Snow at Christmas is a British tradition. You only have to listen to Christmas carols, or look at the snow scenes on Christmas cards and snow-decorated windows and trees to see that we are dreaming of a white Christmas (and lots of presents too!).

Although there is great hope at this time of year for snow on the 25th of December, for most parts of Britain, Christmas unfortunately comes at the beginning of the snow season. This is the reason we don’t normally see much snow until January when cold, winter weather is in full swing.

A white Christmas to those adults in the gambling business does not relate to snow lying on the ground. According to the bookmakers, it actually has to snow on Christmas Day before they will pay out. There can be 20 centimetres of snow on the ground, but if it doesn’t snow on the day then it is not classed as a white Christmas. Bookmakers also class falling sleet as a white Christmas. A white Christmas to a weather forecaster means snow on the ground or light or heavy snowfall on the 25th December. Sleet does not count.

When estimating the likelihood of an event occurring, like snow at Christmas, or that you will tidy your bedroom (not likely!), we can use probability. To a weather forecaster, a probability of 20% of snow doesn't mean that it will snow 20% of the day, or that snow will fall on 20% of your area or that it snowed 20 times out of 100 on that particular day in the past. A 20% chance of snow means that the weather forecasters have calculated that in 100 similar weather situations, snow has fallen 20 times in the forecast area.

If you want to discover the chances of a white Christmas this year, then you could take a look at this table to make a prediction. Remember, we are cheating a little! Weather forecasters work out the chance of snow by looking at a number of alike weather situations. We, however, will be using how many times there has been snow on Christmas day in the past (in London) to work out the chances of a white Christmas.

The table measures the number of white Christmases in London, from 1900 to 1999 (100 years):

Use the table to find out what is the probability of:
a) having a bookmaker’s white Christmas, this year in London?
b) having a weather forecaster’s white Christmas, this year in London?

a) 10 white Christmases out of 100 Christmas days = 10/100 which can be simplified to 1 in 10
1/10 = 0.1 x 100 = 10%
There is a 10% or 1 in 10 chance of a bookmaker’s white Christmas this year, in London.

b) There have been 7 white Christmases out of the last 100 so:
7 white Christmases out of 100 = 7/100 = 7 in 100
7/100 = 0.07 x 100 = 7%
There is a 7% chance of a weather forecaster’s white Christmas this year, in London.