Category Archives: Heatwaves

Diurnal temps present a clothing challenge

The erratic onset of spring in some years often presents that problem of what to wear every morning. During the current warm spell I’ve seen all manner of attire on the school run; everything from T-shirt and shorts to full winter regalia topped off with hat and gloves.

The position of the sun is now bringing in to range the season when the gap in temperature between day and night can be at its greatest.

On Saturday (23/2), virtually unbroken sunshine and ‘thick’ air saw the temperature in Wanstead peak at 16.1C before clear conditions overnight saw the minimum plunge to just 0.2C. The gap of 15.9C represents the fourth highest diurnal temperature range for February in this area back to 1959.

feb diurnal
Top 10 February diurnal temp ranges

 

Looking at the year as a whole the greatest range is 20.8C with the months of May and June the most likely to see the condition.

top 10 diunrals
Top 10 diurnal temperature ranges since 1959

 

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A winter heatwave

It is 21 years ago this month that London and most of the southern half of the UK experienced a remarkably warm spell of weather during what had been a mild winter.

The spell, which saw the record warmest February day on the 13th (19.7C), satisfies the Met Office’s old criteria of a heatwave whereby the maximum temperature is 5C or more above average for five consecutive days.

While these spells are fairly common in summer they are very rare during a meteorological winter. During the last 60 years the only other periods to have experienced a heatwave in winter are December 1966 and 2015.

Weather charts for this week look remarkably similar though, according to the latest forecasts, values will be nothing like they were in 1998.

Since this was published this 2019 spell has satisfied the Met Office criteria. But the anomalies are not yet as impressive as 1998.

1998 2019

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feb 1998 maxes

 

 

40C in London end of July? Probably not

The long range weather models are causing much excitement on various forums with one run predicting an anomaly of +16C on Sunday, July 29th.

july29th

Such an anomaly would see temperatures exceed 40C in London, unprecedented looking back at records to 1841; the highest temperature recorded in the UK was 38.5C at Brogdale, near Faversham, Kent, in Augut 2003.

Though it is improbable it is not impossible. Back in April conditions allowed the temperature to rocket to a monthly record of 29.1C, a positive anomaly of 15.5C!
A repeat of similar synoptic conditions would be needed – these would obviously be helped by the record meteorological drought conditions this area is currently experiencing.

Because reliable thermometer records of heatwaves only go back as far as 1840 it is impossible to quantify whether 40C has ever been exceeded in the UK prior to then.

However, accounts of the heatwave of July 1808 suggest parts of England may have come close. Far removed from images of freezing Georgian winters and miserable summers the July of 205 years ago was among the warmest ever. The monthly mean for July 1808, according to the Central England Temperature series, was 18.4C – the 6th hottest July since the beginning of the series in 1659.

Luke Howard, the ‘father of meteorology’ who at the time lived in Plaistow, referred to the heatwave in his diary on July 13th: “Temperature at 9am 84F. The intense heat of the maximum lasted nearly three hours till about 4pm. At 6pm the temperature was 90F.” Another entry mentions a reading taken nearby. “Another at Plashet, a mile and a half eastward, indicated 96F as the maximum under the shade of a house.”

While Howard’s methods of measuring the temperature ran short of modern standards, his thermometer was hung under a laurel bush, the values still give a valid insight into the heatwave.

Tales of the heatwave, which particularly affected east and north-east England, can be seen in letters sent to local newspapers around the country. Many describe labourers dying from heat exhaustion while working in fields. Farm animals and horses suffered a similar fate.

One letter from Hull, published in the Coventry Mercury, said: “At Sigglesthorne, the honey in some beehives melted, ran out upon the ground, and most of the bees drowned in it. At Sutton, a lamb and a dog belonging to the Rev Mr Croft of Rowley, expired in the heat; and several birds dropped down dead, while flying over the streets of this town.”
Of course it is impossible to know about the health of people and animals that died but that birds dropped out the sky suggests extreme heat.

 

London droughts back to 1871

A thunderstorm on July 27th has ended a dry spell that lasted nearly 7 weeks in east London. As storms go it was a fairly tame affair, just 1.3mm fell, the first rain in 47 days, 20 hours and 14 mins.

drought rankThere are many descriptions of drought but the one I am using here, for sheer simplicity of comparison back to 1871, is the definition used up until the 1990s; that is 15 consecutive days with less than 0.25mm (0.01 inches) rain on any one day.

Meteorological droughts occur in most years though obviously ones that occur in summer are far more noticeable than those in winter. Since 1871 there have been 35 calendar years where no drought has taken place.

The longest drought before this one in 2018, probably not surprising for those who remember it, occurred during the long hot summer of 1976. The fact that summer came on top of a very dry winter, rainfall that season was about a third of what was recorded here last winter, meant that water supplies were in a much worse state, with hosepipe bans common.

Other drought years to feature include 1959, which saw the 3rd sunniest summer on record, 1929 and 1995, a summer which saw one of the hottest heatwaves on record.

For the stats I’ve used local rainfall figures back to 1959 and then stats used at Kew to 1871.

individual droughts
This graphic shows droughts were most common between 2000 and 1921. Apart from this year the length of droughts seems to be declining

Looking at the results more closely I’ve divided them into their meteorological seasons.

spring drought

summer drought

autumn drought

winter drought

 

 

 

More anomalous warmth for late May Bank Holiday?

Weather models are hinting that this coming bank holiday weekend could be very warm indeed, with one WX Charts animation suggesting that a positive anomaly of 12C could be possible on Monday.

