Hours of frost

Frost is a rare beast this year, even in the Wanstead Park frost hollow.

So far this year I’ve recorded just 82.5 hours where temperatures at 4ft (1.2m) were -0.1C or below. That’s just 37 per cent of what is recorded in an average year. And would suggest that frosts in December will be above average.

The lack of frost this year is even more remarkable given that the past seven years, especially the winters, have been so mild.

A closer look at the data shows that winter months are no guarantee of seeing temperatures fall below freezing. The record mild December 2015 and February 2014 are testament to that.

The fact that January 2017 looks the most remarkable month for frost reveals just how mild recent years have been.

Cathedrals of snow

Snow is ever present in the UK in most years though you have to look hard to find it in high summer.

A dedicated group of enthusiasts, led by Iain Cameron, chart and catalogue these snow patches – many of which are hidden or only seen as distant white dots during Scottish Highland walks.

Responding to an appeal in September I met up with Iain and other volunteers to chart the patches in Observatory Gully on Ben Nevis, the UK’s tallest mountain.

Our route up in the shadow of the magnificent north face of the Ben was a warm one; to our left a procession of ant-like figures were tracking the CMD arrete route to the summit, walkers making the most of the amazing weather.

The North Face path up to the Ben was easy to follow under clear blue skies. My previous visits to the fogged-in summit required map and compass.

Out of the sunshine it was noticeably colder in Observatory Gully; being north facing it sees very little of the sun even in mid June, slowing down the rate of melt of the snow which can be tens of feet thick by the end of the snowfall season. Being encased in snow for so long obviously has a chilling effect on the rock.

The walk up on the scree was hard work and I was surprised to see so many debris including parts of large parts of old galvanised chimney cowls, the legacy of the observatory that once sat proudly on top of the Ben. Not many people trek this way, being a bit of a cul-de-sac. To reach the 1,345m summit from here requires a 200m climb at the end.

As we edged higher the dot of snow grew ever larger and is surprisingly big by the time we reached it, Iain was surprised how hard the snow was. From Iain’s reports, published in the Royal Meteorological Society’s Weather magazine, I’d been fascinated by the images of the patches with naturally carved tunnels underneath them; the light inside the ‘cave’ has a gorgeous blue hue to it.

The light inside the snow patch has a gorgeous blue hue to it. It is understandable why so many are drawn to these ‘cathedrals of snow’.

It was at this moment that I realised that this was a large part of the draw of tracking these patches; a fascination with snow in an area where the odds of it existing all year round seem to be forever diminishing.

At the start of Observatory Gully the snow patch is a tiny dot.
On closer inspection the snow patch is a few dozen feet across and is far bigger than it looks from the valley.

Seasons of melt and snowfall

Snow has only vanished entirely in Scotland six times. Three of these occasions were in 2003, 2006 and 2017.

Mean temperature statistics from Cairngorm Summit show just how much the seasons can vary. Of course mean temperature is just one variable that affects snow survival rates. For example the Western Highlands saw huge amounts of snow fall in February and March above 300m, a factor that would have helped snow survive. Though the most recent melt season was 0.4C cooler than 2019 there was probably a far greater volume of snow.

* Because of the unreliable nature of mountain-top records the Met Office data for Cairngorm Summit has some large gaps, it is only the last 5 years that have complete records . I discounted any years that were missing more than 5% of data.

Record rainfall in October 2020

The wettest October since at least 1797 was recorded last month with some 159.2mm collected, 238 per cent of average.

The month started wet with the daily October rainfall record also broken with 49.1mm on the 2nd, the wettest day ever recorded in the UK.

With all that rain sunshine was at a premium; the 51.6hrs recorded was the second lowest October total since 1881. Only 1894 was duller.

The monthly mean was 11.7C, 0.1C below average.

Does any of the above have any bearing on what we can expect this winter? Possibly, but there are so many variables to consider.

Looking at rainfall patterns alone would suggest mean temperatures in November being a degree cooler than average and December broadly average.

Rainfall in November average while December is slightly below.

Make your own grass minimum thermometer

Traditional grass minimum thermometers are expensive. And delicate. Over the years I’ve lost a few of these spirit-filled devices thanks to curious foxes and other wildlife.

But using a £20 datalogger off eBay – units that are used for cold chain supply – and junk found in the garden I’ve found a solution.

As well as the logger and probe you’ll need:
Access to a PC
An old pickle jar with lid, preferably with a wide neck.
1.6m garden cane, cut into 8x 20cm lengths.
Thin garden wire.

list

Once you decide where to place the device (a part of lawn with limited traffic) dig a hole deep enough so just the very top is exposed.

Drill a small hole (big enough for the wire to pass through and the probe to fit snugly) into the top 2cm of each cane before carefully hammering each length into the surrounding turf, leaving 10cm exposed. The last length- that will host the probe – should go in the centre.

Drill a small hole in the lid that is just big enough for the probe to pass through.

You are now ready to place the datalogger into the jar to start recording.

done

First you’ll need to set up the device on your PC. Programs to do this are available to download.

The devices can hold up to 16,000 records. I set mine to 1 minute intervals, allowing for a continuous 11 days of records.

The enclosure is more to stop any foot traffic kicking over and damaging the probe. If wildlife proves to be a problem there is an option of running a very-wide gauge grill over the top.

Two warm April days 128 years apart

The synoptic pattern on Sunday, April 5th, was very similar to the pattern on Tuesday, April 5th 1892.

With so few planes in the sky because of the coronavirus lockdown it offered an ideal opportunity to compare temperatures and sunshine totals between now and then.

