Category Archives: Weather

November 2019: coldest for nine years

The November monthly mean finished 6.4C, that’s 1.6C below average, the coldest November since 2010. It was the third month of 2019 where the monthly mean temperature finished below average.

The wet theme of October carried on into November; the 68.5mm collected was 116 per cent of the 1981-2010 average, slightly drier than last years.

Some 59.7hrs of sunshine were recorded, 102 per cent of average, the dullest for four years.

There were six air frosts.

You can find my thoughts on how winter is going to pan out here.

To view full stats follow this link:http://1drv.ms/1kiTuzv

nov t.PNG

Summary for November 2019 – to follow

London winter forecast 2019/20

For this year’s winter forecast, because of time constraints, I am sticking purely to what the local analogues reveal.

Meteorological autumn was the wettest for 18 years and the coolest for 6 years, revealing some interesting similarities with past climate. Considering data back to 1797 I was able to make the following suggestions on how the next 90 days may unfold.

December is most likely to be on the cold side with rainfall slightly below average.

January is most likely to finish average temperature-wise and wetter than December.

London’s February extremes temperature also look average overall and the precipitation signal also creeps up in comparison to the first two winter months.

Best chance for snow? Impossible to pin down but the coldest weather is likely to be at the beginning of February, the third week of January and just after Christmas.

The warmest period looks to be around January 10th while there’s also a signal for that often phenomena of milder temperatures just before Christmas day.

Most of the probabilities in the above statements are relatively low and are explained in the table below.

Mean temperature 2019/20

December January February
1981-2010 average 5.6 1981-2010 average 5.3 1981-2010 average 5.3
Forecast median 4.4 Forecast median 4.7 Forecast median 5
Rather cold 40% Normal 40% Normal 44%
Normal 28% Rather cold 32% Rather cold 32%
Mild 20% Very cold 12% Mild 16%
Cold 4% Mild 12% Cold 4%
Very cold 8% Cold 4% Very cold 4%

 

Rainfall 2019/20

December January February
1981-2010 average 53.2mm 1981-2010 average 55mm 1981-2010 average 40.1mm
Forecast median 48.3mm Forecast median 55mm Forecast median 40mm
Wet 32% Wet 32% Wet 36%
Normal 20% Normal 28% Normal 36%
Dry 48% Dry 36% Dry 28%

Months of perpetual cold

It’s been a while since this region has seen a sustained colder than average period. Though the past few weeks have seen colder than normal weather we have to go back to the beginning of 2015 where at least three consecutive months were colder than the 1981-2010 average.

The spell, which coincided with winter, was unremarkable with just one day of lying snow. The season finished 55th out of 73 of the most recent winters,  the anomaly for the 90-day period was -0.3C. For deep, lasting snowfall events a monthly mean temperature must be at least 2C colder than average.

For anything ‘snow-worthy’ you have to go back seven years to a 10-month long colder than average spell that began in September 2012 and coincided with probably the last winter I can remember that had more than one cold spell with snow that lasted longer than a few days. The average mean for this spell was -1.3C.

The most potent cold period occurred during the winter of 1984/85, a winter where snow lay nearly a foot deep by the end of January and where seven ice days were recorded – these days we’re lucky to record just one ice day per winter! Only just behind was the 3-month period January to March 1987.

As with previous blogs I have devised a way of ranking these cold periods, by dividing the mean with the length in months. The first nine months of 1986 achieved the highest ranking, a period that included the 5th coldest February in this area since 1797.

rank

And in chronological order…

chron

cold month index

It is looking like the mean this November will finish below average, making it the second month in a row, something that hasn’t happened since March last year. Will December make it three in a row and a new addition to the list?

 

October 2019: wettest since 2004

The wet theme of the final week of September carried on into October; the 100.3mm collected was 150 per cent of the 1981-2010 average, the wettest 10th month for 15 years.

The monthly mean finished 11C, that’s 0.8C below average, the coldest October since 2012. It is only the second month of 2019 where the monthly mean temperature finishes below average.

Only 72hrs of sunshine were recorded, 68 per cent of average, the dullest October since 2002.

