Derek's Weatherwatch: Betting on snow

Heavy snow fell this week in Washington DC

By Derek Brockway BBC Wales meteorologist

Temperatures are falling fast, but are the chances of a White Christmas this year getting any bigger?

We have seen a big change in the weather in the past few days.

After a very mild, but very wet November, temperatures have taken a tumble with a blast of cold easterly winds from Russia.

Christmas is just around the corner and many people are wondering whether it will be a white one this year.

Many of us long for the likes of the scenes depicted on traditional Christmas cards and the annual interest in snowy Christmases goes back a long way.

Wales' hills may not be covered with snow

It has its origins in the colder climate of 1550 to 1850, when Britain was in the grip of a "Little Ice Age".

Winters were particularly persistent and severe - 1813/14 was the last winter that a 'Frost Fair' was held on the River Thames in London.

For Wales and most of the UK, snow is more likely after Christmas, in January and February.

White Christmases were more frequent in the 18th and 19th centuries, even more so before the change of calendar in 1752 which effectively brought Christmas day back by 12 days.

For many, a white Christmas means a complete covering of snow - ideally falling between midnight and midday on the 25th.

However, the definition used most widely, notably by those placing and taking bets, is for a single snow flake (perhaps amongst a shower of rain and snow mixed) to be observed falling in the 24 hours of December 25th.

Snow rare

Last year those who had a flutter were lucky. Many parts of Wales had a white Christmas with a band of wintry showers moving south during the evening although in Cardiff the snow did not lie.

In Greenland, there are about 50 different words for snow, from 'aput' to 'nittaalaq'

On the whole, snow on Christmas Day is a rare event. On average, snow falls on only one Christmas in about nine.

Other countries and cultures have an even greater fascination with snow than we do.

In Greenland, there are about 50 different words for snow, from "aput" (snow on the ground) to "nittaalaq" (air thick with snow). Snow is a surprisingly complex form of precipitation.

Its structure will vary according to the temperature, pressure and humidity conditions under which it formed.

Snow crystals are generally hexagonal and symmetrical but they can form as plates, needles, bullets and columns.

Wet snow

The Met Office recognises several types of snow, including snow pellets (or graupel), snow grains, and diamond dust.

For those who have put a bet on a White Christmas any of these forms will count with the bookmakers. Snow depth is reported in centimetres of level snow.

Big, wet flakes bind together and makes the best snowmen!

If the wind and snow conditions are right then 10 centimetres of snow can drift to a depth of several metres against walls and buildings. At home, too, snow can be a nuisance, if pretty.

Snow in the United Kingdom comes in two basic forms, "wet" and "dry".

The more common "wet" form can produce large snow falls with the temperature just above freezing.

The big, wet flakes bind together. They can obscure road signs and bring down power lines but drifting, unless the wind is very strong, is not a major problem.

And of course this type of snow makes the best for building snowmen!

Cold blasts

When the wind is in the east the air is drier and temperatures are often much lower, perhaps around -2 degrees Celsius.

The snow flakes are much smaller and they do not stick together very well.

It is very difficult to make a decent snowball with them. Whilst 'dry' snow does not stick to road signs, it can produce very large drifts.

If the cold blast of easterly air continues, this snow can last for quite some time as it did on January 7/8th 1982.

Depths of one metre or more were commonplace then with severe drifting and power lines brought down.

Roughly speaking, an inch of rain is equivalent to a foot of 'wet' snow. But that same inch of rain can produce two feet of 'dry' snow.

Spring falls

Trying to forecast amounts of rainfall can be difficult and almost any weather front will deposit varying amounts of rain across quite a small area.

So in snowy weather these differences can be greatly exaggerated.

With odds of 4 to 1 for a White Christmas in Cardiff it might be worth a flutter

In recent years the heaviest snowfalls have occurred in March and April.

So it may be a major nuisance; it may disrupt road and rail networks; it may send us scurrying to the supermarket to panic-buy bread and sugar.

But when those flakes start to fall, their sound-deadening effect makes for an eerie silence. Even the dullest street takes on a wonderful, wintry hue.

So what about this Christmas ? Its still too early to say. In the short term it looks like staying cold with the risk of a little snow in places this week in South Wales.

The longer term is more uncertain but with odds of 4 to 1 for a White Christmas in Cardiff it might be worth a flutter. I mean it snowed last Christmas!

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