Autumn: four things that tell us it’s here

As I walked to work the other morning, there was no escaping the slight chill in the air. It’s that time of year when a jacket is a necessity early in the day, but an excess in the afternoon sunshine.

Autumn is a season that shows just how varied the British weather can be. The warmth of the late summer sunshine wanes and the first eerie fogs blur the landscape.

But what are the sure signs that tell us autumn has arrived?

Nights longer than the days

Today marks the passing of the autumn equinox, when the nights soon become longer than the days in the northern hemisphere.

At face value, this may appear to have little significance, but it is actually quite important. This means that there will soon be more hours of the day with energy leaving 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 that tends to take place – 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.

It 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 eventually 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 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.

There are signs that the beginning of October will see high pressure building across the UK, bringing bright, pleasantly warm days, with chilly, foggy nights.

Don’t forget, you can get more on the Channel 4 Weather website. I’ll also be posting regular updates on Twitter – @liamdutton