What are the chances of a white Christmas this year?

Whether its Bing or Bublé, we will all probably hear more than a few renditions of ‘White Christmas’ in the coming fortnight — but what are the chances of the country actually seeing snow on the day itself?

According to Met Éireann climatologist Aidan Murphy, countrywide snow fell on 17 Christmas days at at least one of its synoptic stations, since 1961 — the last of them in 2010.

The maximum depth of snow ever recorded on the day was 27cm at Casement Aerdrome in 2010. In fact Casement also has the highest depth of snow ever recorded at 45cm on December 31, 1962.

“For many people, a ‘white Christmas’ means a complete covering of snow on the ground. However, sometimes the definition used by bookmakers is for a single snow flake to be observed falling during the 24 hours,” he said, adding that the statistical likelihood of snow falling at Dublin Airport, for example, is approximately once every 5.9 years, as snow fell on Christmas day there 12 times in the last 71 years.

Kirsty McCabe, senior meteorologist at The Weather Channel UK, said already-lying snow on the ground doesn’t count: The snowflake has to fall on Christmas Day.

She also outlined what conditions are needed to get snow.

“The shopping list is pretty short — cold air and moisture in the atmosphere,” she said. @This moisture is in the form of tiny ice crystals in clouds that collide and stick together to become snowflakes and when they become too heavy they fall to the ground.

“Our coldest winter weather comes from the Arctic Circle to the north or Siberia to the east. The longer we are exposed to air coming from these directions, the colder it will get.

“But a plunge of cold air isn’t enough, we need to get moisture from somewhere to produce the snow. The usual suspects are polar lows, showers set off over the sea or a frontal system coming up against cold air. Even just continuous heavy rain can lower the air temperature enough for snow to start falling instead.”

Ms McCabe said the air temperature doesn’t need to be below zero for snow to fall, below 2ºC is cold enough. “In fact, the heaviest snowfalls tend to occur between 0 and 2ºC,” she said. “The slightly warmer air causes the snowflakes to melt around the edges and stick together to become big, heavy flakes.”

Mr Murphy says the accuracy of forecasting falls significantly beyond five days so it’s still difficult to accurately predict snow falling on Christmas Day.

However, at this stage, Accuweather’s prediction for Cork is that it will be 6ºC and raining on Christmas Day — with the temperature rising to 13ºC and thunderstorms on St Stephen’s Day.

And if Paddy Power’s odds are anything to go by, the chances of snow become greater the further north one looks. As of yesterday, it was offering odds of 4-1 for Cork and Limerick, 5-2 for Dublin, and 9-4 for Belfast.