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.
It’s coming to things when you have to write a story about a couple of nights with a touch of frost (fig 1), but it might be something I’ll have to do in the future as the seasons continue to warm as they have since 1772 (fig 2). The last lying snow in our part of Devon was over the Christmas period of 2010 if I remember correctly, and the last few Winters have only produced a few fleeting flakes of snow before it quickly turned to rain. I see that the GFS have already backed off with the two cold northerly outbreaks that they were hinting at earlier in the week, it’s quite obvious already that it will take a miracle to produce any snow in this part of the world.
Autumn seems to have turned much colder in northern Scandinavia in the last week (fig 3), I can remember plotting the upper air ascent for Sodankyla in Finland a few times in my time at Strike in the 1970’s, it just show’s you what a good snow cover and a long clear night can do at this time of the year.
I wrote an article the other day that included a temperature anomaly chart for 12 UTC on Sunday for parts of the UK. But I thought I would look at back at what was the coldest spell of weather in my lifetime, and luckily for me I can include the winter of 1962-63 in that lifetime (even if I was just playing at being a snow plough!) and that was on Monday the 12th of January 1987. Don’t forget that the plotted values in the chart above (fig 1) are anomalies and not actual values for 12 UTC.
As you can see North Hessary Tor, sadly no longer with us as an observing site, had the lowest anomaly of -16.7°C, but many places across England and Wales had anomalies in the range -12°C to -15°C as you can see. 1000-500 hPa thicknesses in the midday ascents were 506 dam at Aughton, and 500 dam at Hemsby. I wrote a more detailed article about this spell of exceptionally cold weather which may well have been the coldest day in 20th century in the UK, and it was absolutely wonderful!
Just a quick paragraph about how I calculated six hourly mean temperatures for 237 observing sites from 1981 to 2010. It took a lot of file IO, parsing SYNOPs from 43,830 files takes some time even on a fast PC. I then plotted individual annual graphs for the 06-18 maximum, the 18-06 minimum, and for the main synoptic hours 00, 06, 12 and 18 UTC. Then, if there was enough data, I then ran a curve fitting algorithm on the results that gave me a formula that allows me to calculate a daily value for any day of the year. This is exactly the kind of work the Met Office should be doing on our behalf, but which sadly you will never see.
I know I’ve gone on about the simplicity of anomaly charts in the past, but I believe they are of far more use that just sticking a map with the highest and lowest temperatures on. Perhaps someone at Meteogroup is reading this particular blog (who am I kidding), I would urge them to be innovative, and make more use of anomaly charts in weather forecasts on TV when they finally take up the BBC contract. I doubt if we’ll ever see anomaly charts in regular use by the Met Office in their ‘Visual Cortex’ system, just a continuation of ‘shrunken’ Scotland in their maps, and ridiculous looking fronts overlaid on MSLP charts.
Here are the 12 UTC temperature anomalies for 12 UTC on the 12th of November (fig 1) and it’s a cold day everywhere, especially the further east and north that you are. Temperature anomalies are generally in the range 2 to 4°C below the 1981-2010 long-term average for 12 UTC on the 12th of November. But the weather is bright enough away from the east coast and the far southwest, where showers have continued in the fresh or strong N’NW wind.
Meanwhile low Numa continues to track SE and deepen across the south of Germany (fig 2). There’s quite an area of snow developed now in the cold air to the north of the low, I should imagine the Alps are in for a pasting in the next 12 hours.
The coldest September in the UK on record, well in the abridged 1910 temperature series from the Met Office that is, was that of 1952, with a mean anomaly of -2.79°C (fig 1), in fact the headline in the Monthly Weather Report for the month read – Unusually cold.
It was also the fourth coldest September in the CET series that started in 1659, and still holds three extreme minimum daily CET records from 1878, and also four extreme low maximum records as well (fig 2).
September 1952 was generally anticyclonic from the 7th to the 16th, which did allow some night frosts to occur, before turning very cyclonic from the 25th (fig 3). You may have noticed that September 1986 was the joint third coldest in the UK since 1910 (fig 1), but unlike 1952 this was mainly due to the anticyclonic nature of the weather (fig 4), in fact 1986 is the joint 10th most anticyclonic on record since 1871 in the Objective LWT series. So September 1952 was cold both by night and day, whilst 1986 was colder at night than day. In contrast to either of those two years, 2017 has so far turned out so far to be generally colder by day than by night.
Colder air has been pushing down across the northeast of Canada and the United States overnight, bringing that part of the world an early taste of Autumn, and starting to turn the leaves a lovely golden brown colour I should imagine. The highlighted station is Mount Washington, where it was already -2.8°C at 06 UTC this morning (fig 1). Further west, warmer air is being pushed up into the middle states from the south by tropical depression Harvey.
Temperatures in the summer of 2017 in the UK crashed on the 20th of July and have never recovered in the almost three weeks since it’s been since then. You can see clearly how the temperature has almost flat lined in the NCEP reanalysis data for the grid point 52.5° north and 2.5° west (just to the west of Birmingham), with almost all the 6 hour anomalies negative since then, negative (fig 1). I remarked in a blog only yesterday about how unusually flat the daily CET values had been since the 20th of July. Its probably all tied up with that ‘ribbon of high wind speed high in the atmosphere‘ that we like to call the jet stream.
Meanwhile in stark contrast to the UK, just to the northeast of Rome in Italy, at the 42.5° north 12.5° east grid point, things have been slightly different. A part from three short cold spells, the temperature anomalies there have all been well above average since early June (and before), with the recent heatwave this month clearly evident.