"Shouldn't be surprised if it hailed a good deal tomorrow - blizzards and what-not. Being fine today just doesn't mean anything. It has no sig - what's the word? Well it has none of that. It's just a small piece of weather."
Now you might think the above is a transcript of a conversation overheard in Met Eireann a few days ago, but it is not. It comes from The House at Pooh Corner by A.A. Milne, part of an exchange between Christopher Robin - usually remembered for his trip with Alice to see the changing of the guard - and his donkey friend, Eeyore. But the passage does, to some extent, synthesise the doubts that exercise, from time to time, the minds of weatherpeople.
Meteorology, as we have often noted in this column, can never be exact. "The aim of an exact science," according to the Scottish physicist James Clerk Maxwell, who coined the phrase in the middle of the last century, "is to reduce the problems of nature to the determination of quantities by operations with numbers". Meteorology tries hard, by processing complex models of the atmosphere on powerful computers, to realise this ambition of exactitude, but it does not quite succeed and probably it never will.
How, for example, does the computer predict hail or snow? Do red lights flash upon the consoles, or does the machine simply display the landscape coloured white, or is it a simple one-word message such as Snow? Naturally, it is not as simple as that. The main output of the computer is a series of charts for various times in the future which are not dissimilar to the weather map for tomorrow reproduced on this page, except that no fronts appear and they are not accompanied by a convenient text. Knowledge and experience are required on the part of the forecaster to translate the output of the machine into a meaningful interpretation of the expected weather.
And snow is particularly difficult in this context. All that is required for rain to fall as snow is that the temperature be low enough - below about 3 degrees. If the computer predicts that temperatures over the country tomorrow will be, say, 3 or 4 degrees, then some places will get snow and others not, and it is a major challenge to identify which will be which. Or it may happen that precipitation falling as rain in the afternoon may turn to snow with the approach of dusk, when the temperature drops below the crucial value - or it may not. No wonder Christopher Robin found it all very difficult to understand.