Coldest day of each year from 1875
Some points about cold days
-25C has been reached or bettered only 4 times since 1955; -20 has been reached 21 times. The earliest date for the coldest day since 1900 is 14 November (in 1919, at Braemar; -23.3); the latest is 20 March (-16.1C at Newport, Gwent, 1930). The highest yearly minimum is just -8.4C, in February 1990, at Grantown.
I take "extemely cold" to be temperatures of -18 ºC or beneath (0 ºF), because that's the temperature your freezer should be set to. That happens about once every three years on average in Britain.
The lowest temperatures usually occur in the morning, often just before dawn, under a clear sky above a layer of fresh snow (which is an excellent insulator and allows the air above it to cool radiatively. An absence of winds prevents warming by mixing of layers of air. The 1995 ALtnharra though happened relatively early in the night; a slight breeze developed and may have prevented the temperature falling even lower. Valley bottoms can act as frost hollows, permitting low temperatures. In Scotland and Northern Irleand the lowest temperatures usually occur in Arctic air with northerly winds. In the south, the coldest winter days happen when very cold continental air is brought in from the east. In the south the easterly air mass has to cross a smaller area of warming water (the Channel) than east Scotland, say, where the North Sea is much wider. On the other hand, the nights are longer in Scotland, and snow cover is more frequent. Although continental air is cold in winer, the exact degree of coldness depends on its origin and the track followed by the air mass from its source. Air that comes straight from northern Russia over Poland and Germany can be exceptionally cold (e.g. as happened most notably in 1987, and most recently 1991). If the easterly blows from the Balkans and east Mediterranean it is less much less cold (about 5C) and the weather tends to be damp and grey.
Low minima in summer happen on still nights with clear skies, over sandy soils, which are also poor conductors of heat.
The coldest nights for each month of the extended winter are:
November. -23.3 Braemar 14 November 1919
December. -27.2 Altnaharra 30 December 1995
January. -27.2 Braemar, 10 January 1982
February. -25.0 Braemar 23 February 1955
March. -22.8 Grantown-on-Spey 12 February and 14th at Logie Coldstone, Grampian, 1958
Frost days
Also of interest in winter are frost days, when the maximum temperature is beneath 0C.
1987 saw 14 consecutive days continuously beneath freezing (14-28 January)
1963 saw 10 consecutive days continuously beneath freezing (16-25 January)
2010 saw 12 consecutive days in December, counting sites in Norther Irealnd and the Scottish Highlands and Islands
The lowest lowland maximum since 1900 is -9.2 at Warlingham (Surrey) on 12 January 1987; Pershore saw a maximum of -8.2 on 19 December 2010, and Altnaharra -15.8 on 22 December 2010.
Cold weather around the world
Malta is the only European temperature never to have recorded a frost (minimum +1.4). Temperatures beneath -50C have been recorded in (European) Russia, Norway, Sweden, and Finland.
The department of Doubs in eastern France has seen some of the lowest temperatures recorded in France, with the coldest being -41.0 at Mouthe on 17 January 1985. For Belgium it's -30.1 (1940) and the Netherlands -27.4 (1942). The lowest temperature recorded in the Republic of Ireland is -19.1ºC at Markree Castle (Co Sligo) on 16 January 1881; since 1900, -18.8ºC at Lullymore (Co Kildare) 2 January 1979; and since 2000, 17.3ºC at Straide (Co Mayo) 25 December 2010. The lowest lowland maximum in Ireland since 1900 is -15.9 at Fyvie Castle on 29 December 1995.
For a nice map of low temperatures in Europe, see viborc.com.
In terms on inhabited places (outside Antartica and Greenland), the lowest temperature recorded was in Siberia, at Oymyakon, with -67.7 on 6 February 1933 (although there is an official record nearby of -71.2 in January 1924). Oymyakon is in the Sakha Republic; the nearest major city is Yakutsk.
The coldest place on Earth is not surprisingly Antartica, with the lowest temperature close to ground level of -89.2 in 1983, and of all time at -93.2 on a ridge between Dome Argus and Dome Fuji (3,900 m up) on 10 August 2010.
