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September weather

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September is often a relatively sunny month in the south east, not infrequently making up for the deficiences of the summer. True, there is a coolness in the air in the early mornings, with dew on grass and park benches. But temperatures are still on the whole very pleasant, and you can still outside and wear light clothing even in the evening.

This fine weather is due to continental highs, which keep the jetstream and its Atlantic lows (the remains of Caribbean hurricanes) to the north and west. If these highs are weaker, as happens in some years, September is a very different month - cool, grey and wet, making it hard to believe that you were doing summery things like going to the beach only a couple of weeks previously.

The golden years

Years when September was mainly sunny and stable include 2001, 2002, 2003, 2005, 2006, 2012, 2014 and 2015. In 2002 and 2003 there was almost unbroken sunshine - with rain on only one day in September 2002, for example. In other years there was some cloud and rain, but widely scattered, with the bias being towards fine weather. 2014 was perhaps a bit more cloudy than other years, with morning cloud often breaking to sunshine as the day wore on and a mix of sun and cloud on other days. But only on five days in the month was there any rain, and then fairly short-lived.

The last week was more unsettled in 2005, 2006 and 2012, though with only a couple of days of rain each case. In 2012 the rain was particularly spectacular, a deep slow-moving low bringing heavy rain on the 23rd and 24th and causing flooding in the north of England. Despite this, September 2012 saw four cloudless Saturdays in a row (from the 8th to the 30th), a record not matched by the Sundays, which were a lot more mixed (and in one case, downright wet).

September 2015 at first looked set to continue the cool and cloudy weather of August, but on the 6th high pressure established itself and produced mainly sunny skies for the rest of the month, apart from interludes of showery low pressure from the 14th to the 18th and 21st to 22nd. This was not a hot September, however, with temperatures in the high teens at best, apart from one brief excursion into the very low 20s from the 9th to the 12th. From the 23rd onwards there was a cool easterly breeze.

Generally when the sun shines, temperatures can still pack a punch in the first half of the month, reaching into the high twenties centigrade (for example 28 degrees on 8 and 9 September 2012), and even late in the month 21-23 degrees is possible. On such days it can seem as if summer will never end. But in cooler, more unsettled weather, temperatures can dip to the mid teens.

Evening temperatures can also fall away noticeably by the end of the month, maybe even into single figures, making a jacket or pullover essential. One exception to this was 2006 when it remained shirtsleeve warm at night almost to the end of the month. The same was true in 2014, though night temperatures did dip into single figures in towns on the 22nd and 23rd, and in rural areas on the 9th-12th as well.

By contrast, both 2012, 2015 and 2017 saw night time temperatures down to single figures as early as the first week. In 2012 the first such night was on the 6th and there was then a series of cold nights from the 12th to the 19th that saw temperatures dive quite widely to 3 or 4 degrees in rural areas. In 2015 night temperatures were at 6 to 8 degrees from the 5th to the 8th, and again from the 23rd to the end of the month: in the latter period rural temperatures again fell to 3 or 4 degrees. In 2017 the first two days of the month saw night temperatures of 8 degrees, and the same was true on the 9th and 10th. 15 to 17 September then saw night temperatures of just 6-7 degrees in towns and as little as 3 degrees in rural places, and the same was true on 20 and 22 September before nights returned to double figures for the rest of the month.

Years when it was more mixed

Years when the continental high was weaker and the weather in September more mixed include 2016, when after a sunny last nine days of August which extended into the first day of September, low pressure intruded from the north from the 2nd to the 5th. A continental high then pushed in, bringing southerly temperatures and some days of hot sunshine, culminating on the 13th with temperatures reaching 32 degrees and a fifty year September record of 34.4 degrees in Gravesend. From 16 September onwards lows were back in charge but the weather remained fairly benign for the rest of the month - often cloudy but with full sun on three days and some sun on four others. In all there were nine fully sunny days in the month, nine with sun and cloud, but also nine with some rain.

In 2007 there was high pressure for the first two weeks, but it brought quite a lot of cloud. There was then a brief sunny interlude from the 13th to the 16th before lows set in on the 17th. Temperatures dipped to 12-15 degrees in the second half of the month. Even so, there were in fact only two days of significant rain.

In 2004 the month started with sunshine, but turned more changeable from mid month, with several rainy days. In 2008 and 2009, it was the other way around: September started wet and windy, before reverting to mainly sunny weather for the rest of the month - on the 8th in 2009 and the 13th in 2008. In 2013 the month started sunny, with highs of 30 degrees on the 5th, before showery lows set in till the 18th. The last twelve days of the month then saw high pressure dominating, with cloudy mornings giving way to pleasant sunshine and temperatures in the high teens in the afternoon.

2011 was an even more extreme version 2008 and 2009. Intense lows (a former hurricane) brought wet and windy weather in the first week, and for the second it was still windy, but drier. High and low pressure then tussled with each other, but from the 23rd onwards an astonishing period of hot sunny weather set in, with temperatures reaching 27 degreees on the 28th, and 29 degrees by the 30th. What made this more extraordinary was that it followed the coolest summer in 18 years, when there had been no settled period of fine weather and temperatures had only twice briefly risen above 25 degrees. The heatwave continued until 3 October, after which temperatures abruptly fell to 14 degrees in just three days.

In 2017 high pressure was in charge on the first two days of the month, while from 22 September a continental high ensured the south east only got some effects of lows to the north. But generally westerlies were in charge all month, including one deep low - Storm Aileen - on 13 September. This was a very changeable month, with rain on 15 days, and full sunshine only on four. Thirteen days had some sunshine, but often this was quite fleeting, or confined to the morning with weather turning more unsettled later

The worst September of all in recent years was 2010, when lows dominated for nearly the whole of the month, at times bringing an almost wintry feel to the weather. This was particularly true towards the end of the month when a high to the west and lows to the east brought cold northerly winds, driving temperatures down into the mid teens. There only two very brief periods of high pressure - from 1-4 and 21-22 September - and a scattering of sunny days in other parts of the month.

Evenings draw in

The evenings in September draw in at an accelerating pace: by mid month, you are shocked to notice that it is dark by 7.30pm, and by the end of the month, lighting up time is 6.50pm, though it feels like it is starting to get dark at 6.30pm. That means you start to have to keep an eye on the clock on days out in the country, though this is not yet a serious problem.

On the plus side, sunlight starts to take on that golden quality, even in the middle of the day. It starts to do that winter thing of shining into your face. In central London, the sun is low enough that most streets are in shadow by mid afternoon, while others have the sun shining straight down them.

T-shirts go back into the cupboard, along with sun hats, whose protection from the sun is no longer needed. Jackets come out. Mid month the bedclothes come rapidly back onto the bed - not just duvet, but blankets too. It starts to feel nippy in the morning when you wake up: you cosy up under the bedclothes before getting up. Later in the month you may start to think about putting the heating on, but you don’t yet. Sometimes you wear a pullover inside, but often on going out you are surprised how mild it is.

Unless it is really dry, plants or gardens no longer need watering. The ground starts can be a bit damp if it rains, but soon dries out again, and there is still very little mud on paths. Colds and other ailments return. Suddenly everyone you know seems to have something and you feel you must be next.

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