A date record looks a formality and, unless there’s a downgrade, new monthly high temperature records at stations across the south-east look a possibility.

While it looks very warm it is too early to start talking about heatwaves. And, considering the pattern since late February, with hot and cold alternating weekly, a brief return to much cooler conditions at the beginning of June can’t be ruled out.

The British record high temperature for May is 32.8C, reached on two occasions:

Camden Square, London May 22nd 1922. The same station recorded this value again on May 29th 1944, along with Regents Park, Horsham and Tunbridge Wells.

The national date record for the 28th, according to TORRO, is 30.6C at Camden Square.

Locally the highest temperature recorded in this area in May is 32.1C on May 27th 2005. The date record I recorded is 26.5C in 2012.

 

 

GIF

April 2018: four seasons in one month

I normally headline these monthly reviews by referring to the most notable weather but this April, often a fairly non-descript mid-spring affair, offered pretty much every type of weather. bluebells

The mean temperature finished 11.7C, 1.9C above average and the warmest for four years.

Rainfall was 55mm, 129 per cent of average and the wettest for 6 years.

Sunshine was just 110 hours, the dullest for 40 years and the 16th dullest since 1881.

Hidden in the positive monthly anomaly was the warmest April day in a local record going back to 1959: 29.1C on the 19th – a figure that represents a positive anomaly of 15.5C and occurring at the start of the warmest April heatwave since 2011. It was remarkable that such an anomaly happened so close to a record negative anomaly the previous month.

Yet, just over a week later, temperatures lurched cold again with one of coldest last days of April on record. Though the 24hr record wasn’t broken the noon-6pm record going back 60 years was beaten.

The wettest day occurred on the 9th with 10mm falling.

Air frosts: 0, Ground frosts: 1

sunset 22nd

18th warmest

29.1

rainapril30
April 30th provided a timely reminder after the heatwave to ne’er cast a clout till May is out. A low pressure system in the North Sea brought a thoroughly miserable day though even more rain fell across the Channel.
april30
It was a month with some amazing sunsets. This one, on April 30th, happened just as the sky cleared before dusk

 

Two April heatwaves compared

The stats are in and the average maxima for the past few days’ heatwave is 1C lower than the hot spell in 2011: a positive anomaly of 10.6C over six days.

Screen Shot 2018-04-23 at 13.34.29Screen Shot 2018-04-23 at 13.34.21

Although this year’s spell pipped the previous April high temp by 0.2C, the 2011 spell saw slightly higher temperatures and were recorded in a month that finished 3.3C above average – the average for this April is likely to finish only 2C above average.

April 2011 also saw just 3.3mm of rain recorded. The rainfall so far this month is already at 36.7mm, with more likely before the end of the month.

The spell in 2011 saw 74 hours of sunshine in total while this spell only saw 58 hours.

In terms of this summer it is quite possible that we won’t see a week with higher maximum anomalies or more sunshine than we saw across the southeast this week. To give you any idea of just how anon. warm last week was here is what the equivalent temperatures would be at the end of July, on average the warmest time of the year.

25th 29.8
26th 35.4
27th 39.2 (102.6F)
28th 37.7
29th 32.1
30th 33.3

* The Met Office definition of a heatwave (the same as the WMO) states:  “When the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5 °C, the normal period being 1961-1990”.

 

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Screen Shot 2018-04-23 at 13.44.09
The large anomalies of the past few days can be seen on this thermo trace

Phew! What a scorcher…

I’ve seen this phrase uttered more than once over the past couple of days thanks to high temperatures and humidity. But ask anyone to define a hot day and you’ll get a different answer every time.

Growing up in the 1970s / 80s redtop newspapers would use the phrase once the mercury was nudging 80F (26.7C). But to ‘scorch’ you need sunshine, preferably at least 10 hours of it. Considering statistics from the Heathrow airport climate station in west London there have been 463 scorchers since 1959, the most recent happening on July 5th with 29.5C recorded and 13.9 hours of sunshine. There have now been 9 scorchers this year, already matching the number that were recorded in 2014 and only 3 short of last year.

But even with last month’s heatwave this year has some way to go, however, to match the amount measured in 1976 and 1995: 31 days!

scorchers

 

Hot spell is minnow among historic heatwaves

Heatwave hype has been building over the past few days and, for once, the media has been justified in its coverage of the current spell.

Temperatures over the past few days have been enough for the Met Office to trigger its Heath-Health Watch whereby the mean temperature (an average of the maximum and minimum) for two consecutive days reaches 23.5C.

A couple of years ago I compiled a list of heatwaves for this area back to 1852 where the mean temperature reaches at least 23.5C three days running. This spell, the only other June heatwave (the other being 1976!), only just scrapes into the list.

Although it felt hot this spell, in terms of Premier League quality, was more Middlesbrough than Chelsea.heatwave table

june heatForecasters say today is likely to be the hottest June day for 40 years though I’d be surprised if we exceed the 34.2C recorded on June 27th 2011. It would be the fifth day running that 30C has been reached in this area, enough to fulfil the World Meteorological Organisation’s (WMO) definition of a heatwave.

Whatever happens today the fact that the heat has been produced by a tropical maritime airmass is remarkable. With France and Spain recording some very high temperatures of late the same pattern in July and August could put more records at risk of being broken.

This week marks the 41st anniversary of the start of probably the finest British summer in history, but anyone hoping that yet another parallel with the 1970s is about to come true should take a look at the latest model runs.

june low
The latest GFS operational run suggests a much cooler, more unsettled end to June – a polar opposite of 1976…