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PARTLY TRUE. Most rain in the UK is part of a frontal system that

will typically blow through in 3-4 hours. This saying has much good

truth in it, for if it rains before seven, more than likely it has been

raining the greater part of the night and the storm is about over.

"In the hours between seven and eleven, old Sol gets in his effective heating which begins to dissipate the clouds, and as he rises high in the heavens toward noon his chances of success in breaking up the entire cloud covering are highly favourable" - Old Folk Lore 1905

The British winter is notoriously unpredictable, sometimes cold and dry and

sometimes mild and wet. This unpredictability is a consequence of the

earth's rotation and the coriolis effect - an effect that I first began to truly

understand, as moving to the right with height, when I first started flying

in hot-air balloons. 

The key factor in Britain, however, is our island location off the coast of

mainland Europe where we sit underneath the boundary between two of

the earth's climate cells. This means that above our heads, during the

winter months, there's a battle going on between two different types of air,

cold polar air from the north and warm tropical air from the south. But

there's a further factor that influences the outcome of this battle between

warm and cold air. The boundary between the cells can move.

This movement can be affected by a

phenomenon that's generated right

at the boundary between the cells

and it's a product of the Earth's spin.

Right at the boundary, high up in the

sky, a wind blows about 10 km up.

It's really, really fast. It can travel at

speeds of up to 450 kilometres per

hour. It coils all the way around

the planet, at about our latitude,

and we call it the jet stream.

The jet stream is crucial because it influences the boundaries between the

cells, and therefore between cold air to the north and warm air to the south.  

In December 2010, the whole of Britain shivered under a blanket of snow

and ice. It was one of the coldest winters since records began. The reason

was that the jet stream had developed a kink. Over the Atlantic, it sat much

further north, near the Arctic. Then it swung down over Britain. This

temporarily shifted the boundary between the cells and brought cold, polar

air across the whole country. Unfortunately for our weather forecasters, it's

particularly difficult to predict the meanderings of the jet stream.

The spin of the Earth makes the weather here in the UK unusually

changeableand particularly hard to predict. The fact that you wake up

every morning and the atmosphere surprises you, just adds to the spice

of life.


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Comment by Michael Grove on September 5, 2020 at 14:57

How £1bn new supercomputer will transform weather forecasting in UK

The Met Office's Cray XC40 supercomputer which was installed in 2016, can perform 14,000 trillion calculations per second and is capable of taking in 215 billion weather observations from all over the world every day. But despite its impressive computational power, and ability to give advanced warning, accurate long-range forecasts on a local level remain out of reach.

"It's the regional details that are important, about where the risks will be, where the rain will fall and getting that information to first responders as fast as we can," says Andy Kirkman, head of government services at the Met Office. That regional data could be provided in the future by the Met’s new supercomputer, which was announced on Monday and will be the most powerful climate supercomputer in the world. The government said that it will invest £1.2bn into the project. The supercomputer itself is expected to cost £854m, with the remaining funds set to go towards investment in the Observations network and programme offices over a 10-year period from 2022 to 2032. The machine will increase the Met Office computing capacity six-fold allowing it to better forecast for airports so they can plan for potential disruption. 

The Met's new supercomputer will look to deliver at least a further three times supercomputing capacity in the last four years of the programme. But, aside from the extra processing power, the Met Office is also hoping the machine will make it easier to deal with the data coming out. “It's valued not just in the accuracy but making that data more available to people to work on,” Kirkman says.

Being able to do this will only become more urgent as global warming starts to change the environment. The computer will use all the data available to allow it to predict everything from reasonable scenarios, to what are termed “black swan events”, ones which are unpredictable and potentially catastrophic.

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