Met Office explains autumnal warmth

Watch as Met Office meteorologist Alex Burkill explains autumnal warmth and the conditions that cause it.

Transcript

00:00Picture the scene. It's autumn. The kids are back at school, the heating's been tentatively

00:05nudged on, and Jack Frost has already nipped a few noses. Trees are turning glorious shades

00:10of amber, crimson and gold, and you're reaching for your scarf.

00:14But then the sun comes out, temperatures rise, and it feels like summer's made a surprise

00:18comeback. What's going on?

00:21Today we're diving into the curious case of autumnal warmth. Why does it happen, and

00:26is it becoming more common?

00:27Let's be honest, if we're talking summer like warmth, I'm not staying cooped up in

00:31the studio. And this is purely for scientific purposes, of course.

00:36Now, you may have heard the phrase Indian summer being tossed around when describing warm spells

00:42during autumn. But here's the thing. It's not an official meteorological term, and in

00:47fact organizations such as ourselves avoid using it.

00:52The origins of the term are pretty murky. Some historians suggest it stems from a period

00:57of warm weather in North America, which allowed indigenous people to continue hunting before

01:02winter set in. The first recorded use was in a 1778 letter by French-American farmer John

01:08de Kravica, who described a warm spell in the Mohawk Valley.

01:13The term made its way to the UK in the 1800s, but the idea of a warm spell in autumn wasn't

01:19new. Across Europe, people referred to St Martin's Summer, which tied in with the feast day on November

01:25the 11th. Despite these poetic names, there's no solid evidence to suggest that we had warm

01:31spells in autumn at the same time each year.

01:37So what causes warm weather during autumn? Well, let's break it down. Usually at this time

01:41of year, the jet stream, that fast ribbon of air moving higher in the atmosphere, strengthens.

01:47That's because the temperature contrast between the cooler poles and the warmer equator increases.

01:53This usually leads to some wet and windy weather for the UK.

01:57But sometimes the jet stream buckles or meanders, a bit like a lazy river. And when that happens,

02:05it can lead to high pressure building. And high pressure means the air is sinking, which then

02:11suppresses the cloud and leads to clear skies and sunshine. Additionally, if the high is

02:17positioned just right, we can be dragging in our air from the south or southeast. And so

02:23we could get some very warm air from the near continent or even North Africa.

02:29And here's a bonus bit. Sea surface temperatures around the UK are at their highest during early

02:35autumn, having absorbed all that summer heat. And that heat can then help boost our air temperatures.

02:41So autumn warmth does happen. And it's all thanks to a wobbly jet stream, a well-placed

02:46high pressure and also some leftover summer warmth. Here's where things get a bit serious.

02:51Thanks to climate change, warm spells during autumn are becoming more intense and more frequent.

02:57And so something that used to be a bit of a surprise is now becoming increasingly common.

03:02In fact, the UK's top 10 warmest Octobers on record have all occurred since the turn of

03:07the millennium. So whilst a warm sunny October day at the beach may sound lovely, it's also

03:14a stark reminder of the shifts in our climate.

03:18So whilst you may only think of them as a chance to sneak in one more beach trip before

03:23the year's out, the science behind these warm autumn days is fascinating and increasingly

03:28important to understand.