Why, oh why, does it keep raining?

Bad weather It has been an extraordinarily wet summer

If you want something to blame for the appalling weather, look up as you raise your umbrella and imagine that high above the rain clouds a great river of wind is flowing through the upper atmosphere.

This is the jet stream and its path is the cause of the repeated flooding being suffered during a British summer that has so far been one of the most miserable on record.

It was first identified by Japanese researchers in the 1920s, and then experienced firsthand by American aviators flying new high-altitude bombers in World War Two.

The jet stream, a massive but mysterious driver of our weather, usually passes along a steady path from West to East across the Atlantic - sometimes a bit to the North of us, sometimes a bit to the South.

As a relatively small island, on the borderline between the Atlantic Ocean and the European continent, the precise location of the stream matters hugely to us and right now we're on the wrong side of it.

Map showing normal jet stream pattern The jet stream normally sits to the north of the UK in summer, directing areas of low pressure and bad weather further north.

This giant flow of air is the result of a constant play of forces across the planet as energy passes from the warmer tropics to the cooler polar regions - and its basic direction is governed by the spin of the Earth.

What matters is where we are in relation to the stream as it surges overhead, particularly when its flow is not a neat curve but a series of massive meanders, like a river approaching the sea.

Unfortunate location

Our misfortune now is to be on the northern side of those meanders where conditions are cooler and wetter which means we in Britain keep getting hit by rain.

The bigger the meanders, the greater the chance of giant pockets of cooler, wetter air being drawn south, starting to rotate and so initiating the process that leads to storms.

Map showing jet stream summer 2012 The jet stream has shifted further south than usual, bringing wet and windy weather to the south of the country.

However if you read this in the US, much of which lies to the south of the jet stream, your temperatures have been soaring because the air on that side of the line is far more settled.

Normally, we would expect the pattern of the jet stream to keep shifting, for its shape to switch every few days and for our weather to change as a result.

Instead for week after week - and possibly for weeks ahead too - the meanders of the stream are sticking to the same shape so repeated rainstorms have become the norm.

Big unknown

The implications are depressing. Without some unexpected force altering the stream's pattern, it looks set to continue for a while yet.

The big unknown is why this current pattern is so static. The high-altitude winds that make up the stream are themselves still racing along but their path remains stuck in place so our battering continues.

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More than 60 years later scientists are still wrestling with the question of how the jet stream operates and what shapes it”

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This is one of the major puzzles for weather specialists and the science behind this is fairly young.

Dr Mike Blackburn of the National Centre for Atmospheric Science at the University of Reading admits that the reasons for a static pattern of the flow remain unclear.

"We haven't discovered why the meanders get locked into position as they are now," he told me.

One attempt at an explanation involves so-called Rossby Waves, named after the Swedish meteorologist Carl-Gustav Rossby whose research was published back in 1939.

This is no comfort as the forecasts continue to be grim, but it is a measure of the complexity of the physics involved - how air moves in waves, why certain patterns form - that more than 60 years later scientists are still wrestling with the question of how the jet stream operates and what shapes it.

Dr Blackburn and his colleagues studied the pattern of the jet stream during the floods in June and July 2007 and found it to be similar in appearance to now.

So it seems that if it gets locked into the wrong position, with a pattern of large waves, heavy rain is the result.

Climate change

On top of this, there is the related question of climate change. Most researchers are extremely reluctant to attribute any single weather event to global warming.

But Dr Peter Stott, a leading climate scientist at the UK Met Office, says that since the 1970s the amount of moisture in the atmosphere over the oceans has risen by 4%, a potentially important factor.

That does not sound like much but it does mean that extreme rain storms may bring more rain than before - with more moisture in the air, what goes up must come down, and the odds are worse.

"That could make the difference between a place getting flooded or not getting flooded," he said.

So there are no exact answers, just some important strands in the science and a lot more research still needed to understand exactly why our weather is so bad.

When I rang the BBC Weather Centre this morning and said I wanted to talk about the rain, a colleague answered with a single word, as if the constant storms were her fault: "sorry."

David Shukman Article written by David Shukman David Shukman Science editor

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  • rate this

    Comment number 117.

    I think we should blame this on Bob Diamond. After all, he's got away with everything else!

  • rate this

    Comment number 116.

    I'm surprised that David Millibandwagon isn't blaming the coalition!

  • rate this

    Comment number 115.

