Santa’s sledge to glide a little less easily this Christmas

What does the forecast hold for Santa this year?

Highlights

• Scotland - There is an increasing chance of colder weather by Christmas Day, with occasional sleety flurries. However, most of any snow will be on high ground, away from the main population centres of the country. On the mountain tops, some heavy falls of snow are possible.

• Northern Hemisphere - Snow cover has been decreasing gradually across the Northern Hemisphere over the past 40 years. Global warming is widely suspected as being the main culprit in causing this reduction among climatologists.

Will there be snow in Scotland this Christmas?

The month of December this year in Scotland is shaping up to be one of the wettest and stormiest for many years. Although there has been some snow at times, it has not lasted long on low ground. The current meteorological prognosis is for a continuation of the very wet and stormy weather for several more days, but with an increasing chance of colder weather by Christmas Day. Most of any snow will be on high ground, however, away from the main population centres of the country.

And what of snow elsewhere around the world? Will Santa be able to glide along in his sledge or will the reindeers have to fly for much of his long voyage on Christmas Eve?

Recently, scientists at Environment Canada and Rutgers University snow laboratory in the United States have been carefully analysing worldwide snow cover data from 1967 through to the present day. Their findings, recently published in Geophysical Research Letters, confirm that the number of days per year with a snow cover has been decreasing gradually across the Northern Hemisphere over the past 40 years. The strongest decreases are found over both North America and Eurasia during spring-time, especially at lower latitudes, but with much smaller decreases during early winter. This may be bad news for Santa, because reduced snow cover (particularly at low latitudes where a higher population lives), means his sledge won’t glide as easily. Among climatologists, global warming is widely suspected as being the main culprit in causing this reduction in snow cover.

Is snow cover in Scotland decreasing too?

The situation in Scotland is complicated by a high variability in the amount of snow from winter-to-winter; some winters are snowy, others considerably less so. Furthermore, changes in the way that snow is measured (such as the changeover from previous manually-made measurements of snow depth to the mass automation of meteorological observations from the 1990s onwards) further complicates any study of snow cover duration due to potential inhomogeneities in the data i.e. we may not always be comparing like-with-like.

However, what is clear from the meteorological records in Scotland over the past 40 years is that the climate of Scotland has warmed by about +0.5degC and there has been a consequent reduction in the number of days with an air temperature below freezing (0°C/32°F). Similarly, the Rutgers University dataset confirms that the total annual number of snowdays per year over Europe has decreased, but with a large inter-annual variability. This overall decrease in snow cover, however, has not been replicated during the month of December - an anomaly that bucks the trend of decreasing snow cover in most other months. Indeed, it appears that December has even become slightly more snowy over the past 40 years across Scotland and Europe as a whole - we only need to recall the very snowy Decembers of 2009 and 2010 to confirm this.

So where’s it snowy for Santa this Christmas?

The above image (left), courtesy NASA/NOAA/Rutgers University, shows the average snow cover distribution (in terms of percentage of cover) for the Northern Hemisphere for Christmas Day, based on the period 1967-2013. One can see that most of Canada and Russia experience a “White Christmas” every year, but with a decreasing likelihood of finding snow on the ground as one heads south.

And how’s December 2013 shaping up?

The image shows the deviation of snow cover from normal for a period earlier in this month (December 2013). Red areas mean the snow cover is below normal, whereas blue areas show areas where current snow cover is above normal. If these same anomalies are replicated on Christmas Day, it means that Santa may have trouble delivering his presents across parts of America, but his travails should be easier over Eastern Europe!

Dr Eddie Graham
Lecturer and meteorologist

Images: All images are free to publish, but please credit “courtesy of NASA/NOAA/Rutgers University”.