11 Jun 2012

Any signs of something more like summer?

Given the heavy rain and flooding that has hit the UK during the past week, you’d be right in thinking that June is the new October. Air pressure has been low, sending blood pressures high in despair of the perpetual downpours that have graced our shores.

Mid Wales was hit hard on Saturday following more than a month’s worth of rain in one day in some places. Aberystwyth, Ceredigion and the surrounding area experienced severe flooding leading to a large rescue effort taking place to save people trapped by flood water.

On Sunday, northern England was hit by thunderstorms, with some places having a lot of rainfall in a short space of time. This led to surface water flooding in parts of Yorkshire, with the M1 motorway closed for a few hours on Sunday night.

The focus of the wet weather then shifted southwards during the early hours of Monday morning as an area of intense rain drifted slowly across south east England. In the space of just 12 hours, 20-30mm fell widely, with a few places seeing around 50mm (two inches).

West Sussex has been badly affected by the wet weather with basement flats in the seaside town of Littlehampton flooded under 1.2m (four feet) of water.

So what is the cause of this stormy weather?
It’s all down to the jet stream, which is a fast-moving ribbon of air high in the atmosphere that determines where our weather systems go. During the first part of June, it has taken a track to the south of the UK and has increased in speed, resulting in low pressure being steered towards us.

How does the jet stream cause low pressure to form?

When the jet stream moves from west to east, as well as travelling in a straight line, it also bends around. In cases where it dips and curves in a broad U-bend shape, something called a trough forms.

As the ribbon of air approaches the base of a trough, it slows down slightly as it goes around the bend then accelerates once it has passed through it.

It’s the acceleration of the jet stream after passing through the U-bend that is a key factor because air high up in the atmosphere is sucked upwards and removed faster that it can be replaced at the surface.

As a result of this process, air has a tendency to rise and pressure falls (forming an area of low pressure), resulting in cloud, wind and rain.

When do low pressure systems fade?

The jet stream varies in speed and position. In the current set up, it’s the speed of the jet stream that is crucial in determining when low pressure systems fade.

Eventually, when the jet stream slows down enough, the rate at which air is sucked up from the surface decreases significantly. This results in air at the surface not being so prone to rising, with fewer clouds forming and subsequently less rain.

Is the jet stream heading north any time soon?

If the jet stream moved north of the UK the weather would settle down and high pressure build, with the rain-bearing weather systems steered towards Iceland and Scandinavia instead of the UK.

However, there are no signs of this happening for the next two weeks, meaning that the unsettled weather is going to stay with us.

The jet stream will vary in speed though, which means that there’ll be bursts of wind and rain when it speeds up interspersed with sunshine and showers when it slows down.

Don’t forget you can get the latest five day forecast on the Channel 4 Weather website and you can contact me at anytime on Twitter – @liamdutton

Tweets by @liamdutton

4 reader comments

  1. barry laughton (@kilkeal) says:

    How far ahead can the jet sream location be forecast?

    1. Liam Dutton says:

      Hi Barry,

      The general trend of the jet stream can normally be predicted 7-10 days in advance, although in some case further ahead than that.

  2. Kate says:

    Liam – much as you look lovely on the box, I have to confess that,in this house, when we see you,we groan.
    We have come to associate seeing you with a forecast of lousy weather. Such is the British summer,or certainly,this one.
    And by the time you’re on, we’ve had virtually an hour of doom ‘n’ gloom anyway – the economy,wars,murders – all manner of “man’s inhumanity to man”. So I think you should finish your piece with a bit of Stand Up or maybe a song or something to redress the balance.
    The nation would be grateful :)

  3. adil says:

    From scientific analysis of summer in the UK it is clear that it’s rationed and occurs either early (this year and last) on time (I can’t remember) or late. The ration is roughly 2-6 weeks. Careful study has revealed the following relationship:

    amount of summer = 1/(number of days of hose-pipe ban * frequency of the word drought in press).

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