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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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