What would Christmas be without windows obscured by fake blizzards, greetings cards depicting arctic snows, and Santa's grottos decked out in bales of cottonwool? But these days - although this year meteorologists say there is a chance of seasonal flurries in the north of England and Scotland - the Christmas snow theme generally looks surreal in a world basking in global warming.
The typical Christmas weather, says the Met Office is "grey, damp, windy and mild". A more authentic Christmas card scene might be torrential rains, floods and wildlife wondering whether to migrate, hibernate or start breeding. Even that emblem of the frozen Christmas landscape, the North Pole, is now growing so warm that the ice is disappearing and polar bears are rapidly becoming homeless. The South Pole fares no better.
Penguins gaze out on seas that for millennia were hidden under vast ice sheets, before global warming cracked them up and they melted away. So the classic frozen Christmas scene is sentimental tosh - and you can blame it all on Charles Dickens. Born in 1812, his childhood would have been plagued by bitter cold. We know his second Christmas, in 1813, was an arctic affair because Jane Austen recorded the conditions in her novel Emma, set in a year of appalling weather.
The snow was falling heavily as Emma went to visit friends on Christmas Eve, and the rest of the season was pretty cold, too. In fact, that winter was the fourth coldest recorded in English weather records until then, and it was the last time the Thames hosted a frost fair when the river froze.
When Dickens was a young boy he would also have witnessed the horrific "year without a summer" in 1816, when northern Europe was plagued with frost and snow through June and July.Out of that wretched summer, another great literary work was created, when Mary Shelley and her poet husband Percy were on holiday with friends at Lake Geneva in Switzerland. The weather was so miserable that they stayed indoors entertaining one another with horror stories - Mary's contribution was Frankenstein.
Altogether, six out of the first nine Christmases in Dickens's life were white, and the decade from 1810 to 1819 was the coldest in England since the 1690s. He would have taken cold weather as a normal part of Christmas, which is probably why it features so large in A Christmas Carol, published in 1843: "They stood in the city streets on Christmas morning, where (for the weather was severe) the people made a rough, but brisk and not unpleasant kind of music in scraping the snow from the pavement in front of their dwellings, and from the tops of their houses: whence it was mad delight to the boys to see it come plumping down into the road below, and splitting into artificial little snow-storms."
What made those Christmases so cold? An unlikely - and early - clue is provided by the painter Pieter Brueghel the Elder. He started off painting jolly Flemish peasants in sunny summer weather. Then in the 1560s he suddenly switched to depicting chilly landscapes, beginning with Hunters In The Snow, featuring lots of snow and a frozen lake. This was during a run of appalling winters, when walnut trees were heard cracking open in the severe frosts. Brueghel became so obsessed with the frozen theme that he repainted many of his earlier pictures, adding images of snow.
Climate historians say these pictures largely belong to a lengthy frozen period called the Little Ice Age that lasted about 300 years, covering Dickens' seminal boyhood winters. It brought some severe winters, bleak summers, failed harvests and led to political upheaval. The Swedish army took advantage of a cold winter in 1632 to invade Germany by marching across the frozen Baltic Sea. The terrible weather of 1788-89 in France triggered shortages of food, and peasant riots that contributed to the French Revolution. Iceland was so devastated by famine that the population was close to extinction. And in the 1790s, the prospect of starvation in Scotland set off waves of migrations to Ireland.
What caused such a calamitous downturn in climate? One answer is that the sun grew dimmer, as shown by a dwindling number of its sunspots. Judith Lean, a solar physicist at the Naval Research Laboratory in Washington, estimates that the sun may have been about 0.25% dimmer during this time. "This may not sound like much, but the sun's energy output is so immense that 0.25% amounts to a lot of missing sunshine - enough to cause most of the temperature drop," she says.
In addition, a spate of volcanoes erupted all over the world during the early 1800s. Cast your mind back to 1991 and the eruption in Mount Pinatubo in the Philippines. It shot out the largest amount of dust into the stratosphere for a century, blanketing the sun, and shaving about half a degree off global temperatures - the biggest single shake-up in the world's climate in recent history, which even masked global warming for a short while.
Yet Pinatubo's smoke was piffling compared to the mega-explosion of the volcano Tambora in Indonesia in 1815, which pumped out 100 times more ash and gave birth to the "year without a summer".
Once the smoke cleared and the sun got back to normal, the climate grew steadily warmer from the 1850s - to its pre-Little Ice Age levels. But the white Christmas bandwagon snowballed and the icy charms of A Christmas Carol struck a chord with the Victorians, nostalgic for the good old, cold days of yesteryear.
