I see the Daily Mail is now suggesting that 2013’s “could be Britain’s coldest March since 1892.”

The nation-wide statistics published by the Met Office only go back to 1910, so the Central England Temperature (CET) record is needed to put current weather in a long-term context.

1892 is an odd year for the Daily Mail to choose, since the CET for March that year was 2.7C, whereas 1962, the year we have to “beat” for it to be the coldest since 1892, saw a mean March CET of 2.8C. We’re unlikely to say this March is “the coldest since 1883”, since if it comes in at 2.8C we’d probably say it’s the “*equal* coldest since 1892″ and if it comes in at 2.7C we might say it’s the “*equal* coldest since 1883″.

Furthermore, given the possibility of rounding, the difference between 1892 and 1962 could be much less than 0.1C, for all I know.

In addition, the difficulties of calibrating temperature readings between 1892 and 1962 make a difference of 0.1C in a monthly mean fairly insignificant (and probably statistically insignificant). To put it another way, the error bars on the temperatures are probably greater than 0.1C. Perhaps we shouldn’t really be quibbling over the difference between monthly means of 2.7C and 2.8C. But then again, we do like our weather records!

If this March is colder even than that in 1892, the next mark is 1883, when March saw a CET of 1.9C. It’s no longer on the cards for it to be as cold as 1.9C this March.

But what are the chances of the CET this March being colder than 2.8C or even 2.7C?

Here’s the state of play at the moment:

This is moreorless in line with my projection of a few days ago. But that was based on ensemble forecasts on 22nd March, and, as I noted yesterday, the forecast for the rest of March has just kept getting colder since 22nd:

Based on the forecast for 22nd March I wrote:

“Ignoring today (22nd) as transitional, it now looks likely that the 5 days 23rd through 27th March will be seriously cold, so let’s knock 0.1C off the monthly average for each of them. That gets us down to 3.1C.

The 28th will most likely be around the new average (3.1C), so it all depends on when the mild air comes in from the Atlantic. The computer model runs (grey lines) differ, and the average (yellow line) for 30th and 31st are for it to be relatively mild. If that’s the case, then we’d need to add on 0.1C for each day, so would roughly equal 1969.”

It’s certainly now not the case that 30th and 31st will be “relatively mild”, so we won’t have to add on 0.1C for each of those days. This March is therefore very likely to be colder than 1969 (3.3C) and therefore the coldest since at least 1962.

But could it be even colder than the 2.8C in 1962?

Here’s a larger image of the current ensemble forecast from the Weathercast site:

The CET mean for March so far is 3.2C. To depress this average the mean for the rest of March would have to be lower than 3.2C, obviously. And since there’s 31 days in the month, each degree it is lower than 3.2C over one day (what we might call a degree-day) will depress the monthly mean by 1/31st of a degree.

The ensemble chart suggests the mean temperature for London for the rest of March will be about 1.5C – your judgement is as good as mine – over 6 days, so that’s very roughly 12 degree days lower than 3.2C (about 2C each day), so dividing by 30 (rounding 31), we might expect the mean for the month to come out about 0.4C lower than it is now, at **2.8C**.

This estimate is very rough and ready since I’ve assumed in particular that London is representative of the CET region. It’s quite possible the region as a whole will be colder than London. Not only might this be the case generally, but there’s a lot of lying snow in more northerly areas, which tends to depress temperature readings (because it resists warming by reflecting sunlight and because its latent heat buffers warming of the ground surface at about 0C, both preventing warming of the air above it, and it is the near-ground air temperature that’s being measured).

Additionally, I’ve noticed the CET is sometimes adjusted downwards before the final figure for the month is published, a few days into the next month. I don’t know why this is. Maybe the data for more remote (and colder) weather-stations is slow to come in. Or maybe it’s to counter for the urban heat island effect, to ensure figures are calibrated over the entire duration of the CET.

By way of a sanity-check, here’s another view of much the same ensemble data as in the previous image, from the Wetterzentrale site:

Note that to depress the average for the month so far the temperature would need to be around 4C less than usual, since the mean CET mean (!) for the whole of March is about 5.7C and it’s near the end of the month when mean daily temperatures around 7C would be typical. On that basis the Wetterzentrale maps suggest that 12 degree-days lower than the mean for the month so far is a reasonably estimate for the outlook over the next 6 days.

If a best guess is that the mean CET for March 2013 is 2.8C (“equal coldest since 1892”), with some uncertainty, it certainly seems possible that it could instead come in at 2.7C (“equal coldest since 1883”). In either case, though, it might be more accurate to simply say it has been one of the coldest 3 Marches since March 1883. I like to be fairly conservative, but I suppose there’s just an outside chance the mean CET this month could be even lower, at 2.6C, say, in which case we’d probably claim it has been *the* coldest since 1883.

Of course, this is all just estimation: the mean CET for March 2013 might end up “only” as cold as say 2.9C, the coldest for 51 years!

Last night was very cold, so it looks like the *provisional* CET for March will come in at 2.85 to 2.9.

I’ve seen a tendency for rounding down between provisional CET and the final values issued 2 or 3 days into the next month. I remember Dec 2010 shocked a few folk – the provisional mean was -0.2 or -0.3 then the final value came in at -0.7 and just a whisker away from the coldest December in 350 years. The rounding down may be bigger during cold, calm weather, but this month has been windy, depressing day more so than night temperatures, so there may be less rounding down than in that exceptional month 2 years ago.

Overall, there’s a chance the final CET might come in at 2.6 to 2.7

Comment by John Smith — March 31, 2013 @ 12:58 pm