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 97.

    What the news neglects to tell you is that quite rightly the last 30 years or so have been very 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, but not as hot as 2003.

  • rate this

    Comment number 96.

    The Summer of 1315??? Ah, I remember it well....

  • rate this

    Comment number 95.

    If there's a hot spell, we're told it's due to AGW. If there's a wet spell, or a cold spell, or windy one, it's down to AGW. If there's a sudden rain of rabbits it must be because of AGW.

    An earlier post mentioned a 7-year wet period starting in 1315AD. It happens, that's all. No need to invoke AGW at all: as the article says, the Jetstream isn't well understood so the cause could be anything.

  • rate this

    Comment number 94.

    What about when dinosaurs roamed the earth??? Then, what is now the UK, had a tropical climate...
    Could this 'Global warming' actually be one of the natrual cycles that the earth takes?
    Naturally warm up and down, up and down....

  • rate this

    Comment number 93.

    # 57.Waint
    # We have to just accept that the British Summer is the worst in the world.

    If it's that bad, emigrate.

  • rate this

    Comment number 92.

    As an aussie living in the uk for my 3rd summer, i see no difference between this summer and the last 2...Im waiting for mid august.. Then the possibilty of 2 days over 23 degrees seems more likely... but i vote keep the jet stream for winter and lets get some snow!

  • rate this

    Comment number 91.

    Despite the effect of this I still find it really interesting. BUT, more amzingly I worked out the hosepipe ban rules...

    You can only use your hosepipe when it's raining, and only then to water plants.

  • rate this

    Comment number 90.

    Love the rain - whisky is made from water so I just look at this weather as storing up on future whisky.

  • rate this

    Comment number 89.

    The rain it raineth every day, upon the just and unjust fella
    But more upon the just because the unjust has the justs umbrella.

  • rate this

    Comment number 88.

    It's easy to just say that this is the British weather and we should stop moaning about it, but have some sympathy for the hundreds of people who are having their homes wrecked because of this.

  • rate this

    Comment number 87.

    It never ceases to amaze me how idiotic the Brits are when it comes to weather. So we've had a bit of rain down south. It's only news when it happens in the south. All we need is a couple of dry days and a warm weekend and everyone will be screaming about heatwaves and droughts. Just be thankful we get as much rain as we do. The outcome if we stop getting enough is to bad to contemplate.

  • rate this

    Comment number 86.

    More moisture above the oceans, unusual weather patterns, ice caps melting are all facts. What is not clear is if this is a just a regular cycle or if man has impacted on the atmosphere. We can gamble and not change anything assuming the global warming scientists are nuts. However if the are correct and we don't change the impact will be catastrophic and change will then be hard to enact.

  • rate this

    Comment number 85.

    @74: 1. The article hardly mentions global warming and certainly doesn't blame it for our current weather. 2. In what sense is it one-sided? 3. Does every article ever produced by the BBC have to have a counter-argument on the same page? In this case there isn't even much of an argument to counter. Essentially the article can be summed up be "We don't know". Hardly controversial.

  • rate this

    Comment number 84.

    Well I suspect the unpredictability of the British weather provides ample work for the weather men and weather forecasters. Imagine if it were like in some of those places where it is constantly hot all year long.

  • rate this

    Comment number 83.

    The terrible weather WAS forecast.

    By the sunspot forecasters who predicted that the Jet Stream would not be kind to us this summer. (see WeatherAction etc.) All due to sun spots apparently! (And nothing to do Anthropomorphic Global Warming and CO2)

  • rate this

    Comment number 82.

    @Paul from Barking. So Climate Change is a means of exploiting the ignorant is it? I really wish it was, but there are plenty of uneducated that try to deny the science. The ones shouting loudest seem to be the fossil fuel industry and the car manufacturers. I wonder why that is? It must be my lack of education. (Beyond an MSc)

  • rate this

    Comment number 81.

    "a British summer that has so far been one of the most miserable on record"

    ...depending on who you ask. England is looking unusually green. Not only the plants, but the insects seem to be loving it too. Doubtless this is all due to the evil carbon dioxide which is destroying the planet as we speak, blah blah blah, drone drone drone.

    ...errm... hang on a minute...

  • rate this

    Comment number 80.


  • rate this

    Comment number 79.

    Yet again the ignorati demonstrate that they don't know the difference between climate and weather.


  • rate this

    Comment number 78.

    I blame the Bankers.
    And the exploitative Water Companies, who put up a drought order at the drop of a safety helmet.


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