no new scientific knowledge is created,
but Members are enabled to exchange information and help themselves - at lea~t in principle.
The occurrence of the SAHEL droughL and the growing pressure to optimize the ~orld food
production substantially changed the general attitude of WMO and led to the preparation of
plans of a WMO controlled Precipitation Enhancement Project (PEP). Two othe~ reasons seem
to have affected this decision: (a) a more active role was indicated in recognition that
WMO is the only agency within the UN with qualifications in the basic aspects of weather
modification, and (b) the successful completion of the data-gathering phase of GATE - which
gave WMO the confidence in its ability to successfully carry out a substantial experiment.

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The first PEP proposal was formulated by the WMO Executive Committee Panel on
Weather Modification (identical with the CAS Working Group on Cloud Physics and Weather
Modification) at its meeting in Toronto, 28 October-2 November 1974, and accepted in a more
final form by the 7th WMO Congress in Geneva in 1975. The aims, achievements and the
present state of the developments are outlined in the following sections.

2. OBJECTIVES OF PEP

It is obvious that even an organization like WMO does not have the capability to
tackle all types of weather modification at the same time; hence, the need t~ set prcio~eJ.,.
Weighing importance from a world-wide point of view and feasibility at the same time, it has
been suggested that the three most important problems are:

(1) pftec.ipitation enhanc.ement,
(2) modetr.a:tion 06 :tftopic.al c.yc.lon~, and
(3) hail J.,uppft~J.,ion.

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Hail is a type of precipitation and its formation is not principally different
from the formation of rain through the Wegener-Bergeron-Findeisen process. Trying to moder-
ate typhoons or hurricanes does also involve the precipitation mechanism as such (except for
direct measures concerning storm surges). Thus, the decision to concentrate on precipitation
enhancement is not surprising. This is also the area in which competence and experience can
most easily be found.

Weather modification has evolved considerably in the last three deyades, but so
have the evaluation criteria (see Warner, 1974). At present, it is accepted that single
clouds can be artificially altered. There are, however, only very few experiments which
demonstrate to the satisfaction of the majority of specialists that precipit~tion of cloud
systems can be enhanced. This sample of conclusive and positive modification experiments
is much too small for a more general application. Considering the still-rudimentary state
of cloud physics, it was considered worthwhile to completely re-think a new experiment in
precipitation enhancement in a textbook type of approach, while still taking~full advantage
of past scientific achievements and technological advances. '

The direction of PEP and its basic philosophy was endorsed in the form of eight
major objectives by the WMO Executive Committee and its twenty-eight's session in 1976.
They are as follows:

(a)

i
To provide Members with reliable information about the probabilities of
successful artificial intervention in meteorological processes with the
object of increasing the amount of precipitation over an area; of the order
of 10 000 km2. The size of the area for the proposed project (Le. the
target and nearby control areas) should be somewhere around 50 000 km2, a
scale large enough to provide adequate evaluation of scientific feasibility
and economic benefit, but small enough to permit the use of adequate methods
for seeding and observations;

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