Time to talk about the coldest December on record in the CET series back to 1659..... December 1890.
December 1890 had a CET of -0.8c, this was only 0.1c colder than the second coldest December on record which was of course December 2010. Despite this, it was a far more unremarkable month in terms of the weather conditions and the actual temperatures outside of this CET. Before 2010, this was the most recent December to have a CET below zero.
This cold December was the continuation of a notable November cold spell in 1890 too - just like 2010. Greenwich had recorded a minimum of -7.6c on 29th November 1890 whilst Addington in Buckinghamshire observed -21.1c on the same day though the latter figure is disregarded from official records. For Ireland, the lowest known temperature recorded was -8.9c at Markree Castle on the 28th. Killarney had an air minimum of -8.3c on both the 28th and 29th. There was occurrences of heavy snowfall during the spell especially in the southeast of England.
The first day of the month doesn't look anything like what the CET would suggest. There is an organised Polar Vortex to the north of Europe with winds from a westerly direction over the UK and Ireland. What was a bit different with this chart was the low pressure over the Mediterranean especially Spain which disallowed the high pressure sinking southwards to here and the Atlantic to throw an onslaught at us.
In fact, the weather became largely high pressure dominated directly after the first day. Here's the 500mb height synoptic for the 4th December 1890 for example with an easterly flow bringing cold air across both countries. Away from the north of Scotland and parts of the west of Ireland, it was just cold and very dull.
Low pressure attempted to attack from the west at times, for instance on the 14th below. However, attempts tended to fail (with the exception of the 18th) as the high to the east squeezed out the low pressures or pushed them back into the Atlantic.
The middle part of the month looked like it was going to pull in some very cold air from the east but it just about failed as the Atlantic attacked leaving the cold air just missing the eastern coast of the UK.
Hereford in England got down to -14.4c on December 22nd whilst Kilkenny Castle got down to -7.8c on the 21st. Both of these temperatures were as a result of an area of high pressure. Around the same period December 21st to 23rd, a place called Stokesay (also in England) observed a minimum temperature of -17.2c.
There was two attempts at quite a bitter easterly flow at the end of December which resulted in some light snow for parts but in general, it was just a very dry, anticyclonic, cold and dull month.
In the easterly flows, the south and east of the UK were extremely gloomy. London had approximately a tenth of an hour's worth of sunshine all month during December 1890 easily making it the dullest month on record here. These dull conditions were also apparent in the east of Ireland with Dublin City having only 27.2 hours of sunshine during the month, just barely above Dublin Airport's record dullest December in 2002. Valentia Observatory at 44.1 hours of sunshine was actually relatively close to average whilst Stornoway in the northwest of Scotland had 50.2 hours of sunshine which is above its December average - remember that's a place where normally low pressure systems cross the majority of the time.
It was a relatively dry month for pretty much everywhere with a Scandi High evident.
Besides the very dull conditions and easterly flows, another reason why December 1890 could have been so cold was the polluted air blocking out the sun (also could be to blame for the very dull conditions especially in urban areas like London). This was more than 50 years before the Clean Air Act of 1956 was initiated in the UK.
Due to the dry nature, it wasn't a snowy month at all despite the cold air being there.