On Wednesday, Denmark recorded its warmest ever January day.

It may not seem an especially high temperature at 12.7 degrees Celsius in the town of Soenderborg, but it hasn’t been seen before anywhere in Denmark since records began, (along with the meteorological service), in 1874.

Denmark is open to weather from the Atlantic, only a little modified in its journey across the UK and the North Sea. Atlantic weather is always relatively mild in the winter, which makes this record warmth all the more surprising.

Denmark's weather in winter is either Siberian cold or Atlantic mild and in the latter case, sea temperatures tend to determine the temperature. In 2005, 12.4C was recorded at the same site in Soenderborg, so this year's figure is a marginal rise, most likely because of a warmer Atlantic, but a new record all the same.

Sea temperatures upwind are about 12C in this part of the North Sea, which is only a little above average. However, the source region for this mild weather was an Atlantic ocean that is currently between 2.5 and 5C above long-term average.

Denmark is the only country so far in northern Europe that has reported an absolute new record for January but is not alone in being abnormally warm. Belgium reported at least one-day maximum/night minimum record with figures of 7C above average.

London, England recorded 14C, 6 above average. Amsterdam, Netherlands at 13.8C was just 0.2C short of a new record for January. Hamburg in northern Germany recorded a similar figure, which is 10C above average.

Warm air carries more water vapour and therefore produces more rain. This wave of mild, moist Atlantic weather is not the first to hit Europe this winter. A series of similar storm systems have left much of France waterlogged.

As the River Seine overflows into Paris, the temperature there too is 6C above average.

Jesper Thielgaard in Belgium contributed to this report