Q&A: Snow chaos in the UK
Snow is causing massive disruption to transport in the UK, but why is it here and why can't its effects be mitigated?
Why is the weather so cold?
During winter, the UK's weather is greatly affected by the nature of a jet stream as it crosses the Atlantic. This current of air travels from west to east in the higher reaches of the atmosphere.
Under "normal" circumstances, this jet stream brings in weather systems from the Atlantic, causing the wet, windy, cloudy weather that is typically associated with mild British winters.
But the path of the jet stream, like this year, can wander, meaning the mild weather systems are not being brought to the UK in the same way.
During these periods of "weakening westerlies" the cold weather from the north moves in.
Will it be this cold next year?
Years of weakening westerlies have come in clumps of three and four in recent decades. So we could well get another very cold winter next year.
But it does not mean the UK is getting colder. The cold winters of the last couple of years contrast with the mild winters that preceded them. But in the 1960s and 1940s there were very cold winters too.
A handful of cold winters means no more than a handful of hot summers.
Why can't we just deal with the snow?
Many people stuck on UK roads over the past few days will have wondered why the UK simply doesn't purchase more gritting and snow plough vehicles, and stockpile more grit.
But the UK does have more than it did last winter, thanks to a review of winter resilience led by David Quarmby. He made a series of recommendations affecting road, rail and air transport, many of which are already in place.
But Mr Quarmby makes the point that in the UK, spending on improving cold weather resilience must always be balanced by the rarity of really severe winters.
In countries that seem to deal better with snow, like Sweden, Norway or Canada, large sums of money are spent on snow ploughs, winter tyres and other coping strategies. But at the same time, heavy snow in these places is a guaranteed occurrence.
Many in the UK might balk at vast sums of government expenditure being directed at tackling weather that only happens on a sporadic basis.
Why can't planes take off in snowy and icy weather?
There are two main issues causing the disruption to air travel.
Firstly, icy or snowy runways can cause planes to skid on takeoff or landing. UK airports, like Heathrow, do have snow ploughs and runway de-icer vehicles, but they cannot cope with the severity of the current cold snap.
Runways can only be ploughed when planes are not using them, meaning delays of some measure are inevitable at airports that operate at or near capacity.
The second problem is the icing of the plane itself. The ice changes the shape and texture of the wings and their flaps, creating a drag effect and making the aircraft impossible to control.
This de-icing needs to be done shortly before takeoff, otherwise the plane ices up again. There can be a window as little as 20 minutes. If a plane has to wait longer while a runway clears up, a chain of delays can occur. At an airport as busy as Heathrow, co-ordinating de-icing and runway clearance can potentially be a logistical nightmare.
In some places, like Sweden's Stockholm-Arlanda airport, planes can be kept flying, but only as the result of an extensive cold weather operation.
Should you grit your own street and clear snow from paths and pavements?
Yes, the government says.
Certain people have suggested in the past that clearing the snow from the pavement outside your house could get you into legal hot water. But even personal injury lawyers have been sceptical about the possibility of such legal action.
And the authorities have made their position clear with a snow code that makes it clear that you can clear paths and pavements without threat of legal action-induced penury.
Other civic duties people might consider include giving blood, as it's running a bit low.
Why do schools close when it snows?
Many parents get irked when schools shut, but shops and offices are open.
There are two facets to school closures - the inability of pupils and teachers to get there safely, and the need for a certain number of staff to assure pupil safety.
Health and safety considerations also apply with icy playground, low indoor temperatures and tap and toilet malfunction all being potential obstacles.
The final decision to close lies with headteachers, barring exceptional circumstances.