A record-breaking month in prospect

Now that we have swapped the long-lasting high to our east for another that has come in from the west, the weather has an altogether fresher feel – no longer are we in a stream of warm, humid air from the tropics. In fact there is almost no wind at all today, but high pressure to the west means air drawn down from the north – not warm, but clearer and drier. We are also seeing plenty of sunshine to keep daytime temperatures up, but nights are likely to be much cooler now.

Early this morning we recorded the lowest temperature so far this month, 5.4C at 0530 – it was also the coldest night (1800-0600) of the month so far with an average of 9.8C. Once the sun broke through the early mist things warmed up rapidly, and at lunchtime as I’m typing this it is a respectable 18C. Expect another cool night tonight, though – autumn has arrived.

The spell of warm, humid weather that lasted almost for the first three weeks of September has given us an average temperature so far of 16.3C – around 2C above normal for September. That may drop a little in the last week of the month, but it is still likely to be our warmest September on record. The maximum temperature so far this September is 26.2C and that is unlikely to be beaten this month, but we did see higher temperatures in September 2013 and 2011. It is the length of the warm spell that will make this month a record-breaker.

The other feature of this month is, of course, rainfall – or the lack of it. So far this September we have recorded just 7mm, with rain recorded on only two days. The driest month we have recorded here is May 2010 when the total was 12.4mm. We are three-quarters of the way through this month with only just over half that total, so a new record is certainly on the cards. However if the warm weather continues and creates some heavy convective rain, or the high pressure breaks down and allows the Atlantic to do its stuff, things could change quite quickly.

Finally, with all this high pressure, September looks likely to be our least windy month by a long way. The wind run currently stands at 92.1nm – at this point in the month a figure of well over 1,000nm would be normal, and over 2,000 wouldn’t be out of the way. The lowest wind run figure we have seen for any month is 691.6nm in October 2007, so we can see that it really has been exceptionally calm this September.

With record rainfall to start with, record high temperatures during the summer and now several records set to fall this month, 2014 is turning out to be quite a year.

Becalmed.

Almost halfway through September the total rainfall for the month is 4.6mm, all of which fell on the 1st: and the average temperature is 16.1C, about 0.7C above normal for this point in the month and warmer than the average for August this year.

The last time we saw such a warm, dry start to September was in 2007 (incidentally, the first full month of operation of this weather station). In that year, the first half of September averaged 16C and we recorded just 1.4mm of rain, all on the 2nd.

It has also been remarkably calm – the total wind run so far this month is just 62.3NM, and most of that was on the 1st. In an average September we might have expected to pass the 1,000NM mark by now.

The common feature of both periods is a consistent spell of high pressure blocking Atlantic lows, but in September 2007 we had seen a wind run of 553.9NM at the halfway point – it was not nearly as calm as the last two weeks have been. In fact there isn’t another fortnight in the records here that comes anywhere near this for absence of wind.

Probably not good weather for yachtsmen or wind turbines, then, but a very pleasant start to Autumn for the rest of us.

August 2014 summary

August provided a real contrast to June and July as far as temperatures were concerned. The first two months of the summer were warm, with our highest June recorded average and second-highest for July. August 2014 produced an average temperature of 15.6C, sharing the title of coolest August with 2011.

Recorded rainfall was 94.4mm, but a fault on the weather station on 6 August means that figure is lower than it should be. The true figure is probably about 100mm, which is well above average but not unprecedented – August rainfall exceeded 100mm in 2008 and 2012.

Although a short spell of high pressure put in an appearance in the middle of the month, August was dominated by low pressure systems. The average pressure was 1011.2hPa, our second lowest August figure – the lowest was in 2008, when we also recorded 125.2mm of rain.

The remains of Hurricane Bertha passed up the Bristol Channel on 10 August, but fortunately for us this deep low pressure system hadn’t really got going when it passed – there were some gales and flooding further east. We saw a few thunderstorms, with one on the 14th producing 10.6mm of rain in 10 minutes.

While the positioning of low and high pressure areas in July often drew in warm, moist air from the south, in August we saw the opposite – a deep low pressure system passed north of the country on 18 August, and then parked itself over Scandinavia while a blocking high pressure system formed to our west. The result was a stream of polar air making it feel decidedly chilly, with a minimum of 6.1C on 21 August – in Dorset it got down to around 3C. Finally, August was quite a windy month, but there were no exceptional wind speeds.

At the end of July were heading for a record summer temperature, but August took the edge off that – the summer of 2014 has to settle for second place with an average temperature of 16.9C, against the summer of 2013 with 17.0C. The rainfall total for summer 2014 was 205.6mm (or about 211mm if we take account of the error on 6 August), which is on the high side but not exceptional.