A deposit of tiny, white ice crystals on a surface. Frost forms through sublimation, when water vapor in the air condenses at a temperature below freezing. It gets its white color from tiny air bubbles trapped in the ice crystals.
I am reliably informed that the spring vernal equinox will occur at precisely 1615 UTC today this 20th day of March 2018, but the weather is in a fickle mood at the moment, and spring, well at least the start of it, looks like it will be a cold affair in our part of the world. The Met Office in their extended outlook till the middle of April (fig 1) is being its usual unequivocal self about the possibility.
In line with that outlook, the GFS model for April 1st (fig 2) is forecasting a northerly outbreak which is not at all unusual for spring across the British Isles.
Cold springs are somewhat of a rare commodity these days, and the last one to produce negative anomalies in Central England was back in 2013 (fig 3), and in the last 30 year there have only been three cold springs (2013, 1996 and 1991).
Yesterday, the 18th of March 2018 was the coldest for that particular day in March since 1853. That’s according to the latest provisional Central England mean temperatures released from the Met Office (fig 1). The maximum for Sunday was 0.6°C and the minimum -2.2°C, and the mean of -0.8°C, was almost 6.6°C lower than the 1961-1990 long-term average of 5.8°C.
Only four other years have reported negative mean temperatures for the 18th of March (fig 2 & 3) in the daily mean CET series that started in 1772, they were 1900, 1853, 1814 and 1812.
Don’t forget that you can keep an eye on the latest daily CET values here
Looking at the observations for yesterday and the reported 06 to 18 UTC maximum temperature (fig 4), many stations across central England and Wales had sub-zero maximums, in fact at a number of stations, the temperature has remained below zero the whole weekend, which just two days from the vernal equinox is highly unusual.
I think the thermograph for the last few days from Rothamsted in Hertfordshire gives a good account of how quickly the cold snap set in last Friday, and just how cold the last 48 hours have been across the country (fig 5).
A slightly premature press release from the Met Office regarding the latest cold spell and today’s snow (fig 1). Maybe it would have been better waiting for tomorrow to release it, in light of the fact that it’s still snowing across large parts of Devon and Cornwall (fig 2).
After reporting yesterday that we’ve just experienced the coldest 28th of February since 1785 in Central England, it now transpires that the first of March provisionally has the lowest mean temperature for that day since 1785 (fig 1), and additionally was the coldest day (lowest maximum of -0.8°C) for any March day since 1878.
The last seven days have been cold, but they still only rank 6th in the all time coldest weeks (23rd February – 1st March) since 1772 (fig 2). So if you think the mean of -0.5°C for the last seven days has been cold, the -2.56°C of 1947 was a magnitude colder. It’s very rare these days to see any new extreme minimum records broken, but setting three records in two days isn’t bad going!
Don’t forget you can keep up to date with the latest daily temperatures in Central England by using the interactive graph on my Meteograph website.
March has certainly come in like a lion this year, well a lion is a beast after all. Nowhere across the country seems to have escaped the snow and freezing temperatures, and at 09 UTC this morning 49 cm of snow is being reported at St Athan in south Wales and 46 cm at Bishopton, Glasgow (fig 1). From what I can see from the chart almost every SYNOP station is also reporting an hourly gust of 25 knots or more, on another day of fresh to strong easterly winds, and although not as cold as the last two days, it’s still subzero in many places.
I would say at this point that not all SYNOP stations have snow depth sensors, and some that do aren’t reporting a depth at the moment, probably as a result of severe drifting, which could also be affecting the snow depths from the stations that are reporting one!
Spare a thought for the hill sheep farmers and their livestock, they must be going through a particular frigid time at the moment. Just imagine how much snow must have fallen on higher ground and the intense cold if conditions are like this on Great Dun Fell (fig 2).
I’ve just noticed that there seems to be a disproportionate number of cold last weeks of February and first week in March in the daily CET records (fig 1). Only someone as crazy as me would be writing code on a lovely sunny Sunday with Mediterranean blue skies outside to discover this trivial and useless nugget of information regarding temperatures in central England since the millennium. Statistically six out of the last 245 years in the top 30 is probably no more or less than what you would expect, but in recent years there have been so few examples of any month that was colder than average (apart from 2010) that it came as a bit of a surprise to me. These colder inter month periods have probably gone unnoticed because you often view climate periods on a calendar month basis, or at least I do.
