Tag Archives: mild

Will it be a White Christmas this year?

Will it be a white Christmas this year? It’s the question most forecasters get asked year after year as the big day looms less than a month away.

The snow probably won't arrive until after Christmas this year
The snow probably won’t arrive until after Christmas this year

It always fascinates me why Christmas and snow are so closely associated with each other when the odds, especially in London, are so stacked against it happening.

Perhaps it is the Christmas card scenes of Dickensian winters that get people yearning for the white stuff. Indeed, the image of snow covered cobbled streets can probably be blamed on Charles Dickens. The backdrop of A Christmas Carol, and a host of other books, were written when winters, and Yuletide, were generally much colder than they are now. Perhaps it was also the cold Decembers, which came during notably cold years as he was coming of age, that sowed the seed of his impressions of life in London. The Central England Temperature (CET)  for December 1829 and 1830 were 1.4C and 1.8C respectively. To give you perspective the average mean temperature for the Wanstead region in December is 5.6C. Of course we had a taste of what a Dickensian December was like three years ago in 2010 when the average mean temperature was 1.5C. Though bear in mind the mean for 2010 was 10.5C – way above the 8.2C and 8.7C mean temps of 1829 and 1830!

Christmas toys of yesteryear also took every opportunity to perpetuate the snow myth
Christmas toys of yesteryear also took every opportunity to perpetuate the snow myth

But back to this Christmas… First of all, what do we mean by a white Christmas? The definition used most widely – notably by the bookies – is for a single snowflake, even if it lands in the midst of heavy rain, to be observed falling in the 24 hours of 25 December at a specified location recognised by the Met Office. Interestingly, the Met Office uses weather observations from Gravesend-Broadness, some 12 miles away, to show current conditions in Wanstead. A lot, however, could be said by us weather anoraks about the different microclimates between here and south of the Thames.

Technically, there has not been a white Christmas in Wanstead for over 30 years. In 2010, we could still see the Christmas card Victorian snow scene in small patches of our gardens, but these were leftovers of a previous dump, so it doesn’t count. Frustratingly, there has been snow on several Boxing Days in Wanstead (1995 and 1996) and in the weeks running up to Christmas, but not on Christmas day itself. The most typical Wanstead Christmas day weather is mild and dry, although it has rained on 12 of the last 33 Christmas days.

So why does it often snow either side of Christmas but not on the actual day? For Wanstead, Christmas is at the beginning of the period when it’s likely to snow. Looking at climate history, wintry weather is more likely between January and March than December.

iced anemOn average, snow or sleet falls in the UK 5 days in December, compared with 7.6 days in January, 6.8 days in February and 6 days in March. 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 back by 12 days. Climate change has also brought higher average temperatures over land and sea and this generally reduces the chances of a white Christmas.

For snow to fall we need moisture in the atmosphere. Snowflakes start their lives as ice crystals thousands of feet up, and when these tiny ice crystals collide they stick together in clouds to become snowflakes. If enough ice crystals stick together, they’ll become heavy enough to fall to the ground.

Precipitation falls as snow when the air temperature is below 2°C. It is a myth that it needs to be below zero to snow. In fact, in this country, the heaviest snowfalls tend to occur when the air temperature is between zero and 2°C. The falling snow does begin to melt as soon as the temperature rises above freezing, but as the melting process begins, the air around the snowflake is cooled. If the temperature is warmer than 2 °C then the snowflake will melt and fall as sleet rather than snow, and if it’s warmer still, it will be rain.

It sounds a simple combination, but getting precipitation on the days when there are temperatures low enough for snow are few and far between.

temple snowChristmas day in Wanstead, on the balance of probability and from previous patterns, is most likely to be a green and mostly cloudy but dry one. Some brightness is possible with temperature peaking at around 10°C. You can read my full methodology on why I think this may happen here.

Though my attempt to find the probability of a White Christmas effectively rules one out, there is still an outside chance that one could happen.

At this time of year the UK can effectively become a battleground between cold polar continental air to the north or east and moisture-laden mild tropical maritime air to the south and west. Where these air masses meet, snow is possible, but a lot depends on which air mass wins the battle. When battleground situations occur, in one location it can be snowing, but just 20 miles or so down the road it can be raining. This is because there is a fine line between the boundary of the warm and cold air.

