FurTech Science

Exploding Rain

This video, from Richard Hammond's Invisible Worlds series, shows how rain drops distort and explode as they fall through the air. Other researchers have shown that the largest stable rain drops have a diameter of about 5mm and fall at 9m/s. Wind speed may significantly alter the behaviour shown here.

Rain Impacts With Hydrophobic Surfaces

The following high speed video shows how the surface tension of a water droplet effects the impact with a water repellent surface.

Reducing heat loss by reducing breathability!?!?

 

A new fabric has become available which is claimed"opens and closes like a pine cone depending on ambient temperature". As the temperature rises from 10'C to 20'C breathability improves by 50% to about 20000g/m2/24hrs. As the temperature falls the reverse process is claimed to reuce heat loss.

There seems to me to be some confusion here about heat loss, venting and breathability:

1. Breathability (or moisture vapour permeability) allows moisture to escape through a fabric without allowing cool air back in - i.e. wind resistance.

2. Venting allows moist air to escape and allows cool outside air back in to the garment.

3. Heat loss is improved by increased venting and breathability, but insulation is also improved by increased breathability because trapped air insulates better when it contains less moisture. The reduction in thermal efficiency due to humidity can be seen here.

Therefore the "pine cone fabric" would be of great benefit if it allowed increased venting in warmer conditions (assuming it could also prevent warm rain from getting in)... but the above figures suggest that this isn't the case and that reduced breathability when the fabric is cold will ultimately reduce the effectiveness of any insulation worn beneath.

These arguments also indicate why polar explorers prefer to use the most breathable fabrics available!

Click here for a review from OutdoorsMagic.

Raindrops Splash Before Hitting the Ground

This report in the New Scientists describes a simulation conducted at Harvard University that indicates that raindrops of a certain size begin to splash before they hit a surface as the air that is compressed in between begins to blow them apart. Other studies have shown how raindrops disintegrate as they get larger and faster due to the increased dynamic pressure on their front face. Please click here for an illustration.

Why is rain cold?

In the temperate regions of the world rain normally forms as ice and snow crystals in the upper atmosphere. It melts as it falls to earth, but can still be cooler than the air it falls into. The high heat capacity of this cool rain can have a noticeable chilling effect on the air at ground level. It can also quickly chill the outer fabric of your clothing system to below the dew point, causing condensation.

FurTech clothing keeps condensation, caused by chilling of the outer fabric, away from the body and allows it to drain away or dry as conditions improve.

Rain and Humidity

You can see from the graphs at this site that relative humidity seldom approaches 100%, even on rainy days. If relative humidity were to reach 100%, sweat would not evaporate and garments wouldn't breathe or begin to dry.

FurTech garments continue to breathe in cold, wet weather because they push liquid water through the outer fabric, utilising wind and fabric movement. With their high hydrostatic head, membrane garments trap liquid water (condensation) inside.

Pervaporation

Pervaporation is a combination of permeation and evaporation. It is a term used in chemical engineering to describe how low temperature and pressure filters can be used to separate liquids, for example the dehydration of organic solvents.

It is relevant to FurTech because our outer fabrics allow condensation on the inside to permeate the outer fabric and dry in the wind (lower dynamic pressure). A similar low temperature and low pressure process.

Can garments breathe in 100% humidity?

 

When relative humidity is 100% the air can't absorb any more moisture and it becomes increasingly difficult to stay dry. However, the outer surface of a garment and the boundary layer next to it, can be slightly warmer than the atmospheric temperature. This boundary layer can therefore absorb moisture from the garment and be whisked away by the wind or even drafts caused by your movement. Unlike an aircraft wing, for example, fabric flapping may help dislodge this layer. This process requires body heat to drive it and once you become chilled less heat is available.

Wind can also reduce the pressure over parts of your clothing, improving breathability. Please see this post.

In the rain, when the outer fabric is saturated, the inside is very likely to reach the dew point, causing condensation. This post explains how FurTech can remove condensation from your clothing, unlike membrane garments.

When relative humidity is less than 100%, breathability can be driven by greater humidity inside the garment and moisture diffuses through the fabrics.

