September weather: what would we normally expect?

As the curtain falls on the disappointing show that was August, today marks the beginning of meteorological autumn – classed as the months of September, October and November.

August was the rogue month of summer, going against the grain of the warmth and sunshine that June and July delivered.

It was provisionally the coolest August the UK has seen for 23 years, with mean temperatures (an average of day and night temperatures) 1.1C below normal.

However, the most notable aspect was the amount of rain that fell, with rainfall for the whole UK 42 per cent above average.

The first part of September looks like it will see a return to summer-like warmth and sunshine, but what would we expect to see in a typical September?

Temperature

As the nights quickly start to get longer in the northern hemisphere during September, the further north you travel, the more marked a notable decrease in temperature becomes.

September can start to see bursts of cooler air taking glancing blows from the north west, giving a stark reminder that the first chills of autumn are not too far away.

It therefore comes as no surprise that the highest average temperatures are found across southern and eastern parts of England and Wales, at 16-20C. Elsewhere, 12-17C is more typical.

Rainfall

Another characteristic of the weather in September is the jet stream starting to power up and steer low pressure towards our shores.

This powering up of the jet stream occurs due to the temperature contrast between the poles and the equator increasing, as hot summer air meets the first bursts of colder air from further north.

Given the fact that most of our rain in the autumn months comes from the west, it is western areas that generally experience the greatest amounts of rainfall.

Hills, mountains and moors in the west of the UK see 100-250mm of rain in a typical September, rapidly falling off to just 40-100mm elsewhere.

Sunshine

As would be expected, the amount of sunshine experienced in September parallels the pattern seen with places that have the greatest amount of rain.

Much of Scotland, Northern Ireland and the hills of north west England see the fewest hours of sunshine for the month, at 80-120 hours.

Elsewhere, there tends to be 120-150 hours of sunshine, with the coasts of southern England and East Anglia faring best.

So, whilst September is usually a decent month of weather for much of the UK, there are usually signs of autumn starting to make their presence felt.

Don’t forget, you can get the latest forecast on the Channel 4 Weather website. I also post regular updates on Twitter – @liamdutton

Maps: Met Office