The area’s first frost arrived 10 days before the median date back to 1959. And 3 days earlier than last year. A shower with >5mm hail was observed on the 9th.

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oct max

Summary for October 2019

Temperature (°C):
Mean (1 minute)  11.5
Mean (min+max)   11.3
Mean Minimum     7.6
Mean Maximum     15.0
Minimum          -0.8 day 27
Maximum          20.4 day 14
Highest Minimum  11.9 day 05
Lowest Maximum   10.3 day 28
Air frosts       1
Rainfall (mm):
Total for month  100.3
Wettest day      16.6 day 05
High rain rate   20.1 day 01
Rain days        20
Dry days         11
Wind (mph):
Highest Gust     21.3 day 25
Average Speed    2.0
Wind Run         1492.7 miles
Gale days        0
Pressure (mb):
Maximum          1026.6 day 29
Minimum          996.6 day 01
Days with snow falling         0
Days with snow lying at 0900   0
Total hours of sunshine        72

September 2019: wettest since 2006

Most of the rain of September 2019 fell in the last week making what was a fairly unremarkable month weatherwise the wettest September for 13 years.

Some 63.9mm of rain fell which is 123 per cent of average.

The monthly mean finished 15.8C, that’s 0.4C above average, the warmest September for 3 years, 0.6C warmer than last year.

Sunshine, at 150.5hrs, 107 per cent of average but over 30 hours fewer than last year.

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sept

Summary for September 2019

Temperature (°C):
Mean (1 minute)  15.5
Mean (min+max)   15.8
Mean Minimum     11.1
Mean Maximum     20.5
Minimum          5.2 day 17
Maximum          26.0 day 15
Highest Minimum  15.7 day 23
Lowest Maximum   16.1 day 09
Air frosts       0
Rainfall (mm):
Total for month  63.9
Wettest day      14.9 day 24
High rain rate   28.2 day 27
Rain days        14
Dry days         16
Wind (mph):
Highest Gust     23.5 day 27
Average Speed    2.5
Wind Run         1809.2 miles
Gale days        0
Pressure (mb):
Maximum          1036.8 day 13
Minimum          992.3 day 29
Days with snow falling         0
Days with snow lying at 0900   0
Total hours of sunshine        0.0

The year snow fell in London in October

Two hundred years ago this October the extremely rare phenomenon of lying snow in October was recorded in London.

A likely plunge of Arctic air on the 21st saw rain turn to snow which lay nearly 8cm deep by the morning and remained for nearly a week.

The month started dry and warm. On the 1st Luke Howard recorded 75°F at his laboratory in Stratford. The warmth remained into the second week with 77°F recorded on the 10th, 11th and 12th.

oct 1819
The values from Luke Howard’s Climate of London.

The wind swung into the north on the 18th and with it arrived the first hoar frosts that were cold enough to kill garden plants.

On the 21st the cold air further dug in and with it rain that turned to sleet. Howard said: “It began to snow about noon falling in very large flakes thick and rapidly for an hour and covering the ground. Some rain followed.

“In the evening the wind rose and it blew hard in the night from NNW. At midnight came a second heavy fall of snow which continued till 6am and though at first much of it melted it lay in the morning a full three inches deep.”

Howard adds that the surroundings ‘took on appearance of mid winter with the single exception of the foliage still remaining on the trees which mingled with an enormous burden of snow presented a very singular and grotesque appearance’.

The weight of the snow was also enough to break off large limbs from fruit trees.

The snow was still lying on the 23rd and, probably caught out by the earliness of the polar plunge, swallows were seen at Stamford Hill. On the 24th a very white frost was observed with a low of 31°F recorded at Tottenham.

More wintry weather followed in November, December and January.

Could snow fall here again in October? The probability is very low but it is not impossible, given the right synoptic conditions. Recent cases of notably positive and negative anomalies following in quick succession somewhat mirror the weather of Howard’s day.

 

 

Collapse of the Weisshorn glacier in 1819

Two hundred years ago this December a huge part of the Weisshorn glacier in Switzerland crashed down several thousand feet to the valley below.

saas fee glacier
A glacier above the village of Saas Grund, Switzerland. Viewed from below I am always struck by how precarious they look.