When it rains is humidity always 100%?

Rain occurs when moisture in the atmosphere condenses on small particles of dust. This seeding of rain drops depends on the factors that effect condensation: temperature, humidity, pressure and surface chemistry. The latter hygroscopic effect may cause rain to initiate when relative humidity is less than 100%. Dust particles may, on occasions, have a lower temperature than the surrounding air and this can also cause precipitation. The seeded droplets are heavier than air and begin to fall, sometimes into air where the relative humidity is less than 100%.

Some sources suggest that the relative humidity is typically between 60% and 100% when it is raining. However, humidity is likely to increase as the air is cooled by the falling rain and the rain water is warmed by the surrounding air, increasing evaporation until equilibrium is reached or conditions change.

For more on cloud seeding try this link.

Does it rain when humidity is 100%?

When relative humidity is 100% the air can't hold anymore moisture (at that temperature and pressure). However, it doesn't necessarily rain. In misty weather relative humidity can be 100% without any rain fall.

Definitions of Hygroscopic, Hydrophilic and Hydrophobic

 

The term hygroscopic refers to the ability of a material to absorb humidity from the air. A hygroscope indicates changes in humidity.

Hydrophilic refers to substances that absorb water.

Hydrophobic materials hate water and repel it.

HardShell Over Wet SoftShell

 

Adding a hardshell over a softshell in wet weather typically means that water is trapped between them. This can virtually paralyse the breathability of the clothing system. The trapped rain may evaporate and diffuse inwards, making baselayers damper and will very often saturate the inside of the hardshell, chilling it to below the dew point and preventing breathability.

Adding a ShellTAover wet hardshells and softshells allows sufficient ventillation between the layers to mitigate these problems, especially if worn over a sack.

Please also see these posts on temperature gradients, humidity gradients and over layers.

Limb Area and Heat Loss

 

This study looks at how heat loss from the limbs effects thermo regulation in humans.

The segmented body diagram shows that:

• Head, neck and torso includes about 39.82% of the total surface area.

• The Arms and hands account for about 21%

• The legs account for about 31%

Venting the legs can have a big effect on temperature regulation because of their relatively large surface area and because the leg muscles can generate lots of heat. This report from Cornell University states that the heat generated by muscles can be 40 times that of all other tissue.

Over Layers and Reverse Breathability

 

Adding insulation over a wet waterproof layer for stops or when it's colder at altitude can be a sensible clothing strategy, especially if taking off your waterproof shell to add mid-layer insulation is going to result in all your layers becoming saturated with rain.

Water trapped between your Shell and Over layer will begin to evaporate in the warmth and will diffuse through your layers to regions that are less humid. The moisture can move outwards and inwards!

This "reverse breathability" can cause your skin and base layers to become damper before the rain water is driven out by body warmth as the temperature/humidity gradient normalises. It is most noticeable when the Overshell is warmer than your skin temperature. To reduce these problems remove as much of the rain water as possible before adding your over layer and put it on before you become chilled. Changing layers inside a ShellTA can help as its hydrophilic fabric absorbs some of the rain, acting like a giant towel.

Reverse breathability can also be noticed if you wear wet FurTech garments inside a car with the heater going, or in a warm pub or mountain hut. However, I often put up with this, as they dry quicker when I wear them.

Humidity Gradients

Evaporation and condensation depend on temperature, pressure and humidity and the breathability of your clothing system depends on all three factors. Evaporated moisture in one layer diffuses to layers with less humidity, but may be slowed or stopped by low breathability fabrics or cold fibres (on which it can condense).

On a damp day the air is humid and this hampers the breath ability of your clothing system.

Temperature gradients in your clothing system can help drive moisture outwards as explained here.

More on breath ability.

Is it condensation or sweat?

It may be worth clarifying what most people mean by condensation in a clothing system. It's fairly obvious when you see it because you will find very small droplets of moisture, often forming on loose fibres that aren't in contact with other layers. It is moisture that was in the air and has condensed again.

Sweat may be a cause of condensation as it evaporates and passes through your clothing system, condensing on an outer layer.

