White Christmas (weather)

The criteria for a "white Christmas" varies. In most countries, it simply means that the ground is covered by snow at Christmas, but some countries have more strict definitions. In the United States, the official definition of a white Christmas is that there has to be a snow depth of at least 1 in or 2.5 cm on the ground on December 25 in the contiguous United States,[3] and in Canada the official definition is that there has to be more than 2 cm (0.79 in) on the ground on Christmas Day.[4] In the United Kingdom, although for many a white Christmas simply means a complete covering of snow on Christmas Day, the official definition by the British Met Office and British bookmakers is for snow to be observed falling, however little (even if it melts before it reaches the ground), in the 24 hours of 25 December.[5][6] Consequently, according to the Met Office and British bookmakers, even 91 cm (3 ft) of snow on the ground at Christmas, because of a heavy snow fall a few days before, will not constitute a white Christmas, but a few snowflakes mixed with rain will, even if they never reach the ground. In the United Kingdom the most likely place to see snowfall on a Christmas Day is in North and North Eastern Scotland, in Aberdeen, Aberdeenshire or the Highlands.[7]

Although the term White Christmas usually refers to snow, if a significant hail accumulation occurs in an area on Christmas Day, as happened in parts of Melbourne on 25 December 2011, this can also be described as a white Christmas, due to the resulting white appearance of the landscape resembling snow cover.[8]

In the United Kingdom, white Christmases were more common from the 1550s to the 1850s, during the Little Ice Age; the last frost fair on the River Thames, however, was in the winter of 1813–14.[17] The shift from the Julian to the Gregorian calendar in 1752 also slightly reduced the chance of a white Christmas, by moving Christmas earlier in the winter.[18] Before 2006 for betting purposes, a Met Office employee was required to record if any snow fell on the London Weather Centre over the 24 hours of Christmas Day; after 2006 computers were used.[19] An "official" white Christmas is defined by the Met Office as "one snowflake to be observed falling in the 24 hours of 25 December somewhere in the UK",[18] but formerly the snow had to be observed at the Met Office building in London.[18] By the newer measure, over half of all years have white Christmases, with snow being observed 38 times in the 54 years to 2015.[18] A more "traditional" idea of snow-covered ground is less common, however, with only 4 occasions in the 51 years to 2015 reporting snow on the ground at 9am at more than 40% of weather stations.[18]

Although most places in the UK do tend to see some snow in the winter, it generally falls in January and February. However white Christmases do occur, on average every 6 years.[17]

Christmas 2009 was a white Christmas in some parts of Britain,[20] with thick lying snow which easterly winds had brought over the previous week. Travel over much of Britain was badly affected by ice and snow on roads, and was made more slippery by partial daytime thaw followed by overnight refreezing. It was the first white Christmas anywhere in the United Kingdom since 2004.[21] There was another white Christmas in 2010,[22] it was also the coldest Christmas Day ever recorded in the United Kingdom.[23] In 2014, parts of the Northern Isles had a white Christmas and again in 2017, Northern England and Southern Scotland had a white Christmas.[24]

In Ireland, the prospect of early winter snow is always remote due to the country's mild and wet climate (snowfall is most common in January and February). Bookmakers offer odds every year for a white Christmas, which is officially lying snow being recorded at 09:00 local time on Christmas Day, and recorded at either Dublin Airport or Cork Airport (bets are offered for each airport). Since 1961, countrywide, snow has fallen on 17 Christmas Days (1961, 1962, 1964, 1966, 1970, 1980, 1984, 1990, 1993, 1995, 1998, 1999, 2000, 2001, 2004, 2009 and 2010), with nine of these having snow lying on the ground at 09:00 (1964, 1970, 1980, 1993, 1995, 1999, 2004, 2009 and 2010). The maximum amount of lying snow ever recorded on Christmas Day was 27 cm (11 in) at Casement Aerodrome in 2010.

At Dublin Airport, there have been 12 Christmas Days with snowfall since 1941 (1950, 1956, 1962, 1964, 1970, 1984, 1990, 1993, 1995, 1999, 2000 and 2004). The statistical likelihood of snow falling on Christmas Day at Dublin Airport is approximately once every 5.9 years. However, the only Christmas Day at the airport ever to have lying snow at 09:00 was 2010 (although no snow actually fell that day), with 20 cm (7.9 in) recorded.[25]

White Christmases in Romania have been rare in recent times, and they will likely continue so, due to the geographic position of Romania and climate change. In recent years, blizzard and snow falls usually start in January and they usually end at the beginning of March. However, at high altitudes, white Christmases are common.

The last White Christmas in Kraków was in 2010. Rzeszów had a White Christmas in 2016, the first since 2011.[26]

In recent decades, white Christmas in Zagreb - defined as the presence of snow cover on the ground on Christmas morning - has been observed in 1996, 1998, 1999, 2001 and - most recently, as of 2018 - in 2007.[27][28] The probability of white Christmas in Zagreb has been estimated at 25%.[27]

In continental Croatia, the estimated probability of white Christmas is highest in Gorski Kotar and Lika (50-70%), followed by northwestern Croatia (40-60%) and Slavonia (20-40%).[27] Climate projections suggest white Christmases in Croatia might become rarer in the future.[28]

In Europe, snow at Christmas is common in Norway, Sweden, Finland, the Baltic states, Russia, Ukraine, Belarus, and northeastern Poland. In general, due to the influence of the warm Gulf Stream on European climate, chances of a white Christmas are lower farther west. For example, in southern France a white Christmas is rare, while in Bucharest, Romania, which is at a similar latitude, it is much more likely. Northern Italy and the mountain regions of central-south Italy may also have a white Christmas. In cities such as Turin, Milan or Bologna a Christmas with falling snow or snow on the ground is not a rarity. White Christmases are also common in the Carpathian Mountains, as well as the Alps.

Because Christmas occurs during the summer in the Southern Hemisphere, white Christmases are especially rare events there, apart from Antarctica, which is generally uninhabited. A white Christmas elsewhere in the Southern Hemisphere is approximately equivalent to having snow in the Northern Hemisphere on 25 June. Some places like Ushuaia and Stanley, Falkland Islands have received measurable snowfall on Christmas Day on numerous occasions.

In 2006, a snowstorm hit the Snowy Mountains in New South Wales and Victoria, arriving on Christmas morning and bringing nearly 12 in (30 cm) of snow in higher areas.[29]

In New Zealand's Southern Alps / Kā Tiritiri o te Moana, snow can fall any day of the year, and a white Christmas is possible. The same situation can be seen in the Andes at elevations above 4,000 m (13,000 ft), with some locations on the Bolivian altiplano, such as El Alto, having the theoretical possibility of a white Christmas.

• The dictionary definition of white Christmas at Wiktionary
• Are you dreaming of a white Christmas?