17 Sep 2012

What makes autumn weather autumnal?

As the autumn equinox approaches this Saturday, there is no denying that the nights are rapidly drawing in, and despite some pleasantly warm days, the mornings and evenings now have a distinct chill in the air.

Whilst our seasons here in the UK don’t always conform to how we expect them to be (take the summer we’ve just had as an example), there are a number of things that happen each season that cause the weather to “generally” behave in a particular way.

Autumn and spring are closely related in the sense that they are transitional seasons. Spring marks the transition from winter to summer and autumn the transition from summer to winter.

For these reasons alone, autumn and spring can experience huge ranges of weather, with frequent overspills from neighbouring seasons showing their hand they battle it out for supremacy.

So what is it that makes our weather in autumn autumnal?

Shorter days and longer nights

We are currently at a time of year when the hours of day and night are roughly equal, but beyond the autumn equinox on Saturday, the nights will progressively become longer than the days.

The significance of this is quite important, as the amount of energy received from the sun in the northern hemisphere decreases. This is because more energy is radiated away each day than is received, which results in a net cooling effect.

This net cooling effect becomes more pronounced as autumn progresses, initially in the Arctic, but increasingly so further southwards, slowly dragging us into winter.

Increased temperature contrast

As a result of sudden cooling towards the Arctic, the difference in temperature in the northern hemisphere from equator to poles becomes more marked, where summer heat further southwards tries to hang on.

This is important, because the jet stream that has a huge influence on our weather and elsewhere at mid-latitudes is driven by this temperature contrast.

In a nutshell, the greater this north to south temperature contrast, the stronger the jet stream tends to be.

A stronger jet stream means that areas of low pressure develop more often and more intensely, bringing wind and rain – a trait that is typically associated with our weather during autumn.

What can we expect in the coming weeks?

The next few weeks look set to fit in with the autumnal theme. As the jet stream picks up from time to time, there’ll be areas of low pressure bringing wet and windy weather to our shores.

In the quieter interludes when areas of high pressure are over us, there’ll be pleasant days, but chilly nights with an increasing risk of mist, fog and frost.

One thing that seems to be a pretty consistent theme is that it is going to be quite cool. There are no signs at this stage of any bursts of late September warmth, so we may have to keep our fingers crossed for an Indian summer in October.

Don’t forget, you can get the latest five day forecast on the Channel 4 Weather website and you can follow my regular updates on Twitter – @liamdutton

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