Category Archives: UK climatology

How cold will November end?

Twitter and other forums have been at fever pitch with excitement for a ‘nailed-on’ cold spell. 

But with many getting carried away over tales of a repeat of March’s ‘Beast from the East’ it is important to remember that snow in November is extremely rare in the south-east; despite predictions of the incoming upper air being very cold the ground is still very warm and will take time to cool.

However, as seen in November 1993, a week of hard frosts were enough to chill the ground enough for a decent week-long cold spell. Similarly, although the snow didn’t arrive in London until December, November 2010 was also cold enough.

The weather of late has been in an ‘average mood’ with the mean temperature in September and October both finishing at 0.2C below. The mean for the this month is currently (on the 15th) running 1.8C above average, therefore for the mean to finish the same as September / October would require a big cool down, as hinted by the models.

Will we see another an average, 1993 or a 2010 end to the month. My hunch is it will be something between the three.

Below is a graph that shows this November so far with average, 1993 and 2010 ends.
2018: 0.4
1993: -0.8
2010: -1.4

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East London’s highs and lows of the last 60 years

The chart below shows every maximum and minimum temperature recorded in this area since January 1st 1959, some 21,860 days!

The extremes range from -12.7C recorded on January 31st 1972 to 37.5C on August 10th 2003.

After peaking in 2003, summer maxima seems to be in decline. Winter minima, by comparison, has been increasing since 2012

maxmin

November: an Indian summer before turning cold?

With the mean temperature of both October and September finishing 0.2C below average it is probably safe to say that the weather is in an average kind of mood.

Conditions during the first part of November look changeable, according to the Met Office’s 30-day forecast . After mid month, however, the agency says the forecast is uncertain.

Now that much of the UK has had its first frost any warm spell in November will, correctly, be called an Indian summer. A singularity called the St Martin’s Summer occurs in 66 per cent of years, occurring between 15th and 21st and peaking on the 18th.

And, as if by magic, the GFS model today has this chart for the 16th, an Atlantic ridge of high pressure with daytime temperatures about 6C to 8C above average. Though warm during the day I would imagine there being a risk of fog forming at night

16th

Beyond that there could be a tendency for much more unsettled weather at the end of the month. The early December storms singularity occurs in 98 per cent of years, starting between November 24th and December 14th, often peaking on December 9th.

November, the last autumn month, can often surprise with its extremes, though it can also often be characterised by days of anticyclonic gloom. The warmest, coldest and wettest November conditions in London back to 1959 can be found here.

 

 

Perfectly average months are extremely rare

Readers of this blog know that I often described a month as fairly average – a look at local statistics shows that many months come in very close to the monthly mean.

But when you combine mean temperature with rainfall that is 100 per cent average it becomes very rare and non-existent with 100 per cent sunshine hours considered.

Of the 2650 months since 1797 just one, February 1972, saw a mean temperature and rainfall precisely average. However, sunshine hours were just 40 per cent of the 1981-2010 average.

To extend the data I included monthly rainfall totals that were between 90 per cent and 110 per cent of average. Even then just five more months were revealed: September 1806, February 1876, January 1883, October 1886 and March 1972.

Although on paper February 1972 was perfectly average a look at the weather for the month reveals typically variable weather. The website London Weather explains:

The maximum temperature was only 3C on the 1st as mild Atlantic air slowly displaced the very cold continental air eastwards. There were outbreaks of rain, and during the remainder of the first week, although mild, it was often dull and wet. During the second week, active frontal systems crossed the country bringing strong winds but with sunshine between the rain bands. After mid month, east winds returned, and although not cold, it was frequently dull.

Considering the Met Office forecast the mean for this month, October 2018, is set to finish precisely average. However, rainfall looks set to finish well under the average for the time of year.

London’s October extremes since 1959

October is one of those months that can see both ends of the spectrum; from calm ‘mists and mellow fruitfulness’ and, rarely, frost, to wet and wild systems whistling in off the Atlantic, best known being the 1987 Great Storm and, more recently, the St Jude storm.

I’ve put together a few top 10s of stats for Wanstead, St James’s Park and Heathrow for the month of October.

october extremes

October SJP

Heathrow oct

Some national UK October values according to TORRO

Hottest: 29.4C March, Cambs – 1st 1985
Coldest: -11C Dalwhinnie 28th 1948
Wettest: 208.3mm Loch Avoich 11th 1916

In terms of climatology October maxima, considering the 1981-2010 average, shows a decline through the month, though around the 8th and 20th there is often a spike. This would reflect the October singularities; early October storms, between 5th and 12th, peaking on the 9th, occur in 67 per cent of years. St Luke’s summer, between 16th and 20th, peaking on 19th, also has a 67 per cent probability.
Mid-autumn storms occur between 24th and 29th October, with a 100% probability.

october average graph

The average rainfall graphic shows that downpour amounts are variable through the month. A tendency for dry weather around the 17th and 18th before the wettest days on the 20th and 21st.

October rainfall

 

 

 

Wild equinox brings memories of childhood

Wild weather in the run up to the autumnal equinox was frequently a staple of the Septembers of my childhood but it has been absent in recent years.

Since 2013 I have been recording the ‘wind run’ data on my AWS, a stat generated by the amount of times the anemometer spins.

wind run

The results show there has been nearly two-and-a-half times more wind than the 5-year average.

Stormy weather over the equinox is one of the less frequent recurring singularities. The meteorologist Philip Eden a few years ago noted that ‘Mid-September storms’ during the period of 17th to 24th September had a frequency of 60 per cent.

20th
The wind run was the highest on 20th

When did September become a summer month?

The answer is around 1993. A look back at mean temperature and rainfall statistics for east London over the past hundred years reveals that the ninth month has, since that date, slowly become warmer and drier.

Putting all the arguments of meteorological and astronomical summer aside, many people of a certain age regard September as an autumnal month, but as recent years have shown it can very often be an extension of summer; September 2016 was the second warmest on record in the local area, warmer than many previous summer months!

Looking back even further, over the past 100 years, the September mean has trended upward, though many peaks and troughs reveal how the month has ebbed and flowed from being summery to autumnal.

The prognosis to the end of this month suggests that air pressure will be anomalously high – so the pattern in the south-east for settled, summery weather in September doesn’t look like ending any time soon.

sept means
The September mean trend has crept generally upward.

sept rainfall
A look at rainfall back over the last 100 years shows that wet Septembers have been on a general decline since 1994.

sept chart.gif

London’s September extremes since 1959

September in recent years has often been a more summery month than August. But this month in some years can see a rapid onset of autumn.

I’ve put together a few top 10s of stats for Wanstead, St James’s Park and Heathrow for the month of September.

september extremes

SJP september

heathrow sept

Some national UK September values according to TORRO

Hottest: 35.6C Bawtry, South Yorkshire – 2nd 1906
Coldest: -4.5C Dalwhinnie, Highland, – 26th 1942
Wettest: 190.7mm West Stourmouth, Kent – 20th 1973

In terms of climatology September maxima, considering the 1981-2010 average, shows a slight decrease through the month, though around the 1st and the 22nd there is often a spike. This would reflect the September singularities; early September warmth occurs in 82 per cent of years while the ‘Old Wives Summer’ has a 64 per cent probability.

september mean

 

The average rainfall graphic shows that downpour amounts are variable through the month. A tendency for dry weather to the 8th prevails before the trend increases, the wettest days usually 13th and 19th.

sept rain