Even experts using advanced computers find predicting snow showers especially tricky, writes Brendan McWilliams
Despite remarkable advances, there remain several reasons why weather forecasting can never be an exact science. For one thing, to predict the weather by computer it is first necessary to define the existing state of the atmosphere precisely. The picture is assembled from weather observations at places perhaps 100km apart, or inferred from satellite images with a resolution of, say, five or ten kilometres. This presents, at best, a rough approximation; the fuzzy snapshot is more a Monet than a Caravaggio.
Secondly, the equations used to describe the behaviour of the atmosphere and predict its future state are not exact; they are approximations, if for no other reason than to match the necessary calculations to the capabilities of the machine. And finally, even if the computer predicts the future pressure pattern with absolute precision, knowledge and experience of local topography and geographical features are necessary to translate the output of the machine into a useful weather forecast for any particular spot.
Both showers and snow are particularly difficult in this regard. The computer will identify 24 hours in advance a general area where showers are likely to occur, but it cannot pinpoint exactly when and where the showers will happen. In addition, a slight miscalculation of the thermal structure of the atmosphere affects the frequency and intensity of the predicted showers.
And as regards the kind of showers, all that is required for a shower to be of snow is that the temperature be below about three degrees. If the computer predicts that temperatures over a showery Donegal tomorrow will be, say, three or four degrees, then some places will get snow and others rain, and it is a major challenge to identify which might be which. Indeed 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 then again, it well may not.