September weather summary

11 10 2013

September opened with some fine, warm, sunny weather. After a brief stormy period mid-month, the second half was quieter and more typical of autumn.

The provisional UK mean temperature was 12.8 °C, which is 0.1 °C above the 1981-2010 average. The UK overall received 73% of the average rainfall amount for this month and there was provisionally 93% of the long-term average hours of sunshine. Visit our climate section for a full written summary of the month.

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Stats reveal so-so September

2 10 2013

Provisional Met Office statistics show September has been fairly average for the UK, with lower than average rainfall being the only notable feature.

The UK mean temperature for the month was 12.8C, which is just 0.1C above the long-term (1981-2010) average. It’s the same story with maximum and minimum temperatures, which at 16.6C and 8.9C respectively, are both 0.1C above the long-term average.

September UK rainfall as a % of 1981-2010 average

September UK rainfall as a % of 1981-2010 average

There were 115.8 hours of sunshine in the UK during the month which adds up to 93% of the long term average – again, fairly normal. However, there were some big regional differences. Sunshine amounts were below normal in the west and south, but near or above normal in the north and east. For south Wales and south-west England this was provisionally the dullest September since 1994.

When it comes to rainfall, it has been a drier than average month for the UK. There was 70.7mm of rain during September, which is 73% of the long-term average. Parts of Wales, eastern England and Scotland had less than half the normal amount. For Scotland overall this was provisionally the driest September since 2003.

The drier than average September continues a theme for 2013 as a whole, with every month this year apart from May registering below average rainfall.

You can explore Met Office statistics on our UK Climate pages.

Mean Temperature Sunshine hours Rainfall  
September Actual Diff to Avg Actual % of Avg Actual % of Avg
  degC degC hours % mm %
UK 12.8 0.1 115.8 93 70.7 73
England 13.7 0.0 126.1 92 54.5 78
Wales 13.1 0.3 104.0 81 79.6 68
Scotland 11.2 0.3 104.9 100 95.1 70
N Ireland 12.7 0.3 98.1 86 69.3 76




How wet has this September been?

1 10 2012

The latter part of September saw some exceptional rainfall in parts of the UK which caused disruption and flooding at times.

With such a great deal of rain falling in a short period of time, some people have asked whether it will make September one of the wettest in our national records going back to 1910.

Provisional early statistics up to 26 September show this isn’t the case, however, with the month looking set to be slightly wetter than average – but by no means a record breaker.

Up to the 26th, UK rainfall is 96.3 mm – which is 100% of the full month average. After 26 days we would, assuming rain falls fairly evenly through the month, expect this to be around 87%.

Of course, rain doesn’t always fall evenly throughout a month – as we saw this September. The first three weeks saw relatively little rain in many areas, but then a particularly active weather system brought four days of persistent heavy rain.

Northern parts of England were particularly badly affected by this, as you can see in the rainfall map below. In the map you can see a band of blue colours across northern England denoting above average rainfall for the month, whereas much of the country is coloured white to denote near-average amounts.

Two brown areas, one across central Scotland and the other in East Anglia, show it has been drier than average here – even despite the heavy rain in the latter part of the month.

 

Temperatures up to the 26 September are also fairly ordinary, being slightly below average. Mean temperature for the UK is 12.2 °C, which is 0.5 °C below the long-term average for the month.

While September looks set to be slightly wetter and cooler than average, the good news is sunshine hours were slightly up – with the UK having seen 126.1 hours of sunshine, 101% of its whole-month average.

Again, we’d expect it to be around 87% after 26 days, so we’re ahead – but not by a record-breaking amount.

So this September is set to go down as a fairly average month overall, but – as is often the case – this belies some very stark contrasts and some less-than-usual weather.

Met Office provisional 1-26 September figures
mean temperature sunshine duration rainfall
Actual Difference from 1981-2010 average Actual % of 1981-2010 average Actual % of 1981-2010 average
degC degC hours % mm %
UK 12.2 -0.5 126.1 101 96.3 100
England 13.2 -0.5 145.2 106 80.6 116
Wales 12.1 -0.8 124.5 97 115.3 99
Scotland 10.4 -0.5 98.6 94 117.2 86
N Ireland 11.8 -0.5 105.3 93 94.0 103
England & Wales 13.1 -0.5 142.3 105 85.4 112
England N 12.4 -0.4 124.0 98 121.4 150
England S 13.7 -0.5 156.4 110 59.0 93




Storm caused by most intense low to cross UK in September in 30 years

26 09 2012

The low pressure system that has brought heavy rain, strong winds and flooding to the UK is the most intense to cross the UK in September for more than 30 years, with the lowest air pressure of 973mb being recorded on Tuesday morning.

