If there is to be American military activity in Afghanistan, it will run into a problem that has always plagued armies there - the winter weather.
The Taleban's winter ally news.bbc.co.uk
Afghanistan's ongoing civil war usually stops from November to April.
Snow is the norm over much of the centre and north of the country, arriving in blizzards.
In the mountains, temperatures can fall to -40C (-40F). The snow drifts to two or three metres (10ft) deep.
The wind chill makes matters even worse. At -40F a wind of just 15mph can take the temperature down to -85F - in which exposed flesh freezes in less than 30 seconds.
Effect on troops
Troops slow down - everything takes longer and requires more energy.
The cold regions research laboratory of the US Army Corps of Engineers estimates that it takes 80% longer for even a motivated team to repair a damaged runway, for example, on a snowy, windy day, than it would in summer.
Snow makes movement heavy going
"The effects on soldiers include increased time to perform tasks, reduced dexterity and accuracy, reduced grip strength, and failure to achieve adequate concentration on the task at hand," it says.
Well-equipped modern armies have clothing "systems" based on the principle of insulation, layering, and ventilation.
Inner layers are made of fabrics which retain heat but draw perspiration away from the skin, and an outer layer which lets perspiration evaporate, while repelling water on the outside.
When the Russians were fighting in Afghanistan in the 1980s, soldiers found their cotton sleeping bags were a liability - it is said their prize trophy was a light, waterproof and warm Western-made bag captured from mujahideen fighters.
Food intake
But cold-weather clothing is cumbersome, so it greatly increases the energy needed to do anything.
Soldiers must eat - and drink - more than usual
The US Army's Research Institute of Environmental Medicine says soldiers need to eat 4,500 calories a day - the average in normal circumstances would be 2,000 to 2,500.
And they need to drink three to six litres - more water in the bloodstream improves circulation, sending more blood to the hands and feet.
Caffeine dehydrates the body. A better choice is something warm, sweet and non-caffeinated, such as hot cocoa.
"Soldiers must drink even when they are not thirsty," the institute warns, noting that below -10F their standard-issue canteens and five-gallon metal water containers can freeze.
Natural defences
Snow can be thawed to provide water - but cold also drastically reduces the effectiveness of chlorine or iodine water purification.
The human body's way of keeping warm includes shivering and vasoconstriction.
Shivering burns energy - hence the need to consume more calories than normal.
Vasoconstriction is the tightening of blood vessels in the skin. This reduces blood flow and conserves heat - but can lead to numbness, loss of dexterity in hands and fingers and eventually frostbite.
People do not acclimatise to cold as well as they do to heat, so the best way to cope is to adjust mentally and emotionally, through training.
Equipment problems
The armies of Indian and Pakistan are very adept at fighting at high altitudes and in very cold weather as a result of their conflict in Kashmir, yet many of their casualties have been as a result of cold injury.
Soldiers also have to take greater care of their equipment. The action of a weapon can jam.
The British Army's Standard combat rifle, the SA80, is being modified because its complexity meant it was prone to jamming in extreme climates - hot or cold.
This would be unlikely to trouble American troops whose M16 ArmaLite is more reliable - which is why the British SAS special forces use it in preference.
Snow-covered or wet ground presents obvious difficulties for heavy military vehicles.
And in extreme cold, fuel and hydraulic fluids can freeze.
Icing on aircraft
Icing is always a problem for aircraft. Frost, snow and freezing rain can affect aerodynamic surfaces, engine inlets and windscreens.
It can take up to four hours to prepare a single aircraft for flight.
The composite materials on newer aircraft can be damaged by scraping and the use of heat to get rid of ice.
The usual approach is to use glycol-based solutions. But glycol is potentially damaging to some parts as well environmentally unfriendly and expensive.
Flying restrictions
Once airborne the problem does not go away. If melted ice freezes again it can jam control lines.
Aircraft are at risk in extreme cold
Aircraft which fly at relatively low altitudes and speeds, such as helicopters and turboprops, are most at risk.
Because weather forecasters cannot predict precisely where icing might occur - only in general terms - some aircraft simply cannot take off, just in case.
Experiments are being carried out to try to develop detection systems - with a view to getting a ground-based system within five years and an airborne one within 10 years.
Disorientation of the pilots in a blanket of whiteness is also a danger. During the Falklands War, two of the Wessex helicopters sent to recover SAS troopers from South Georgia crashed in a "whiteout".
Radar problems
David Jordan of the Centre for Defence Studies at King's College, London, said snow could also cause ground mapping radars to malfunction.
It presents what is known as a "high clutter" environment, scattering the reflected signals by which radar works.
The saviour for modern aircraft is GPS - satellite positioning systems are not affected.
But the targeting radars of "smart" weapons could be, and infra-red targeting suffered major problems in winter, he said.
"Some of the likely targets, such as training camps, are 'area targets' - there's very low risk of collateral damage.
"So the Americans might use conventional, 'dumb' or iron bombs."
Long-range bombers flying from bases such as Diego Garcia in the Indian Ocean could deliver a heavy load of these - the B-1B could carry 84, 500-lb bombs.
"The problem then really is only getting to the target area and shovelling large amounts of unguided ordnance onto it.
"Whether that's what they would do is another matter. They might prefer to use special operations forces on the ground."
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Internet links:
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