Anyway the original reason why I started doing this little spot of research was to find out just what were the coldest end of February start of March periods since 1772. I could then gauge statistically just how cold the next couple of weeks of cold had been in the series.
The upshot of that work confirmed what I already thought might be the result, and that is that this coming cold spell of very cold weather is going to have been pretty spectacular if it’s to equal or surpass the exceptional same period in 1947 (fig 2).
According to results that I’ve generated, the mean temperature for the period 21st February to the 7th of March was -2.2°C, that ‘s -6.6°C below the long-term average (1961-1990). That’s a clear 1.4°C colder than 1786 the next coldest such like period. Even if we do have a couple of ice days in this coming week in central England I can’t see how the mean temperature can get anywhere near that of 1947, although it might push the means for 2005 and 2006.
This period has shown only a very slight warming over the years of just 0.006°/decade, which equates to a small warming of 0.15°C in the last 245 years (fig 3). Perhaps this is the last vestige of how the temperatures were during the late 18th and 19th centuries in central England, now that’s another crazy thought.
Very cold night across much of Europe with an anticyclone of over 1050 hPa across Norway. I notice that overnight temperatures fell to a minimum of -32.5°C (-26.5°F) at Drevsjo in central Norway (WMO 01393) (fig 2).
Unsurprisingly, it was also where the highest MSLP pressure was to be found (fig 3). It would be interesting to see the adjustment tables they use to calculate a QNH pressure from the station QFE for a station that’s already 674 M above sea level.
Meanwhile back in good old blighty, Dunkeswell in Devon did rather well to get down to -5.0°C overnight (fig 3), to make it the fourth coldest low-level station in WMO block #03 (fig 4).
Apologies for the weird title of this blog – it just came to me after listening to American Pie on Spotify!
It’s interesting to see how this impending cold snap has got many people excited about it, even those that don’t usually give a hoot about the weather most of the time, it’s almost like the announcement of a very early Christmas. I saw a good example of this in a tweet from Weather Outlook (fig 1) saying how the ‘thickness’ would fall below 500 dam in NE Scotland – really?
Just to check that I hadn’t just entered the Twilight Zone I looked at one of the few sites that does include a 1000-500 hPa partial thickness forecast chart these days from NOAA (fig 2).
As you can see 1000-500 hPa thicknesses for next Wednesday are expected to be below 528 dam (the old blue or snow line) across most of the country except the west of Ireland and NW Scotland, and the 510 dam (the old brown line) has just about engulfed Belgium. To put this cold spell into some kind of perspective here is the thickness chart from the 13th January 1987, in what I reckon was the coldest couple of days in the British Isles of the whole 2oth century (fig 3).
I have seen in the last few days people displaying forecast sequences of 850 hPa temperature charts on social media – which I also like to do – and people on Facebook and Twitter thinking that the -15°C isotherm at 5000 feet is the forecast surface temperature!
So in the scheme of things a good cold snap, even very cold for a day or so, but not record breakingly cold. One thing that would make this cold spell memorable though is the length of time it persists into March – which at the moment is still up in the air!
There’s good inter-model agreement now at T+72 for the start of the anticyclonic easterly brought about in part by the recent SSW event over the North Pole (fig 1).
Early next week looks particularly cold with a cold vortex of -19°C at 850 hPa temperatures across central Germany (fig 2). Those low temperatures at 850 hPa equate to 12 UTC surface temperatures below freezing for much of the UK (fig 3).
If this meteogram is correct it looks like as well as being dry, windy and very cold in the southwest, it should also be sunny by day and clear by night (fig 4), and there’s an interesting blip in the hyetograph by day 10 to give us something to look forward to as well.
This item just wouldn’t have been the same without the help of the excellent NWP products from wxcharts, lets hope they don’t start charging us for the privilege anytime soon! This is exactly the kind of free service that we should expect from our own Met Office – it’s going to get very cold in the next few weeks – but hell would have to freeze over before that ever happened.
St Valentines day, the 13/14th of February is the coldest night of the year in Central England, this is based on the lowest mean daily minimum temperature record that started in 1878, the mean for that day is just 0.67°C, with a 40% chance of an air frost, and a 4.3% of an ice day. The lowest mean daily maximum on the other hand occurs around four weeks earlier on the 17th of January with a temperature of 6.1°C (fig 1).