How Significant is the Cold UK March of 2013 in the CET?

Well, few UK citizens can still be unaware that March 2013 has been the coldest since 1962, though I’m still baffled why the Met Office jumps the gun on reporting data. There were 4 days to go when their announcement arrived in my Inbox – and clearly that of every newspaper reporter in the land.

Overall, though, the Met Office analysis – which, remember, is based on a series going back only to 1910 – suggests 2013 has been less of an outlier than it is in the Central England Temperature (CET) record.

This is what they say:

“This March is set to be the coldest since 1962 in the UK in the national record dating back to 1910, according to provisional Met Office statistic.

From 1 to 26 March the UK mean temperature was 2.5 °C, which is three degrees below the long term average. This also makes it joint 4th coldest on record in the UK.”

They provide a list:

“March – Top five coldest in the UK

1 1962 1.9 °C
2 1947 2.2 °C
3 1937 2.4 °C
4 1916 2.5 °C
5 1917 2.5 °C”

The discrepancy with the CET is presumably partly because Scotland, although colder than England, has not been as extreme compared to the cold Marches of the 20th century. The Met Office note:

“Looking at individual countries, March 2013 is likely to be the 4th coldest on record for England, joint third coldest for Wales, joint 8th coldest for Scotland and 6th coldest for Northern Ireland.”

Still, I’m rather puzzled why this March is reported as only the 4th coldest in England, particularly when I read in a post the Met Office’s blog that in most English counties it’s been the 2nd coldest after 1962.

It may be that the overall ranking for England will change over the next few days, which would add to my bafflement as to why the Met office makes early announcements. I’d have thought such behaviour was fine for mere bloggers like me, but not what is expected from an authoritative source. Isn’t the difference the same as that between City company analysts and the companies themselves? The former speculate; the latter announce definitive results.

Anyway, it’s also possible that the CET region has been colder than England as a whole relative to the previous cold Marches. I notice on the Met Office blog that this March has not been the second coldest for Yorkshire, Northumberland and Durham. If these are outside the CET area, their significant area would explain the difference in the Met Office rankings for England as a whole.

Focusing just on the CET, it’s still possible that March 2013 could be as cold or colder than 1962, and therefore the equal coldest since 1892 or 1883 (or even the coldest since 1883, though that seems unlikely now).

Although daily maximum temperatures have increased slightly to 6C or so, we’re also expecting some serious frosts (in fact some daily minimum records look vulnerable on 30th and 31st), and the CET website implies it is a (min+max)/2 statistic (as included in the screen-grab below).

Here’s the latest CET information for March:

It’s now very easy to work out what the mean temperature will be at the end of the March, due to the happy coincidence of the mean being 3.1 so far and there being 31 days in the month (regular readers will have noticed that I much prefer ready reckoning methods to those involving calculators or spreadsheets). Obviously, spread over the whole month the 3.1C so far would be 2.7C. That is, if the mean temperature for the remaining 4 days were 0C, that for the month would be 2.7C, the same as 1892 (and lower than 1962s 2.8C). Every 3.1 degree days above 0 (that is ~0.75C mean for the 4 days) adds 0.1C (over 2.7C) for the month as a whole. So if you think it’ll average 1.5C for the rest of the month in the CET region, the mean for the month as a whole will be 2.9C.

Obviously rounding could come into it, so it might be worth noting that the mean to 26th was also 3.1C. If you think (or find out – due to time constraints, I haven’t drilled down on the CET site) that 27th was colder than 3.1C (which seems likely) then just a bit less than 1.5C for the rest of the month – say 1.4C – would likely leave the overall mean at 2.8C.

Here’s the latest ensemble chart for London temperatures from the Weathercast site to help you make your mind up:

My guesstimate is 2.8C, so on that basis I move on to the main point of this post. Just for a bit of fun I put together a chart of the entire CET record for March, with running means:

The picture is not dissimilar to that for the unusually cool summer of 2011. Although this March has been the coldest for “only” 50 years – one might argue that a coldest for 50 years month will occur on average every 50 months, i.e. every 4 and a bit years – global and general UK (including CET) temperatures have increased significantly over the last few decades.

As can be seen from the chart above, this March has been around 3.5 degrees colder than the running mean (depending which you take).

I say this with the health warning (as I gave for summer 2011) that the running means may be dragged down if March is also cold over the next few years – the significance of extreme events can only be fully appreciated in hindsight, and it may be that the warm Marches of the two decades or so before this year will look more exceptional and this year’s less exceptional when we have the complete picture.

Health-warning aside, there aren’t really any other Marches as much as 3.5C colder than the prevailing March temperature. The period 1783-6 stands out on the graph, but isn’t really comparable, because the eruption of the Icelandic volcano Laki gave the country a sulphurous shade, significantly reducing the Sun’s warmth. 1674 looks notable, too, but the monthly means back then seem to be rounded to the nearest degree centigrade, so we can’t be sure it actually was as cold as 1C (at least without considerable further research).

It’s all very curious. After December 2010 (for which I should prepare a similar chart some time) and now March 2013, one wonders whether, when we do get cold snaps, it’s going to be even more of a shock than in the past. Does global warming have much less effect on cold UK winter temperatures than on the long-term average? Or would this March have been even colder had the same weather conditions occurred before the global warming era? March 1883 averaged 1.9C, but was only about 3C colder than prevailing Marches. Perhaps this year’s weather conditions would have resulted in a monthly mean of 1.4C back then! The trouble is we now have no idea whether this March has been a once in 50 years, once a century or once a millennium event.

And has melting the Arctic ice made cold snaps more likely?

Confusion and unpredictability abounds when it comes to extreme weather events. Preparing for the worst – the precautionary principle – is called for.