5 Nov 2013

Why autumn weather is so changeable

Autumn is a fun time of year for us weather folk. It provides such a wide range of meteorological delights to get our teeth into.

However, for everyday life, it can prove something of a pain – especially when it comes to deciding what to wear.

A winter coat that is necessary for a chilly morning can soon become redundant on a balmy autumn afternoon. On the other hand, if winter decides to strike early, the shopping list suddenly fills up with soup, de-icer and gloves.

Living on an island that sits at the eastern edge of the Atlantic ocean, means our weather is varied enough already. But what makes it so changeable in autumn?


Summer versus winter

Autumn is a transitional season that takes us from summer into winter. This is why we can get such a large range of weather at this time of year.

The remnants of summer warmth battle it out with the lurking beginnings of winter – and, where the two mix, the weather turns pretty lively.

Contrasts are what drive the weather on our planet, with differences in temperature, pressure and humidity all contributing to what happens where and when.

Jet stream

I regularly mention the jet stream – the fast-moving ribbon of air fives miles above our heads – because it determines the weather that we experience at the surface.

mountaintop_snow_g_wpIts position is driven by the temperature contrast between the cold poles and warm equator, with it taking a path where the zone of greatest temperature difference occurs.

During autumn, this tends to be close to or over the UK due to cooling taking place to the north and the legacy of late-summer warmth to the south.

Low pressure versus high pressure

When the jet stream moves from west to east, as well as travelling in a straight line, it also bends around forming a wave that travels around the earth at mid-latitudes.

barometer_closeup_g_wpDips in the wave are known as troughs and result in low pressure forming, with peaks in the wave known as ridges and result in high pressure forming.

This is why the weather is so changeable in autumn. As this wave ripples around, it brings alternating patterns of ridges and troughs, giving areas of low or high pressure.

One thing to note is that there doesn’t necessarily have to be a balance of low or high pressure. The wave can get stuck in a rut, with the same weather type prevailing for extend periods of time – causing extremes.

Cold air versus warm air

As well as determining whether we get low or high pressure, the jet stream also steers huge masses of cold and warm air around the earth.

When the jet stream heads northwards towards the poles, it scoops up cold air and transports it southwards.

Conversely, when the jet stream heads southwards towards the equator, it scoops up warm air and transports it northwards.

z_typical_autumn_jetstream_wpJet stream (white arrows), warm air (orange arrow), cold air (blue arrows)

This is why temperatures can vary so much at this time of year, as scoops of warm and cold air are thrown around the northern hemisphere.

So, the next time you’re frustrated at autumn’s melange of meteorological mischief, at least you’ll know why.

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

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