We are losing moisture through our sweat glands all the time. This is called insensible sweat. When we are hot the skin can be wet with sweat which is much more obvious.

Cavity Walls and Waterproofing

Have you ever looked inside a cavity wall? You will often see hundreds of droplets of water beading on the inside of the outer wall. This is usually condensation as the inner walls breathe and damp accumulates on the colder outer brick work. The void in between is sufficiently ventilated to allow the condensation to escape as conditions improve.

So, why isn't outdoor clothing constructed in the same way? Well, some of it is. The FurTech jackets have a two layered construction with a furry "void" in between. This keeps the outer "wall" from touching the inner one.

Sharing Body Heat

Making some crude calculations and assuming that each person is a cylinder, then ignoring the losses from the ends, heat loss is proportional to the circumference of each circle (cylinder cross section) divided by the area of the circle. So the heat loss coefficient for each is 2/radius.

Assuming that each cylinder huddles, one next to the other in line, then, crudely speaking, the heat loss coefficient for the line is 1/radius.

Summarising, these crude and incomplete calculations suggest that two people huddled together may have something approaching 1/4 of the heat loss from their surface area than two people insulated separately (2 times 2/r).

I don't actually believe these calculations, because of the assumptions made and the other types of heat loss ignored, but they do indicate why mitts keep your fingers warmer than gloves and why a group shelter or Blizzard Tube is exceptionally effective.

...And some credence may be given to the concept if you have experienced how much warmer it is in the middle of a large crowd on a cold day.

Duvet Blankets

Duvet blankets are sometimes rated in Togs:

• Light weight summer duvet - 4.5 togs
• Spring/Autumn duvet - 9.0 to 10.5 togs
• Winter duvet - 12 to 13.5 togs

However, duvet blankets don't fit as well as most sleeping bags or duvet jackets and require more insulation to be as effective. Click here for some comparisons of insulated jackets.

R values and U values

R and U values are used in the building industry. The R value is a measure of thermal resistance and is equal to 10Togs. In SI units it is measured in Km2/W.

The U value is the reciprocal of the R value and measures how well building materials conduct heat. U values are measure in W/m2K.

A Watt is a unit of power, or the rate of change of energy.

Tog & Clo

The tog is a unit of insulation created by the Shirley Institute in Britain. 1tog is equal to 0.645Clo or 0.1m2K/W.

One Clo represents the amount of clothing required to keep a resting person comfortable in a windless environment at 21.1'C. (1Clo = 1.55tog=0.155m2K/W)

Dew Point Creep

The dew point is the combination of temperature, humidity and pressure when condensation occurs. In a clothing system this typically occurs first in the outer layer where moisture laden air from the body meets cold, damp fabrics. However, condensation in these outer layers can cause them to saturate (they can appear to change from hydrophobic to hydrophilic - a sentiment change) causing a loss of insulation and allowing the dew point to creep closer to the body. This is why you often find wetness inside perfectly waterproof garments. In civil engineering it is sometimes called cold bridging.

FurTech jackets are designed to resist Dew Point Creep. The furry lining fabric (the synthetic fur faces away from the body) directs liquid water outwards and is designed to stay warm when wet and dry quickly. However, there are, inevitably, times when these systems can be overcome. It is then time to add an OverShell or warm up in a Shelter.

Cold Bridging

Cold bridging is a term used in civil engineering and damp proofing. It is the situation when the inside walls of a building become cold enough for condensation to occur.A well insulated and ventilated building can avoid this problem.

You may see the analogy with clothing. Cold bridging occurs easily on all thin, non-insulating waterproofs in cold and wet conditions (particularly when the outer fabric saturates).

Click here for more on condensation in buildings.

Cooling the Neck

This study, conducted in Japan, compared cooling of the neck and chest under controlled lab conditions. What is interesting is that effective cooling of just 2.2% of the body surface reduced sweating by between 16% and 22% with a significant drop in core temperature. Cooling the neck was a little more effective than cooling the chest, perhaps because the major arteries are close to the skin.

Wearing a wet multi-tube can be very effective in hot weather and effective neck insulation can make a big difference in the cold.