See here for the coldest day of each winter in the UK from 1900 (December, January, February).
The coldest temperatures in Britain from 1875
1875 -14.7 Hawes (N Yorks.) 1 January
1876 -12.2 Buxton (Derbys.) 11 February
1877 -15.6 Braemar 3 January
1878 -18.9 Gainford (Durham) 13 December
1879 -26.7 Kelso (Borders) 3 December
1880 -17.7 Braemar 27 December
1881 -26.7 Kelso (Borders) 17 January
1882 -22.4 Braemar 15 December
1883 -15.1 Alston (Cumbria) 15 March
1884 -12.5 Kingussie (Highland) 3 January
1885 -14.9 Braemar 16 January
1886 -18.9 Alston (Cumbria) 7 March
1887 -15.3 Marlborough (Wilts.) 17 March
1888 -17.2 Lairg 16 February
1889 -14.8 Logie Coldstone 5 March
1890 -17.4 Stokesay (Shrops.) 22 December
1891 -17.2 Ketton (Leics.) 18 January; Lairg 12 March
1892 -22.2 Scaleby (Cumbria) 19 February
1893 -20.0 Braemar 6 January
1894 -20.2 Hodsock (Notts.) 6 January
1895 -27.2 Braemar 11 February
1896 -12.8 Braemar 23 December
1897 -21.7 Braemar 4 February
1898 -16.1 Braemar 24 February
1899 -16.1 Braemar 21 March
1900 -19.4 Braemar 11 February
1901 -18.8 Swarraton (Hants.) 9 January
1902 -18.9 Lairg 14 February
1903 -17.2 Braemar 13 January
1904 -14.4 Appleby (Cumbria) 27 November
1905 -15.0 Braemar 18 November
1906 -16.8 Woodbridge (Suffolk) 30 December
1907 -15.0 Llangammarch Wells (Powys) 25 January
1908 -18.3 Liphook (Hants.) 30 December
1909 -17.8 Marlborough 3 March, Balmoral 7 December
1910 -23.3 Balmoral, and Logie Coldstone 28 January
1911 -12.2 Garforth 1 February
1912 -20.6 West Linton 4 February
1913 -15.6 Braemar 31 December
1914 -13.9 Braemar 25 December
1915 -13.3 Balmoral 5 December
1916 -16.7 Braemar 20 February
1917 -20.0 Benson (Oxon) 6 February
1918 -19.4 West Linton (Borders) 14 January
1919 -23.3 Braemar 14 November
1920 -18.3 Raunds 13 December
1921 -15.0 Braemar 7 March
1922 -13.3 West Linton, Eskdalemuir, Garforth 18 January
1923 -14.4 Braemar 25 Decembe
1924 -16.7 Balmoral 3 March
1925 -16.7 Braemar 20 December
1926 -16.7 Luton 17 January
1927 -17.8 Braemar 18 December; Logie Coldstone 17, 18 December
1928 -13.3 Braemar 1 January
1929 -18.3 Usk (Gwent), Ross-on-Wye 14 February; Houghall (Durham)17 February
1930 -16.1 Newport (Shropshire) 20 March
1931 -17.2 Braemar 3 March
1932 -11.7 Dalwhinnie 12 March
1933 -15.6 Logie Coldstone 19 January
1934 -12.2 Braemar 14 March
1935 -15.6 Braemar 24 December
1936 -15.0 Braemar 5, 13 February
1937 -21.7 Braemar 13 December
1938 -15.6 Braemar 22 December
1939 -17.2 Dalwhinnie, Braemar 4 January
1940 -23.3 Rhayader (powys) 21 January
1941 -21.1 Houghall (Durham) 5 January; West Linton (Borders) 6 January
1942 -19.4 Braemar 8 March
1943 -13.9 Braemar 5 January
1944 -15.0 Braemar 28 February
1945 -19.4 Dalwhinnie 26 January
1946 -14.4 Dalwhinnie 26 February
1947 -21.3 Elmstone (Kent) 30 January
1948 -15.6 Logie Coldstone 24 January
1949 -17.8 Braemar 5 January
