Elements for Life

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1. Introduction

1.1 Every day around the world, the NMSs and the

private sector meteorological service providers of the

Member States and Territories of WMO provide

hundreds of thousands of forecasts and warnings of

weather and climate conditions and events. These

forecasts and warnings provide information for the

benefit of the community at large and for a wide

range of specialized user sectors, on a broad spec-

trum of atmospheric phenomena ranging from those

with timescales of seconds to minutes and space

scales of metres to kilometres, such as severe storms,

through to those, such as El Niño-related drought,

with multi-year and global impact. The forecast infor-

mation provided is used to inform and improve

decision making in virtually every social and

economic sector and the globally aggregated

economic benefits of meteorological services are reck-

oned to be of the order of hundreds of billions of

United States dollars

1.2 The capacity to provide these socially- and econom-

ically-beneficial services to the citizens of the 185

Members of WMO results from the operation of the

unique international system of cooperation of the

WMO World Weather Watch Programme which is

based on:

(a) The collection and international exchange of the

global observational data that are essential to describe

the current (initial) state of the atmosphere (and the

underlying land and ocean) at any point in time

(b) The fact that the physical and dynamical processes

governing the behaviour of the atmosphere and ocean

can be represented in numerical models which are

capable of providing forecasts of daily weather condi-

tions with significant skill out to several days from

the ‘initial’ state as well as useful indications, in

certain circumstances, of general trends of climate

for months and seasons ahead

rological system of global, regional and national

data-processing and modelling centres producing

real-time products from which skilled professional

forecasters are able to prepare forecasts and warnings

in forms that are relevant and useful to the user

community;

(d) The ability to monitor extreme events in real-time

and to issue warnings by combining classical mete-

orological observations, model output and

information from remote-sensing systems such as

satellites and radar.

1.3 The scientific understanding and technological capa-

bilities underlying this globally cooperative system of

weather and climate forecasting have made enormous

progress over the past 25 years as a result, in partic-

ular, of such cooperative international research

programmes as the WMO/ICSU Global Atmospheric

Research Programme, the WMO World Weather

Research Programme and the WMO/ICSU/IOC

World Climate Research Programme. The skill levels

and utility of the resulting forecasts and warnings

have steadily increased. Indeed three-day forecasts of

surface atmospheric pressure are now as accurate as

one-day forecasts 20 years ago. But the observational

database necessary to describe the ‘initial’ state of

the atmosphere will always be limited by considera-

tions of scale and measurement accuracy, the

processes governing the behaviour of the atmosphere

are non-linear and the phenomenon known as chaos

imposes fundamental limits on predictability. While

new techniques are emerging which help potential

users of weather and climate forecasts to understand

better, and make allowance for, the inherent uncer-

tainties in the forecasts, the WMO Executive Council

believes it is important that all those who make use

of such forecasts in decision making should be made

better aware of both their scientific foundation and

their scientific and practical limitations. It therefore

requested that CAS prepare a statement on the

current status of weather and climate forecasting.

1.4 This statement has been prepared by CAS with input

from other WMO and external scientific organiza-

tions and programmes including the World Climate

Research Programme. It was approved by the thir-

teenth session of CAS in Oslo in February 2002 and

TATEMENT OF SCIENTIFIC BASIS

(EC-LIV, 2002)

WMO statement on the scientific basis

for, and limitations of weather and

climate forecasting