From the trail of devastation left by the ‘Great Storm’ of 1987, it’s clear that it was an unusual event.
Analysis of the storm suggests there had been nothing like it since 1703 and that it was an event so rare you would only expect a storm of that magnitude once every 200 years.
That does need clarifying, however, as we have seen storms as powerful as that before and since then – but they have affected areas which are more used to stormy weather such as the far north of the UK (like the north coast of Scotland) and far South West (like the Isles of Scilly).
So what was truly unusual about this storm was that it affected the South and East of England – which had an important bearing on the impact of the storm.
But how did the storm develop?
Most Autumnal storms head in from the Atlantic to the west of the UK, but this storm developed over the Bay of Biscay to the south.
It started as particularly warm tropical air and very cold polar air collided, forcing the warm air to rise and creating an area of low pressure.
The big difference in temperature between the warm and cold air helped to cause rapid ascent and therefore particularly low pressure – at one point it measured 951mb over the English Channel.
Crucially, just to the west of the low, pressure rose rapidly (due to descending air), to leave a big differential in pressure. You can see the difference in pressure in the tightly packed isobars in the (hand-drawn) chart from the early hours of 16 October 1987, below.
The atmosphere naturally tries to even out this pressure imbalance with the air flowing from the high pressure towards the low pressure – what we feel as wind
Much like water flowing down a plughole, that air doesn’t rush in straight lines but spins around the centre of a low pressure until it reaches the middle due to the Coriolis effect.
The bigger the difference in pressure between the high and the low pressure, the faster the flow of air is – and in this case that big differential led to hurricane force winds.
A sting in the tail
We now know that the strength of the storm was boosted by a phenomenon known as the ‘Sting Jet’, where cold dry air descends into storms high in the atmosphere.
Rain or snow falling into this jet of air evaporates and cools the air further, adding more energy which translates into stronger winds. By the time this ‘sting in the tail’ reaches the ground it can produce winds of 100mph which are concentrated over a small area.
In 1987, no-one knew sting jets even existed, but now they are well understood and included in forecast models. The storm which affected Scotland in December 2011 was boosted by a sting jet, explaining the maximum gust speed of 164mph recorded on top of Cairngorm.
The combined impact
It’s clear that several factors came together to make this storm particularly ferocious, but it was the track of the storm which was perhaps most significant.
Arriving on the south coast of the UK, it tracked north and east over the course of several hours before reaching the Humber estuary at about 5.30am.
This path took in a large, built-up and very populated part of the UK which exacerbated the damage caused.