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 257.

    Why is there no mention on the news of the 2001 NASA report showing a correlations between solar activity and jet stream migration?

  • rate this

    Comment number 256.

    It's an accepted fact that warming adds moisture/energy to the weather, culminating in greater precipitation and wind, at our latitude and particularly at our geographic location. This is just a foretaste of worse to come. Add to that the Americans' beaming megawatts of microwave energy, with their irresponsible HAARP facility, at the jetstream itself, there's no wonder the weather's in a mess.

  • rate this

    Comment number 255.

    The current positioning of the jet stream has had a pretty noticeable effect on the Summer weather... what changes to the weather would we experience if this was the position of the jet stream during Autumn and Winter?

  • rate this

    Comment number 254.

    Its FANTASTIC weather, our well normally runs dry for 3-4 months of the year meaning we have to buy water in at great expense, not any more. Its Bath Heaven

  • rate this

    Comment number 253.

    Things were better under Michael Fish. Apart from the '87 hurricane, he was pretty good at controlling the weather. Nowadays they get paid a fortune and can't even give us sunshine in the Summer. I say bring back Fish!

  • rate this

    Comment number 252.

    The current path of the jet stream looks suspiciously like the extent of the ice sheet in the last ice age. I believe all of the melting ice and permafrost in the arctic circle is releasing vast amounts of freshwater into the arctic ocean and is affecting the salinity of the water. The change of the jet stream is a pre-cursor to a change in the gulf stream. Be prepared for 10m of snow this winter.

  • rate this

    Comment number 251.

    The terrible weather is simply a judgement from the Gods, aka the Bankers, who have been losing out on bonuses and reputations.

  • rate this

    Comment number 250.

    I want to know where they have hidden the global warning geeks that they wheel out when the sun shines for more than two weeks

  • rate this

    Comment number 249.

    High or Low, Science or spirituality, Math or politics, Humane or nature all is same and inter-related. It is truly said where Science be silent spirituality speak. We all are made of five natural elements and nature or weather can't be differing from us. The weather is wet here as people cry or say women cry too much and no one is to care. Be leading in humanity or spirituality can bring change.

  • rate this

    Comment number 248.

    @Andy Don't worry, it's not all your fault. I took a career break in March so the weather was bound to be awful. If you want some good news, I'm going back to work in August - it'll be the hottest on record. Just don't say BBQ...

  • rate this

    Comment number 247.

    Some of my colleagues come from India and were commenting that our reputation is to talk about the weather ALWAYS. They were joking the other day that now they find themselves talking about the weather every day! Successful acculturation or just human despair?

  • rate this

    Comment number 246.

    How often have we heard from the newscasters that floods, heatwaves, droughts etc. are the worst in 150 years or 200 years, or some other unconfirmed period of time? These statements simply confirm that the conditions are not new. They have happened before. There must be millions of combined elements which effect the weather. Man is most probably not even one of them.

  • rate this

    Comment number 245.

    It's Camerons fault. He's probably sold the summer to one of his cronies. Well, tell me it ain't so.

  • rate this

    Comment number 244.

    Humans are a mere blip on the time scale our Earth works on, parasites hanging onto the crust of this planet, whatever happens regarding climate/weather/whatever will not be influenced by humans or ever fully understood. We can't as a species even predict lottery numbers what chance do we have of predicting climate/weather/whatever ? arrogant..... yes, clever.... sometimes, stupid...... mainly

  • rate this

    Comment number 243.

    There is a bright side to this, we have been getting some really spectacular clouds. Sunshine is great, but there is a lot of joy to be had just looking up sometimes for seriously beautiful skies. Clouds aren't all bad. That said, I have not suffered with the terrible flooding and my thoughts go out to all those that have.

  • rate this

    Comment number 242.

    This lovely chat about the weather is very nice but could we now talk about the 25 REMPLOY factories that are run for the disabled to find employment in which are being closed throwing said disabled on the scrap heap! (and in the middle of a recession your caring gov in action)

    I suppose THEY wont get a termination package akin to that of Mr Diamond.

    So how about it BBC?

  • rate this

    Comment number 241.

    Last time I looked at a globe, the UK was a tiny little Island off the coast of Europe, a lot closer to the Arctic than the Equator.

    We are blessed by the Gulf Stream, it keeps us from freezing our butts off in winter. Occasionally, the Jet Stream will head North and give us a summer, but lets be realistic, we are never going to have Spanish summers, we are more likely to have another Ice Age.

  • rate this

    Comment number 240.

    Oh dearie me,

    It's wet in London. They're having "a British summer that has so far been one of the most miserable on record."

    Well, I live in Britain. Up here in western Scotland we've had a terrific summer.

    Thank you and goodnight.

  • rate this

    Comment number 239.

    HooHum the reason people mark you down is you are conflating an article about why the jet stream has become fixed in place, which we don't understand, but has nothing to do with taxation

    And Global Warming a climatic affect which the majority of evidence suggests human CO2 is expediting, and so we are taxing CO2 emissions

    FYI Since 2008 not one paper published in a reputable journal refutes AGW

  • rate this

    Comment number 238.

    I blame, the government, the weather forecasters, the moonies, the oil companies, the americans, the greek economy, the great sky pixie and most importantly a small butterfly flapping it's wings on Fiji!!!

    Damn inconsiderate I say.

    Someone/thing is to blame but it wern't me.



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