Sunday dawned sunny and clear and stayed that way until dusk, some 11 hours of sunshine recorded, exactly the same as 1892!

The temperature in Wanstead reached 22.3C, 0.9C cooler than what was recorded at the Royal Observatory, Greenwich, in 1892. This maximum was reached after an overnight minima of 5.3C, the same as the 41.5F recorded at Greenwich all those years ago.

Looking further afield, and at the spell over 3 days… markmpcc

4th

5thww

6th

 

 

 

 

London’s colder than average months

Last November was on the cold side prompting me to investigate whether we were about to record a third month in a row below average. December turned out to be mild and wet, the lack of snow especially stark in Scotland.

The findings of that study showed that any sustained period of colder than average months was more likely to happen during months of March, April and May, nothing unusual there, especially considering H.H Lamb’s weather types.

I decided to scrutinise further all the colder than average months in this area, considering the 1981-2010 average, back to 1981. This gave the below results.

neg anoms

The dataset covers 399 months, of which 200 were colder than average.

The overall picture shows that negative anomalies are becoming more and more rare, though with notable exceptions being March 2013, December 2010 and January 2010.

The only month that has showed any sort of consistent general decrease in negative anomaly is November.

*For good snowfall at this station needs a negative anomaly of 2C during the months of November, December, January and February.

Winter 2019/20 forecast review

For those interested my winter forecast this year was way out. A prediction of a mean of 4.7C was 2.3C too low. The chief culprit for the 7th mildest winter back to 1797 was most probably the strong polar vortex which has often been at record strength over the past three months.

Rainfall prediction was also over 100mm too low. The wettest winter for 25 years with 248.1mm recorded places it 13th in wettest winters. The only crumb of comfort I can take is that the stats indicated an uptick in precipitation in February! The 90.8mm recorded made it the wettest February for 10 years, just short of 2010. Before that you have to go back to 1951 to find a wetter February. It places 9th in wettest Februaries since 1797.

Over the past months I have looked into the method of seasonal prediction and found some interesting results. For example, the winter of 1989/90 – a winter that was very similar to this one – led to a prediction that was 2.7C too low. Similarly the winter of 2013/14 was predicted 2C too low.

Rainfall looks to be far more random. Though it is obvious that we are now in a wetter than average spell experience shows that it is impossible to tell how long this will last.

Pattern matching and singuarities can be helpful in long distance forecasting. The spanner in the works, however, can be a sharply positive or negative ENSO (El Nino / La Nina) or, as in this year, a very strong polar vortex.

You can find the original forecast here.

anom
The coldest anomaly was at the start of winter.

 

The relentless wind

Damage from Storm Ciara was a lot less notable than further north though the relentless wind saw three records broken locally. Though the gusts were nothing like the St Jude storm in October 2013 the sustained wind blew at its greatest 1, 2 and 3-day rate since this particular automatic station was reset in November 2012.

wind run

Over the 3 days the wind direction was locked in a south-westerly, from 199 to 203 degrees.

9th10th11th

Dynamic early final warming and spring

In my search for some winter weather a tweet by Amy H Butler about dynamic final warmings piqued my interest.

According to the atmospheric scientist a winter where there was no major disruptions of the polar vortex (SSW) we are more likely to see a dynamic early final warming. A table published by Wiley shows the final warming dates.

FW
* means there was a SSW, bold means late FW (after the mean date of April 15).

So what could this mean for the weather in the London area? Considering all the above years with no SSW gives an average date of April 19th for a dynamic final warming.

I then looked at the TMax anomaly for those years for 60 days following a DFW and came up with the following graph.

june cold

The results suggest temperatures in April will be heading down in the final week for a  cold end. The average to cool theme continues into May before temperatures lift in the final week for a warm end, with anomalies up to 5C above average. June, however, looks shocking with temperatures nearly 6C below average by the 16th.

This winter has so far been very similar in type to 1990. The dynamic final warming that year was among the latest in the list and led to a cool and dull June with anomalies in the second week nearly 7C below average!

1990

2019 Wanstead annual weather review

This year finished as the 17th warmest on record – some 0.4C warmer than the 1981-2010 average. Although mild it was the coolest for six years.

After a dry start a wet autumn saw annual rainfall finish above average, the wettest for five years, 105 97 per cent of average.

Sunshine was down on 2018 though the total was just above average.

2019

2019 temp

2019 rain

2019 sun

For a review of each month, click JanuaryFebruaryMarchAprilMayJune, JulyAugustSeptemberOctoberNovemberDecember

You can read the national review of weather events at the Met Office blog here.

 

Summary for 2019

Temperature (°C):
Mean (1 minute)  11.7
Mean (min+max)   11.7
Mean Minimum     7.3
Mean Maximum     16.1
Minimum          -6.5 day 30/01
Maximum          36.8 day 25/07
Highest Minimum  19.6 day 23/07
Lowest Maximum   1.3 day 23/01
Air frosts       43
Rainfall (mm):
Total for year  667.4
Wettest day      20.6 day 19/07
High rain rate   65.8 day 09/08
Rain days        158
Dry days         207
Wind (mph):
Highest Gust     36.5 day 10/03
Average Speed    2.5
Wind Run         22211.5 miles
Gale days        0
Pressure (mb):
Maximum          1053.4 day 09/08
Minimum          969.8 day 12/12
Days with snow falling         5
Days with snow lying at 0900   2
Total hours of sunshine        1451

Meteorology-based musings about east London and beyond

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