<Tr> <Td> Bradford </Td> <Td> 14% (since 1971) </Td> </Tr> <Tr> <Td> St Mawgan </Td> <Td> 10% (since 1957) </Td> </Tr> <P> 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 . The shift from the Julian to the Gregorian calendar in 1752 also slightly reduced the chance of a white Christmas, effectively moving Christmas earlier in the winter . 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", but formerly the snow had to be observed at the Met Office building in London . By the newer measure, over half of all years have white Christmases, with snow being observed 38 times in the 54 years to 2015 . 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 . </P> <P> 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 . </P>

When did the uk have a white christmas
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