White Christmas (weather)



A white Christmas refers to the presence of snow at Christmas; either Christmas Eve or Christmas Day depending on local tradition. This phenomenon is most common in the northern countries of the Northern Hemisphere. Because December is at the beginning of the Southern Hemisphere summer, white Christmases there are extremely rare, except in Antarctica, in the Southern Alps of New Zealand's South Island, and in parts of the Andes in South America. The Irving Berlin song, "White Christmas", sung by Bing Crosby from the film Holiday Inn, is the best-selling single of all time and speaks nostalgically of a traditional white Christmas.

Definition
The definition of "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 at 7:00 a.m. local time on Christmas morning, and in Canada the official definition is that there has to be more than 2 cm on the ground on Christmas Day. 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. Consequently, according to the Met Office and British bookmakers, even 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 snow flakes 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.

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.

Canada
In most parts of Canada it is likely to have a white Christmas in most years, except for the coast and southern interior valleys of British Columbia, southern Alberta, southern Ontario, and parts of Atlantic Canada – in those places Christmas without snow is not uncommon in warmer years, with the British Columbia coast the least likely place to have a white Christmas. The definition of a white Christmas in Canada is 2 cm of snow-cover or more on Christmas morning at 7 am. Environment Canada started to analyze data from 1955 to 2013 for a total of 59 years, It shows the chance of a White Christmas for several Canadian cities.

The year 2006 saw some of the warmest weather on record, with such places as Quebec City experiencing their first green Christmas in recorded history. In 2008, Canada experienced the first nationwide white Christmas in 37 years, as a series of pre-Christmas storms hit the nation, including the normally rainy BC Pacific coast.

United States
In the United States, there is often – but not always – snow on the ground at Christmas in the northern states, except in the Pacific Northwest, with the northern Plains the most likely to see snow on the ground at Christmas. Some of the least likely white Christmases that have happened include the 2004 Christmas Eve Snowstorm, which brought the first white Christmas in 50 years to New Orleans and caused the first recorded white Christmas in Houston, Texas. The 2004 storm also brought the first measurable snow of any kind since 1895 to Brownsville, Texas, and its twin city of Matamoros, Mexico. The Florida winter storm of 1989 also occurred immediately before Christmas causing a white Christmas for cities like Pensacola and Jacksonville. The same storm buried Wilmington, North Carolina and the rest of Southeastern North Carolina under 15 in of snow.

In the United States the notion of a white Christmas is often associated in the American popular consciousness with a Christmas celebration that includes traditional observances in the company of friends and family. White Christmas is an Irving Berlin song reminiscing about an old-fashioned Christmas setting.

According to research by CDIAC meteorologist Dale Kaiser, the United States during the second half of the 20th century experienced declining frequencies of white Christmases, especially in the northeastern region. The National Climatic Data Center based the probability of a white Christmas (1 in of snow on the ground) at selected cities upon the 1988–2005 numbers from stations with at least 25 years of data.

White Christmases in the United Kingdom
In the United Kingdom, white Christmases were more common from the 1550s to the 1850s, during the Little Ice Age. The shift from the Julian to the Gregorian calendar in 1752 also slightly reduced the chance of a white Christmas. During the 20th century there were only seven official white Christmases in the United Kingdom.

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.

Christmas 2009 was a white Christmas in some parts of Britain, 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.

White Christmases in Ireland
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 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 recorded.

White Christmases in Poland
White Christmases, once common across the country, are becoming rare in Poland. Some northwestern coastal towns barely see White Christmases any more, even though many other regions of east-central Poland do, including the cities of Rzeszów, Suwałki and Białystok. In the Tatra Mountains of southern Poland, including the winter capital of Zakopane, snow is usually present throughout the season.

White Christmases in other parts of Europe
In Europe, snow at Christmas is common in Norway, Sweden, Finland, the Baltic states, Russia, Ukraine, Belarus, and Poland. In general, due to the influence of the warm Gulf Stream on European climate, chances of a white Christmas are lower the further west. For example, in southern France a white Christmas is very 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.

Southern Hemisphere
Because Christmas occurs during the summer in the Southern hemisphere, white Christmases are especially rare events there, apart from Antarctica which is generally uninhabited. In 2006, a snowstorm hit the Snowy Mountains in New South Wales and Victoria, arriving on Christmas Morning and bringing nearly 12 in of snow in higher areas. This was an especially rare event because it occurred during Australia's typically warm summer. In New Zealand's Southern Alps snow can fall any day of the year and a white Christmas is very possible. A white Christmas in the southern hemisphere (specifically those close to Antarctica) is approximately equivalent to having snow in the northern hemisphere on 25 June, and in some ways is even less likely because the Northern Hemisphere has population centers farther from the equator than does the Southern Hemisphere.