At 6am on December 27th 1819 the villagers of Randa, near Zermatt, were awoken as millions of tonnes of snow and ice swept away boulders, rocks, gravel and mature larch trees. Though the debris missed the village the force of the slide created a blast of air that moved entire buildings and their contents, burying 12 people, all but two of whom escaped with their lives.

Eyewitnesses described the noise of the falling mass as the loudest thunder and said a bright flash accompanied the slide before darkness once again enveloped the village.

First light revealed the utter devastation of the avalanche that have covered an area of pasture 2,400ft by 1,000ft by 150ft high.

It was not the first avalanche to bring disaster to Randa. In 1636 the village was destroyed by a similar avalanche when 36 people were killed. It is said that that occasion saw a much greater chunk of the flacier fall from the slopes of the mountain, at 14,783ft the 5th highest in the Alps.

Two other less serious falls happened in 1736 and 1786 but not precisely in the same place. This time only a small part of the glacier fell down.

Could a similar disaster happen again? With climate change and the nature of the Alps being constantly on the move it is possible. Earlier this month it was reported that part of a glacier on the Mont Blanc Massif, just 40 miles away as the crow flies, was on the brink of collapse.

I don’t have local figures but a look at the recent climatology in London shows that anomalies during the past couple of years – warmer than average summers and low rainfall – have been similar to what happened in 1819.

summer mean

summer rainfall

Of course the difference between now and 1819 is that we have early warning systems in place that can help prevent loss of human life in the event of a catastrophic avalanche.

weisshorn
The Weisshorn in the far distance seen from the slopes above Grächen

August 2019: warmest for 4 years

Apart from the hot four days in the last week August 2019 was fairly unremarkable.

Overall the monthly mean finished 19.9C, that’s 1.4C above average.

Rainfall at 37.7mm, was 75 per cent of average,  the driest for 3 years. Sunshine, at 180hrs, the sunniest for 3 years.

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august max

Summary for August 2019:

Temperature (°C):

Mean (1 minute)  18.9
Mean (min+max)   19.3
Mean Minimum     13.9
Mean Maximum     24.7
Minimum          8.8 day 20
Maximum          32.6 day 27
Highest Minimum  19.5 day 07
Lowest Maximum   17.9 day 14
Air frosts       0
Rainfall (mm):
Total for month  37.7
Wettest day      12.6 day 14
High rain rate   65.8 day 09
Rain days        11
Dry days         20
Wind (mph):
Highest Gust     22.1 day 10
Average Speed    2.6
Wind Run         1941.4 miles
Gale days        0
Pressure (mb):
Maximum
Minimum
Days with snow falling         0
Days with snow lying at 0900   0

Two Paris heatwaves two centuries apart

In July 1808 Paris wilted in a heatwave. The average maximum for the 13-day spell that began on the 10th was 31°C, higher than a similar spell last month that saw the all-time record for the French capital broken.

The temperature at the peak of last month’s hot spell reached 42.6°C, some 6.4°C higher than the peak of the 1808 spell but, as the graph below shows, maxima fell back more quickly than 1808.

paris heat line

The more ‘pointed’ nature of maximum temperature during the spell last month backs up findings of the changing jet stream;  the wriggly nature amplifying the heat.

Tmin

mean

The average mean and minimum temperature of both spells showed a difference of just 0.2°C.

Since Paris recorded its hottest day ever there have been just two days where the temperature has exceeded 30°C.

paris july 1808
The values for the 1808 spell were listed in Luke Howard’s Climate of London.

The heat in Paris in 1808, like in 2019, was also felt in London. Luke Howard noted the following in The Climate of London.

“Very hot from July 12th to 19th. On the 12th a thermometer in perfect shade in a window in St James’s Park was 81.5 degrees at 3pm, and on the 13th at the same hour, 94 degrees. On the same day four men and seven women were killed by sunstroke in various parts of the Midland counties, and numerous coach and other horses were also killed. On the 15th a very violent and destructive thunderstorm in Gloucestershire, Monmouthshire, and surrounding counties.”

* Values for 2019 were taken from the station Montsouris.
** Though there is no way of knowing how accurate the 1808 values were previous studies have found that historic temperatures can be as much as 3°F too high.