    97.Russ Wirral
    the news neglects to tell you is that quite rightly the last 30 years or so have been warm. This has been due to the strength of the solar cycles, which as we are moving into a solar minimum - will lead to cooler weather. Winters are expected to be colder and windier, summers sunny and showery..
    That big yellow thing is not part of the C02 models, so it cant be relevent :)

  • rate this

    Comment number 114.

    @73 . You need to live in Scotland to understand what that comment meant!!!
    Anything in England affected by the weather gets a mention on the main news.
    Anything affected by the weather in Scotland, only gets told on the local, or Scottish news. It seems the 6pm news considers its viewers to be English, wanting English related news, us lot in the north can wait for the local news.

  • rate this

    Comment number 113.

    When we've 10,000 years of reliable weather data to feed into our climate model... our computers still won't be able to deal with the complexity to a sufficient enough degree of accuracy to predict the weather.

  • rate this

    Comment number 112.

    I remember reading, many years ago, that 19th-century scientists explored the possibility of separating the Labrador Current and the Gulf Stream by building a 200-mile barrage out into the North Atlantic from Newfoundland. Alas, the technology wasn't available for such a massive undertaking. It was believed that Europe's climate would be changed to the extent that grapes could be grown in Norway!

  • rate this

    Comment number 111.

    I'd like to blame Michael Fish...oh, wait a minute, this isn't 1987 anymore is it??

  • rate this

    Comment number 110.

    When it eventually does stop raining, could the weather forecasters desist from saying it is "DRY" when it is palpably damp, dismal and very depressing. I don't call the non-raining periods anything but that. Dry is dry. Damp is damp. Can't these satellites detect what's on the ground?

  • rate this

    Comment number 109.

    94. KM0606 "when dinosaurs roamed the earth??? Then, what is now the UK, had a tropical climate"

    Well it would have done so as it was far far nearer the equator!

    See continental drift etc etc.

  • rate this

    Comment number 108.

    I am a Nobel Prize winning climatologist and can assure you this wet weather has absolutely nothing to do with Climate Change FACT. Anyone who disagrees with me is ignorant and quite possibly a neanderthal FACT.

  • rate this

    Comment number 107.

    Nobody can deny that our climate is changing:- it has done so in natural cycles for millennia.

    The question is whether human activity is having a significant effect on these changes, and nobody has yet proven either side of that argument to everyones satisfaction.

    The fact that our politicians milk the situation for everything it's worth certainly does not help rational debate!

  • rate this

    Comment number 106.

    As far as I can recall we haven't had a decent summer for about 5 years. The long hot summers we used to know seem to be gone. Could be climate change, but it's a pattern that seems to be more and more familiar.

  • rate this

    Comment number 105.

    Quote from foreigner visiting from a sun-drenched land: "I love visiting your country in the summer because everything is so green. Such a shame it rains all the time."

    Cause / Effect? Or is this apocalyptic weather attributable to female bishops / gay marriage / Barack Obama?

    Failure of sodden crops will push up food prices, which is bad for consumers. But will we bail out the poor farmers?

  • rate this

    Comment number 104.

    Thank God. The inter-continental 'water wars' they predicted in the 1970s probably won't be starting this year then...

  • rate this

    Comment number 103.

    Bring back 1976!

  • rate this

    Comment number 102.

    Just waiting for the usual cognitively challenged few to blame it on global warming...

  • rate this

    Comment number 101.

    Did you read the article?
    1. 'On top of this, there is the related question of climate change.'
    2. Where was the counter argument?
    3. Yes it does. It's not the daily mail or the Guardian. It is supposed to report the facts not make judgments. But hey that went by the side many moons ago.
    4. Some people 'Gamble' on religion and people like you decry them.

  • rate this

    Comment number 100.

    Is the Met Office simply an umbrella organisation?

  • rate this

    Comment number 99.

    ~6 months ago I remember reading articles of why Britain is getting drier & heading towards bigger droughts in the future. Now, we are dealing with floods and the articles cover new/alternative theories to cover the rapidly changing situation. Weather prediction is clearly too complex for our brains/computers, e.g. are affects of the sun included, e.g. solar flares and ionisation?

  • rate this

    Comment number 98.

    Well if it continues like this for years, everyone will be requiring 4x4 high clearance vehicles, for wading through the summer floods and getting to work in the deep winter snow.


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