Heavy snow occurs only in a narrow range of weather conditions - which is why it can be so difficult to predict accurately.

The conditions most likely to produce snow in Britain are a good supply of moist air, a temperature of about 1C and a slight wind. Any warmer, and the snow will fall as rain; any colder, and the air is likely to be very dry Polar Continental. Although Canada often experiences snow in temperatures of -15C or lower, most snow in this country falls in temperatures between -2 and +2.

By following the changes in weather forecasts issued yesterday, we can see the difficulty in predicting snow. In the early hours of the morning, it was: "Blizzards predicted to sweep south". The huge snowfalls that Scotland had suffered would spread down the whole of eastern England and drivers were advised not to make journeys unless absolutely necessary.

That was at 3.39am; but by the time we were getting up, some snow had indeed fallen, but blizzards were already looking unlikely. And shortly after ten o'clock, the forecasters took off their winter woollies and told us: "The cold snap which has brought snow and freezing conditions to much of Britain will make way for milder weather today ... there will be some light snow showers on the east coast today, but they will soon be over and will be fairly insignificant." The difference between "insignificant" and a blizzard can be very small indeed.