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A volunteer is sought to assume complete editorial control of the Club’s quarterly Newsletter from the end of 2011, soliciting content, selecting that which is most appropriate, liaising with our proof-reader and interfacing directly with the graphics designer who takes care of layout. If this challenge is of genuine interest to you, please contact the Club Secretary via the AAC(UK) Office for further details.  

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Windchill Mythology

by Mel Owen

We have all seen articles through the years on windchill, and we have all experienced it for ourselves in the mountains, but the usual discourses on the subject fail to provide the complete picture, and the usual tables of windchill figures with which we are usually blinded are no substitute for practical advice on how to interpret them and what to do about it. The following thumbnail images provide links to some of these tables on the web:

Thompson Higher Education  

CSG Network

Dundee University

NCAR

                                                                                                                                         

NWS, Kansas

NovaLynx

Outdoor Education

UKWIS

Laws of Physics

Fundamental to interpreting these (or any similar) tables is the following simple understanding. If an object (such as a human body) is surrounded by air, then the heat flow between the air and the object depends on their relative temperatures and the speed of any air flow. There are 3 cases to consider:

  • Air Warmer. If the air is warmer than the object, then heat will be gained by the object. The greater the rate of air flow, the greater the rate of heat gain.
  • Both at the same temperature. If both the object and the surrounding air are at the same temperature, there will be no heat transfer between them, irrespective of the wind speed.
  • Air Cooler. If the air is cooler than the object, then heat will be lost from the object. The greater the rate of air flow, the greater the rate of heat loss.

This simplistic explanation ignores evaporative losses from objects which are wet, but the many tables of windchill figures also ignore this factor, making no reference at all to the combination of wind and rain. Perhaps rain is relatively unimportant in this context?

The Mountaineering Situation in Adverse Conditions

Windchill tables apply solely to bare flesh, which is invariably minimised in really adverse conditions, when we will probably be wearing effective thermal base and mid layers inside a windproof shell garment. In such circumstance the external shell will adopt the same temperature as the air, irrespective of the wind speed. This means that the temperature gradient across our insulating layers from our skin to the outside surface, and thus our rate of heat loss, will be unaffected by wind speed.

Minimising Heat Loss in Adverse Conditions

Most of the body can easily be protected by the use of good insulation inside a windproof shell. The weak points in this ‘armour’ are usually heat loss from the head and especially from the face, but also a lack of draught-proofing principally at the neck but also marginally at the wrists and ankles. The combination of wearing a reasonably draught-proof balaclava plus woolly hat in conjunction with a waterproof hood and a pair of ski goggles, a neck scarf, gaiters and efficient wrist seals at the junction of sleeves and gloves genuinely makes a very significant contribution to one’s level of comfort if caught out in a wintry gale. They  should always be carried in one’s rucksack if one is venturing far in the winter whenever strong winds cannot be ruled out.

Shell Garments: Airtight or Breathable?

If our outer shell is genuinely airtight it cannot breathe. And if it is made from a breathable material which is saturated by rain it cannot breathe either, which is why some wearers of breathable shells think that their shell garment is leaking when it rains heavily. In neither case can perspiration escape, and it is this perspiration which causes the dampness.

  • If our outer shell garment can breathe, then there are two factors to consider:
    • Some of the external wind might penetrate to the insulation layer, but it will be far less serious than windchill tables suggest, since the speed will be reduced to a very small fraction of the external wind speed. Wearing a second lightweight breathable cagoule just inside our principal breathable cagoule comes very close to solving this problem altogether.
    • If the air temperature is below freezing, then there will be a layer within the insulation at which the temperature gradient crosses zero; at this point any perspiration vapour will form a layer of ice crystals and the perceived need for the shell to be breathable probably vanishes. [This is why if you are backpacking in subzero temperatures your sleeping bag, if promptly packed away in a stuff-sack each morning, progressively gets damper and thus colder each night, even if breathable]. 
  • If your outer shell garment cannot breathe, then evaporating perspiration cannot escape and your insulation layer will gradually get wetter and wetter and thus become much less effective as a heat insulator, particularly if you are exercising strenuously for a protracted period. The only guaranteed answer to this is to add a second airtight shell inside the insulation close to the skin, but obviously if you are working hard you might soon be swimming in perspiration .... but at least the insulation layer will still be dry, so you should still be warm even if wet! Being wet only makes you cold if the wetness has means to evaporate, but under these circumstances, it cannot evaporate, so you remain warm.  This is also why some backpackers in very low temperatures use poly bags inside their sleeping bags, although a technique occasionally encountered is to swathe your bare skin in cling film! One key benefit is that you can usually manage with a lighter sleeping bag ... or in lower temperatures.

There is no ideal answer; we are up against the laws of physics.

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Last updated 24 January 2011