24 Sep 2014

What makes autumn feel like autumn?

As I walked to work today, there was a real sense of autumn in the air. Golden leaves lay scattered on the ground, the smell of dampness filled my nostrils and there was enough of a chill to need a jacket.

Autumn is a season that demonstrates just how varied the British weather can be. We can quickly descend from the warmth of late summer, to gales and rain, only to be followed by fog and frost.

Such a melange of autumnal surroundings led me to ask myself the question: what makes autumn feel like autumn?

Nights longer than the days

Having just passed the autumn equinox in the early hours of Tuesday, the northern hemisphere is now at a point whereby the nights are longer than the days.

Whilst at face value, this may appear to have little significance, it is actually quite important. This means that there will soon be more hours of the day with energy leaving the earth’s atmosphere than entering it.

As we move deeper into autumn this net loss of energy from the northern hemisphere will drive us towards winter, with the air at the north pole getting colder and colder.

The building of colder air at the north pole has significance for the next change – the jet stream powering up.

Jet stream powers up

The jet stream is the fast-moving ribbon of air around five to six miles up in the atmosphere that determines the weather that we experience at the surface.

jetstream_fast_WZ_wpIt travels around the earth at mid-latitudes, travelling from west to east at speeds as high as around 200mph.

The jet stream is driven by the big temperature contrast between the poles and the equator. The greater this temperature contrast, the faster the jet stream and thus the more unsettled the weather.

The build up of increasingly colder air at the north pole mentioned above increases the temperature contrast between the poles and still relatively warm equator. This causes the jet stream to power up and deliver us autumn rain and gales.

Sun lower in the sky

As autumn progresses, the height of the sun in the sky gradually lowers as the winter solstice approaches in late December.

This means that even on days that are sunny, the amount of incoming sunlight isn’t as great or as lengthy as it would be during the summer.

As a result, temperatures exhibit a downward trend and there can be a definite chill in the air, even when the skies are blue.

Leaves change colour

One of the most significant visual changes that we experience in autumn is the changing colour of leaves on the trees, turning from green to a beautiful selection of browns, reds and oranges.

leaves_change_g_wpLeaves changing colour is a tree’s way of getting ready for winter, as the amount of sunlight for photosynthesis (a plant’s food source) decreases as the days become shorter.

Chlorophyll is not only a chemical that helps photosynthesis happen, but also gives leaves their green colour.

Therefore, when trees shut down their food-making process during autumn, the chlorophyll and thus the green colour disappears from the leaves, leaving the array of autumnal hues.

So there’s a variety of things that make autumn feel like autumn, each one playing its own significant role in the bigger picture.

If you capture any autumnal pictures in the coming weeks, send them to me on Twitter – @liamdutton

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