Once, a strip of seaweed or a rheumatic hip were all that were needed to forecast the weather.

Yesterday, however, the Met Office unveiled a £33 million super-computer that it hopes will be able to do the job with a little more precision.

The IBM machine is more powerful than 100,000 standard PCs combined. For the technically minded, that means it will have a speed of one petaflop, placing it among the world’s top 20 most powerful computers. It is capable of performing 125 trillion calculations per second.

For those less than technically minded, that means it may no longer be necessary for forecasters at the Met Office to look out of the window to find out what the weather is doing.

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The Met Office’s current computer can provide forecasts accurate to an area of 4km sq — roughly 2½m sq. The new machine’s resolution will be a grid of squares 1.5km by 1.5km, providing far greater accuracy.

The obsession in Britain with the weather is largely the result of its geographical position, sandwiched between the Atlantic in the west and the continental landmass of Europe to the east.

In few other places in the world is the weather as changeable or as unpredictable. If you live in the south of Spain, for example, the outlook is warm and sunny for the next six months.

The IBM occupies two halls each the size of a football pitch at the Met Office’s headquarters in Exeter. It will use 1.2 megawatts of power, enough for a small town, provoking criticism from environmentalists. Although the machine was switched on this week, it will take two months to boot up fully and will not be running at peak performance until after an upgrade in 2011.

The Met Office says that the machine will improve day-to-day forecasting but will still not guarantee accuracy. In theory it could save millions of lives by predicting long-term trends in global warming and forecasting extreme weather events, such as floods, typhoons and hurricanes.

Steve Foreman, chief technology officer at the Met Office, said: “This computer will allow us to make the most accurate weather forecasts we have ever produced. People should be able to see a noticeable improvement in the accuracy of forecasting.

“Not only will it help us tell you what the weather will be like today and tomorrow, it will help create a much better long-term picture. Obviously, we can never predict the weather 100 per cent accurately, but this will help considerably.”

The machine will process data from hundreds of weather stations, balloons, satellites and atmospheric observations around the world. Forecasters will be able to focus on local conditions anywhere, such as Afghanistan, where British Forces need accurate information.

John Hirst, the Met Office’s chief executive, said: “The effect of extreme weather events is becoming more severe and the potential impact of global warming is ever more apparent. The Met Office plays an increasingly vital role in researching and forecasting these events. The new supercomputer is an important step forward.”

Computers are developing at a staggering speed. The Met Office machine will have just a twentieth of the power of one currently being built at the Lawrence Livermore National Laboratory in California, called Sequoia, which will be able to simulate the effects of a nuclear test.