Any forecast of future weather, or of the longer-term climate, must, ifit is to be realistic, depend on knowledge and at least some understanding of the physical processes at work. With a public increasingly aware of how science contributes to our competence, this is much more widely appreciated than it used to be. Yet there are still many who do not distinguish between a well-grounded forecast and the most ill-informed guesswork of an amateur. Sometimes, sad to say, such guesswork seems to be featured very publicly on the media - maybe just for the sake of its 'joke value' - though this is really unkind to the amateurs themselves, who are thereby exposed, and to others among them who have a better understanding. Predictions based on mere extension of a recent trend, or on supposed cycles which cannot be related to any known physical process, are worthless.