1950 -18.3 Dalwhinnie 15 December
1951 -16.1 Dalwhinnie 11 March
1952 -17.2 Logie Coldstone 30 January
1953 -12.8 Braemar, Dalwhinnie 3 January
1954 -20.0 Welshpool 2 February
1955 -25.0 Braemar 23 February
1956 -18.9 Perth 3 February
1957 -16.1 Sutton Bonnington (Notts.) 20 February
1958 -22.8 Logie Coldstone (Grampian) 14 March
1959 -17.8 Braemar 15 January; Kincraig (Highland) 4 February
1960 -22.2 Grantown-on-Spey 18 February
1961 -22.2 Cannich (Highland) 30 December
1962 -21.1 Corwen (Clwyd) 2 January
1963 -22.2 Braemar 18 January
1964 -15.6 Caldecott (Leics.) 29 December
1965 -21.7 Corwen (Clwyd) 3 March
1966 -20.0 Braemar 15 Februar
1967 -14.4 Carnwath (Strathclyde) 10 December
1968 -18.3 Braemar 21 February
1969 -20.6 Grantown-on-Spey 18 February
1970 -21.1 Carnwath (Strathclyde), West Linton (Borders) 7 January
1971 -12.8 Braemar 4 January
1972 -18.5 Moorhouse (Cumbria) 31 January
1973 -21.2 Carnwath (Strathclyde) 15 February
1974 -10.2 Lagganlia (Highland) 14 February
1975 -11.4 Balmoral 8 February
1976 -16.2 Carnwath (Strathclyde) 5 December
1977 -18.7 Lagganlia (Highland) 17 January
1978 -22.0 Keith (Grampian) 20 February
1979 -24.6 Carnwath (Strathclyde) 13 January
1980 -17.5 Clashnoir (Grampian) 2 January
1981 -25.2 Shawbury (Shrops.) 13 December
1982 -27.2 Braemar 10 January
1983 -14.2 Lagganlia 19 February
1984 -23.6 Grantown-on-Spey 20 January
1985 -23.4 Lagganlia (Highland) 27 January
1986 -21.2 Grantown-on-Spey 27 February
1987 -19.1 Warsop (Notts.) 13 January
1988 -11.6 Carnwath (Strathclyde) 22 November
1989 -14.0 Tummell Bridge 15 December
1990 -8.4 Grantown-on-Spey 15 February
1991 -16.0 Cawood (N Yorks.) 14 February
1992 -14.2 Grantown-on-Spey 21 December
1993 -15.8 Altnaharra 26 December
1994 -16.1 Braemar 17 February
1995 -27.2 Altnaharra 30 December
1996 -13.0 Aviemore 25 December
1997 -14.5 Santon Downham (Suffolk) 3 January
1998 -17.0 Altnaharra 1 March
1999 -14.0 Sennybridge (south Wales) 20 December
2000 -15.0 Dalmally (Argyll) 30 December
2001 -21.7 Kinbrace (Sutherland) 3 March
2002 -16.1 Grantown 2 January
2003 -18.3 Aviemore 7 January
2004 -15.2 Kinbrace (Sutherland) 19 December
2005 -13.2 Ravensworth (North Yorks.) 29 December
2006 -16.4 Altnaharra 2 March
2007 -13.0 Aboyne 22 December
2008 -12.9 Aviemore 30 December
2009 -18.4 Aviemore 9 February, Braemar 29 December
2010 -22.3 Altnaharra 8 January
2011 -13.0 Althnaharra 8 January
2012 -18.3 Chesham (Bucks.) 11 February
2013 -13.4 Marham (near Norwich, Norfolk) 16 January
2014 -9.0 Cromdale (Morayshire) 27 December
2015 -12.5 Tulloch Bridge, Glascarnoch 19 January
2016 -14.1 Braemar 14 February
2017 -13.0 Shawbury (Shropshire) 12 December
2018 -14.2 Faversham (Kent) 28 February
2019 -15.4 Braemar 1 February
2020 -10.2 Braemar 13 February and Dalwhinnie (30 December)
2021 -23.0 Braemar 11 February
2022 -17.3 Braemar 13 December
2023 -16.0 Altnaharra 9 March
2024