28 Aug 2013

Subtle hints of autumn as September approaches

Following the final bank holiday of the year, September is just a few days away and it’s hard not to notice the chillier mornings as well as the nights swiftly drawing in.

In the past few mornings, there’s some dense fog around – a sure sign that autumn is just around the corner. With nights becoming longer, there is more time for cooling to take place, allowing moisture in the air to condense and give those grey starts.

There is often much debate as to when autumn officially begins. Astronomically, it’s on the autumn equinox, which is on 22 September this year.


However, in the meteorological world, autumn starts on the 1 September, running all the way until the end of November. It allows easier upkeep and comparison of weather records by taking the whole month.

So, as September approaches, what signs will the weather offer us to signify the change of season?

Jet stream strength and position

The jet stream is driven by the temperature contrast between the cold poles and warm equator, following the path where the greatest difference in temperature occurs over the shortest distance.

This greatest difference in temperature tends to occur at mid-latitudes, which is why the weather tends to be most variable here.

As the nights lengthen, more cooling takes place, with the legacy of summer heat becoming confined to more southern parts of Europe.

This causes the zone of greatest temperature difference to also move southwards. As this drives the jet stream, it follows.

It’s the gradual sliding southwards of the jet stream that delivers more unsettled weather, with low pressure, rain and brisk winds.

Chilly nights and fog

wales_mist_g_wpEven though the trend through autumn is often for the weather to turn more unsettled, there are still occasions when the jet stream heads north, allowing high pressure to build.

When this happens, there can be some pleasantly warm and sunny days, but with the longer nights, there is more time for the temperatures to fall.

This leads to those chilly morning that can be accompanied by mist and fog. It also gives a dilemma of whether to take a coat or not, as the morning chill can soon turn into a warm day with temperature in the mid-20s.

What can we expect?

In the coming weeks, we can expect a combination of the above taking place, as the weather starts to change.

Low pressure, wind and rain is most likely for the northern half of the UK, as the jet stream speeds up and dips southwards at times.

For the southern half of the UK, whilst there will be some rain, high pressure looks set to retain an influence, bringing some pleasantly warm days along with cool, occasionally foggy mornings.

As ever, you can get the latest forecast on the Channel 4 Weather website. I’ll also be tweeting regular updates on Twitter – @liamdutton

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