Pressure chart at 6am on 25 September 2012

To find a similarly intense low pressure system that affected a wide part of the UK in September you need to go back to 1981, when pressures below 970mb were reported across central parts of the UK.

Like this week, this low pressure system brought unsettled weather as it crossed the British Isles – tracking east over the Isle of Man before heading north to Cumbria, Northumberland, eastern Scotland, Orkney and Shetland.

But what do we mean by ‘the most intense’? The intensity of a low pressure system is measured as the lowest pressure recorded at the centre of the system, as this gives an indication of how active it may be. This will relate to the rainfall amounts and wind strengths associated with it.

However, pressure is only one indicator of how much wind and rain there will be, so it is possible that other systems have resulted in stronger winds or heavier rain in some places than we have seen over the last few days.

Although the storm we have seen this week is certainly unusual in that it crossed central parts of the UK, some parts of the UK have seen pressure systems of this kind of intensity many times before at this time of year. In fact, Met Office records show some 31 occurrences of pressure below 975mb being observed in the UK in September, but the vast majority of these were confined to north and west Scotland, Northern Ireland or the far west of England.

For example a deep low affected the northwest of Scotland with pressure as low as 972mb as recently as 12 September 2011, whilst the Isles of Scilly and part of Cornwall saw pressure as low as 966mb on 7 September 1995. So, with regard to the system which has recently affected the UK, the key to what makes it remarkable is that it has tracked over a wide area of the UK rather than those areas which are more used to storms of this intensity.





When does Autumn start? Defining seasons

20 09 2012

Seasons are fundamental to how we understand the UK climate and the environment around us, but how do we define when they start and end?

In meteorological terms, it’s fairly simple – each season is a three month period. So, Summer is June, July and August; Autumn is September, October and November, and so on.

Of course, this is fairly arbitrary, but provides a consistent basis for the Met Office, as the holder of the UK’s national weather and climate records, to calculate long term averages and provide seasonal climate summaries from year to year.

Mike Kendon, of the Met Office National Climate Information Centre, said: “Defining seasons in this way means we can compare weather from one season or year to the next. It also has the advantage that each season is roughly the same length, neatly dividing the year into four quarters.

“Looking at longer timescales, our recently updated 30-year averages can show us how ‘normal’ seasons are changing over time, giving us clues about trends in the UK’s climate.”

Astronomical definitions of seasons also exist – using the Earth’s position relative to the Sun as the cue for separating one season from another via equinoxes and solstices.

So the Summer begins around the Summer Solstice, when daylight hours are at their longest (around 21 June), and ends around the Equinox, when days and nights are of equal length (around 21 September, on 22 September this year). Thus astronomical Autumn begins, continuing until the Winter Solstice, when daylight hours are at their shortest (around 21 December), and so on. Astronomical seasons therefore are about three weeks behind the meteorological ones.

One thing both methods have in common is that the dates are fixed by the calendar and don’t take into account what is actually happening in nature, which is after all how most of us understand the notion of seasons.

So comes the third method, which is based on phenology – the process of noting the signs of change in plant and animal behaviour.

In this distinction, Autumn may be deemed to have arrived at the first tinting of oak or beech trees, the appearance of ripe sloes or elderberries and the arrival of winter migrant birds such as redwings and fieldfares. Winter begins when native deciduous trees are bare, and so on.

For more than a decade The Woodland Trust has been using observations from thousands of members of the public to build a phenological record for the UK, called Nature’s Calendar. This builds on records going back over much longer periods of time.

It aims to give a comprehensive view of how nature defines the seasons in a record which takes into account how weather in individual years or longer term changes to climate may affect natural signs from one year to the next. As such it is a more fluid, natural definition of our seasons.

Dr Kate Lewthwaite, project manager for Nature’s Calendar for the Woodland Trust, said: “Taken individually the observations of what’s going on in nature provide only anecdotal evidence, but taken as a whole and analysed with temperature data, they offer a powerful insight into local and national impacts of environmental and climatic change.

“For example, our data shows that, on average, native trees are producing ripe fruit 18 days earlier than a decade ago, with a potential consequence being that animals’ food reserves could become depleted earlier in the winter. In contrast, leaf fall, indicating the end of the growing season, is often much later nowadays than in the past.”

Ultimately, however you choose to define them, it is weather and climate which govern the perception of the passing of seasons for plants and animals, including us humans.

So, like our weather, the exact timing of when we ‘feel’ one season is over and a new one has begun will always be liable to change. Whereas, in contrast, the meteorological seasons always remain fixed by calendar month.

Between the Met Office’s climate records and our forecasts up to a month ahead, you can stay up-to-date with what’s going on with the UK’s weather and climate.








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