There's nothing like an open coal fire. It's even better when it is cold enough outside to have one
There’s nothing like an open coal fire. It’s even better when it is cold enough outside to have one

In years gone by, Wanstead and the surrounding area has often ended up in the cold air mass or the warm air mass. In these situations dry and mild or dry and cold weather is often the result. Of course, in a cold air mass situation there is always the chance of showery activity of the North Sea. The weather so far this winter has not been anything out of the ordinary, so I’m afraid there is nothing to suggest a white Christmas is likely.

If it does turn very cold on December 24, pray for clouds to appear and we could be in with a chance. But at the end of the day we still need that vital combination of temperature and moisture. Snow, like Christmas in that sense, requires some magic.

Winter Forecast 2013/14 (with no apologies to the Daily Express)

Much has been printed in the tabloids over the past month or so that we are in for a severe winter. Not a week goes by without the Daily Express splashing that the ‘Worst winter EVER is on the way’. Just this morning the same rag told us that three months of ‘exceptionally cold’ weather are due. On closer inspection the story elaborated the scene with quotes from James Madden of Exacta Weather, one of the ‘experts’ feeding these fantastical stories.

The Temple, Wanstead Park, always looks that much more stunning with a covering of snow
The Temple, Wanstead Park, always looks that much more stunning with a covering of snow

Quite how Mr Madden and other experts arrive at these forecasts is a bit of a mystery. The mystery has deepened further since I decided to crunch a few numbers and try to predict what is in store for the months ahead. Looking at data for this area stretching back over 130 years to 1881 I decided to calculate a seasonal average and arrived at a final figure using singularities – basically looking at the weather patterns we’ve had during October and November.

Many professionals would scoff at this method of pattern-matching, so I’ve incorporated a couple of other ‘now’ factors and taken on board current variables such as sea surface temperatures in the Atlantic and Pacific.

The figure I arrived at, taken as an average of the closest matching autumn periods, is a mean temperature of 4C with rainfall totalling 133mm over the months of December, January and February – that’s about a degree colder than average and 90% of average rainfall. The probability of a winter with a mean temperature of between 4C and 5C is 37% – the most likely outcome. With this in mind a winter in the form of 1986-87 is possible – though whether we would see the same extremes of temperature and snowfall that we experienced in January 1987 is open to question.

Here is a link of my method to predict the coming winter.

The Met Office, with all their computing power and expertise, seem to be hinting at something similar – ie slightly colder than normal. The opening couple of weeks to my forecast seem quite plausible, looking at tonight’s models. Look at the similarity in the synoptic charts for December 8th here.

Shoulder of Mutton lake, Wanstead PArk
Shoulder of Mutton pond, Wanstead Park

Over the last few years the snow lovers among us have been spoiled after a run of very mild winters during the late Nineties and early Noughties that prompted climate expert Dr David Viner to utter the immortal words that one day “children just aren’t going to know what snow is”. It was only a matter of a few years before the words from the senior research scientist at the University of East Anglia seemed a bit hollow.

Mark Twain, born this day in 1835 , once uttered the famous phrase: “Climate is what we expect; weather is what we get” – there is always a chance we could suddenly lurch back to milder winters. This winter could spring a surprise, be it very cold or very mild. But judging what’s happened over the last 130-odd years – a 1962/63 or a balmy 1989/90 look very unlikely.

* Forecasting models use probability on any given outcome. Millions of observations are fed into the Met Office database (and other countries’ weather agencies) every day. Supercomputers then crunch through this data to give probable outcomes. With the volatility of the atmosphere it is not surprising that certainty of any outcome often falls away rapidly. Forecasting has improved greatly in the last 20 years – though anything the models churn out beyond three to five days should be handled with caution. Long range models can give *some* idea of general trends for the months ahead – but changing just one variable can vastly alter an outcome at the end of the run.

* *Over the past few months I have been collating data for the area around Wanstead. This data is freely available from the excellent Met Office library and is emailed via Excel spreadsheet. Rainfall stats include a near-complete daily archive, stretching from 1961 to 2003, from City of London Cemetery . Sadly the rainfall station, along with many others, ceased to supply the Met Office after cuts were made shortly after the turn of century. Prior to 1961 I have used monthly figures taken at the Greenwich Royal Observatory stretching back to 1881. Though this is 6 miles away the difference in temperature between the two areas would be miniscule compared with rainfall data and so can be used. I use my own stats for the period after 2003.