For the UK, a record-breaking mild and wet December 2015 will go down in history. But it wasn't just here that records were set as 2015 was the warmest year on record globally.
The UK was in a warm, moist tropical air mass for most of December as unseasonably mild winds came from the south or southwest carrying very high levels of moisture.
It was extraordinarily wet and often windy, with frequent storms including Desmond, Eva and Frank delivering record-breaking rainfall over Scotland, Wales and northern England. Records were broken across Europe and North America in very warm temperatures for the time of year which were more like spring than winter.
The UK mean temperature for December was record breaking at 7.9 °C, which is 4.1 °C above the long-term average. The previous record was 6.9 °C in 1934. In the Central England Temperature series, the December value of 9.7 °C was 5.1 °C above the long-term average. This is by far the largest difference above average for any month in the series, which extends back to 1659.
The UK mean temperature for 2015 overall was a notably warm 9.2 °C compared to the warmest year of 2014 which was 9.9 °C.
Along with remarkable warmth, there was almost a complete lack of air frost across much of England. Although there were frosts in places, particularly over high ground and Scotland, the UK had fewer than three days of air frost on average which is eight days below the long term average.
It was not only the wettest December for the UK on record, but also the wettest calendar month overall on record in our archive which goes back to 1910. Several stations recorded over 1 metre of rain in December, and Capel Curig, in Snowdonia, recorded over 2 metres for November and December combined.
2015 overall was provisionally the sixth wettest year for the UK in the series and notably 2012, 2014 and 2015 all fall into the top six. 2015 was also the second-wettest year for Scotland and equal third wettest for Northern Ireland in series back to 1910.
Severe flooding affected Cumbria in early December, and became widespread across North Wales, northern England and Scotland after Christmas, impacting many people and their homes. November was also a very wet month in the flood-affected areas, and the saturated ground conditions greatly contributed to the severity of the flooding in December.
We supported partners and government to ensure warnings and advice were consistently communicated. Our National Climate Information Centre confirmed the wettest 24 hour period ever of 341.4mm recorded at an observing station in Honister, Cumbria.
We are now contributing to the National Flood Resilience Review, due to report to Parliament in June 2016, which is considering future flooding impacts to help inform risk assessments of critical infrastructure to better protect the country from flooding and extreme weather.
Links to El Niño
El Niño - a natural intermittent warming of the Pacific Ocean - was very strong in 2015 which meant that the air contained more moisture than usual. This atmospheric moisture fed storms that formed on a stronger than normal jet stream.
The potential for December to be stormy and wet was picked up in our three-month outlook and was consistent with what we expect when there is a strong El Niño. However, analysis suggests that December's extreme weather might be linked to the detailed structure of this El Niño, the warmth of the northeast Pacific Ocean and their combined effects on atmospheric circulation.
In addition, climate change could have intensified the naturally arising period of very wet and stormy weather. Warming of the oceans due to climate change increases the moisture content of the atmosphere. This extra moisture provides additional energy to weather systems, enabling more moisture to be drawn into systems therefore increasing rainfall over the UK.
2015 warmest year on record
The average global temperature in 2015 was 0.75 °C higher than the long-term average between 1961 and 1990. It was also considerably higher than the previous record of 0.57 °C in 2014.
2015 was 0.75 ±0.1 °C above the long-term (1961-1990) average, a record since at least 1850. When compared with the pre-industrial period, the 2015 average global temperature was around 1 °C above the long-term average from 1850 to 1900.
2016 forecast to be among warmest years
The global mean temperature for 2016 is expected to be between 0.72 °C and 0.96 °C above the long-term (1961-1990) average of 14 °C, with a central estimate of 0.84 °C. The forecast considers man-made global warming, combined with the effect of El Niño from unusually warm waters in the tropical Pacific Ocean.