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 277.

    8 Minutes ago
    I believe this is a Warning from God, for sinners and for the nation to come to repentance. I dont believe in global warming its rubbish.

    Just mythical sky daddies for you eh?

  • rate this

    Comment number 276.

    I'm loving the wet weather. Keeps the local yobs indoors.

  • rate this

    Comment number 275.

    its about high pressure weather systems ,we have changed them for worse and now the World is doomed ,back in the middle 1990s a high pressure weather system sat for over a month to the east of the mississippi river it caused massive flooding all the low pressure weather systems just stacked up and dumped a large amount of rain in a short time ,clearly these weather systems are here to stay .jj7

  • rate this

    Comment number 274.

    It is unfortunate that we are faced with this wretched situation controlling our weather.
    It is even more unfortunate that our media forecasters are not a little more '

  • rate this

    Comment number 273.

    We need this rain. Other less fortunate nations would be fed up with our whinging about it.

  • rate this

    Comment number 272.

    If I have to moan, it's that the jet stream should be north of us in summer (warm summers from the south) and south in winter (cold, snowy winters from the north).

    I'm not really miffed about the rain. Let it pour.

  • rate this

    Comment number 271.

    If only Bruce Willis was running the world, he'd have done something about it already. A 100 Gigaton airburst somewhere high over the Bay of Biscay maybe?

  • rate this

    Comment number 270.

    It's important that we come to understand these anomalies so that we can better predict if they will continue, get worse, or whatever.
    I remain highly suspicious that some country(s), having achieved the ability to manipulate the weather, tectonic plates, etc. - inclusive of earthquakes & tsunamis - are indeed doing exactly that. (In the US its called HAARP. Russia has capability too.)

  • rate this

    Comment number 269.

    Its just the Earth getting its own back. Better get used to it if we continue to upset the natural order in the way we have been doing for the past 4 centuries or so. And it is not global warming but climate change.

  • rate this

    Comment number 268.

    In the days of my youth, in '63 I think, we had a winter/spring so cold and so long that for 6 weeks it never went above freezing. All the pundits were talking about the comming of the next ice age.

    Ah well ...

    Now it's all talk about global warming.

    Fashions in metrology change, almost as fast as the Brittish weather. Get used to it.

    (And what happened to acid rain, is that still arround ?)

  • rate this

    Comment number 267.

    Ok so it's raining a lot.Big deal.
    In many parts of the world people die from freezing,broiling,earthquakes,hurricanes and the list goes on of weather related deaths.
    We have it good here.
    Stop whining.

  • rate this

    Comment number 266.

    What gives me the heebie-jeebies is the possibility that the jet stream is "echoing" the southern sweep of cold fresh water pouring out of the Arctic. If those two huge forces combine... Suppose the latter slows down the North Atlantic Drift while, at the same time, the jet stream brings Northern Europe heavy precipitation and lower temperatures...
    Yikes. The opening round of an Ice Age?

  • rate this

    Comment number 265.

    Variations in climate and mini ice ages, hot and wet periods are the norm. A stable predictable climate isn't.

  • rate this

    Comment number 264.

    I was expecting to be told it was labours fault....

  • rate this

    Comment number 263.

    I believe this is a Warning from God, for sinners and for the nation to come to repentance. I dont believe in global warming its rubbish.

  • rate this

    Comment number 262.

    I think we should just move to the south of France................ All of us, everyone in the world................... just a thought


  • rate this

    Comment number 261.

    Doody doo doo doody doody doo doody doody

    Doody doo doo doody doody doo doody doody

    I'm singing in the rain
    Just singing in the rain
    What a glorious feeling
    I'm Happy again !!!

  • rate this

    Comment number 260.

    I am so fed up with forecasters and other "experts" calling rain "bad" and sun "good". I don't fancy living in a dry dusty desert, and find rain beautiful, pleasant and refreshing. Snow even more so. Stop anthropomorphising weather. That said, this summer has been the driest for years in the West Highlands where I live, I can't wait for some rain. Go figure....

  • rate this

    Comment number 259.

    It's getting annoying reading how it's a good thing that it's raining for the farmers sake, I don't buy that for one minute. that then means anyone south of the jet stream must really suffer, I am pretty sure Spain and France are not having those issues.
    Fact is, this weather is making the country miserable, olympics will be a washout, science needs to fix it so we have some sun back.

  • rate this

    Comment number 258.

    The Asian Tsunami shortened the earth's day by 1.8 microseconds and shifted the planets axis by 25 centimetres I am sure the it must have some effect on the weather and maybe why the gulf stream has moved.


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