Why is snow harder than any other weather to predict?

The view from Defynnog, Powys, after heavy snow Image copyright Getty/AFP
Image caption The view from Defynnog, Powys, after heavy snow

As snow forecasted fails to materialise again, why is it so difficult to predict?

When rain is forecast it normally falls. When we are promised blue skies we can usually get out the sunglasses. But snow? Now that is a different story and so much harder to guarantee.

But why?

On Tuesday evening, up to 10cm (3.9in) of snow was predicted to fall in parts of Wales, with yellow warnings from the Met Office telling motorists of possible travel disruption.

But when curtains were drawn open on Wednesday morning, instead of being greeted by the crisp white stuff, the sight was of wet pavements.

So where were the predicted snow flurries and why was the forecast so wrong?

The Met Office admits forecasting snow "will always be a challenge" in the UK as there is often a "fine line between whether it will rain or snow". This means a difference in height or air mass can completely alter the outlook.

BBC weather presenter Rhian Haf said: "Predicting snow is harder than anything else.

"Most precipitation falls out of the sky as snow to start with, but it melts to rain."

Snow is more likely in higher areas as the temperature is cooler, meaning rain may fall as snow in mountainous areas giving the dusting of snow on peaks which is commonly seen at this time of year.

Equally, snow is less likely to fall in urban areas because of the warmer surroundings. In central London it will rain sometimes, while a relatively short way away just outside the M25 there is heavy snow.

"It only takes a tenth of a degree for it to be either rain or snow," said Rhian Haf.

"When we predict snow it will be falling somewhere, it is just hard to say whether it will be in one place or another."

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