Why has Britain had such extreme weather? Scientists reveal how it's down to the 'North Atlantic Oscillation'

It's hard to believe amid the warm weather Britain has been experiencing for the past several weeks that only two months ago, parts of the country were still covered in snow following the Beast from the East.

And scientists have now revealed how the rapid transition from cold to warm weather was caused by the flip of a phenomenon called the 'North Atlantic Oscillation', which measures the strength of westerly winds off the Atlantic.

The flip of the NAO from a strong negative to a strong positive phase coincided with the transition towards warmer continental air at the time of year, which resulted in the quick change in Britain from sub-zero to balmy conditions.

This graphic shows anomalies in surface pressure for March. The month was dominated by higher than normal pressure to Britain's north, and lower pressure to the south, bringing in anomalous easterly winds. This is a negative North Atlantic Oscillation (NAO)

Simon Lee, a researcher at the University of Reading's department of meteorology, told MailOnline how the story of the extreme conditions began in mid-February with an event known as Sudden Stratospheric Warming (SSW).

This rapid warming of temperatures between six and 31 miles up in the stratosphere generated 'atmospheric blocking' - when the usual west-to-east flow of the UK's weather at its latitudes was deflected or reversed.

This manifested itself in a high pressure system north east of the UK, which brought in a feed of easterly winds that became the 'Beast from the East' – combining with Storm Emma from Portugal to bring deadly conditions to Britain.

This was represented as a negative phase of the NAO, which means the westerly winds reversed to easterlies - bringing cold weather in winter. The Atlantic jet stream sunk south, so that Britain became on the cold pole-ward side.

This graphic shows anomalies in surface pressure for May so far. It shows how Britain has seen the reverse weather pattern of a positive NAO. But Britain has still been influenced by continental air, thanks to high pressure to the east yet again, which this time is warmer

Continental air becomes very cold in winter, due to the low heat capacity of the land surface – which also means it heats up quickly during spring and summer.

This means that weather patterns which generate cold weather in the UK in winter tend to generate warm weather in the spring and summer months.

Mr Lee said: 'In late April, the NAO transitioned into a strong positive phase. This is associated with a strengthened Atlantic jet stream, which shifts pole-ward and leaves us on the equator-ward, warm side, under high pressure.

'The flip of the NAO from strong negative to strong positive phase, coinciding with the transition toward warmer continental air at this time of year, led to a rapid transition from cold weather to warm weather.'

Hammersmith Bridge in West London is blasted by the Siberian snowstorm nicknamed the 'Beast from the East' on March 1 this year

Mr Lee continued: 'It's a complicated picture, but the basic result is that we've been under the influence of high pressure systems both in March - with the Beast (from the East) - and then in April and May.

'And the simple change of season has resulted in a switch from cold to warm. Amplification of patterns in the jet stream, probably due to the SSW, have helped us see both extremes.'

He added: 'It's a case of perfect timing, in a way. Had the SSW happened earlier in the winter, we likely wouldn't have seen the rapid change as we have had.'

Mr Lee also pointed out that flipping between extremes is common in the UK, with a prolonged absence of rainfall for central and southern England prompting a hosepipe ban in April 2012 - before the wettest April-to-June period on record.