Click here to see the MultiTube and here to see a fleece neck warmer.

This infra-red video clearly shows significant heat loss from the neck:

Do you get less wet running in the rain?

This humorous article on the BBC explains the scenario. The author doesn't mention how much wetter wind driven rain is, but the effects are similar!

Rain Velocity

Rain reaches a terminal velocity as it is slowed by aerodynamic drag. Large drops fall faster than small droplets, catching and absorbing them until the aerodynamic forces become too great and blow them apart again. This is illustrated in this link: http://www.shorstmeyer.com/wxfaqs/float/dropdeform.html

The largest stable drops are about 5mm in diameter and travel at 9m/s, though larger drops can exist for a period of time before they disintegrate. Here's a table of drop diameters and velocities. According to this source typically 4% of rain drops are 5mm with the vast majority (63%) being in the range 0.85mm to 3.2mm.

Interestingly the dynamic pressure on the 5mm rain drop at terminal velocity may be calculated to be about 50Pa. Equivalent to just over 5mm hydrostatic head (but acting over perhaps more than 1 second).

Using Vapour Barriers in the UK

The March issue of TGO magazine included an article showing that eliminating the evaporation of insensible moisture from the skin, even in relatively mild UK winter conditions, did in fact increase warmth. The impermeable vapour barrier was placed next to the skin, creating a warm clammy layer, and insulating layers were placed on top. (Please note that if a non-breathable layer is placed over your insulation the increased humidity can lead to chilling.)

Respiratory Heat Loss

In certain cold, dry conditions as much as 50% of your total heat loss may come from breathing as the warm moist air from your lungs is exhaled to be replaced by cold dry air.

In an excellent  TGO magazine article, Eddy Meechan writes of his experiments using a heat exchange (dust) mask while camping in typical British winter conditions. The article highlights the benefits of breathing through a face covering which traps heat and moisture ready to warm and humidify the inhaled air. Though his experiments utilised a standard DIY dust mask, breathing through a Multi-Tube or Fleece Neck Gaiter should have similar benefits.

Dynamic Pressure

This is the pressure exerted by a fluid impacting a surface at 90 degrees to the flow. It is equivalent to the fluids' kinetic energy and is sometimes considered useful when looking at wind loadings on a fabric for comparison with hydrostatic head tests. However, it only relates to a homogeneous fluid (all air or all water for example) and can not be used to calculate the force exerted by a rain drop.

The pressure exerted by a rain drop depends on a multitude of factors:

• drop size, shape, mass and velocity
• drop impact angle
• drop impact area (depends on how it's oriented with wind velocity)
• how the fabric moves to absorb the drops momentum
• how much water penetrates the fabric
• how much water splashes and at what angles

More on Dynamic Pressure

Wind Chill and Compression of Insulation

Very high winds can compress your insulation and reduce its protection. The more compressible and packable it is, the less it will insulate in the high winds that are sometimes capable of knocking you off your feet. Also, very lightweight down insulation may be compressed when worn beneath heavy waterproof clothing - one reason that lightweight shell fabrics are used in the construction of these types of garments.

Tests conducted at Manchester University show that FurTech fabrics maintain more than half their thickness under loads equivalent to windspeeds of 286 miles per hour (460km/h)!!!

UK Wind Speed Map

This map shows the increased wind speeds on high round. Click to enlarge or click here for the original version.

Click here for speed conversion software.

UK Temperature Map

This picture shows the different annual average temperatures across the UK, with the cooler mountainous and northern regions in blue. It's worth considering how you dress for trips to different areas.

Please click on the image to enlarge it.

Humidity and Insulation

Using calculations for the thermal conductance of air and water, regardless of evaporation, shows that the insulating effectiveness of the airspace in your clothing diminishes dramatically when a small amount of water is introduced (% of Water in Air refers to the percentage volume of air replaced by liquid water):

% of Water in Air	Conductance (Wmֿ¹K ֿ¹)	Increase in Conductance	% of Insulation of Dry Air
Dry air	0.025	1x	100%
10%	0.0825	3.3x	30.3%
20%	0.14	5.6x	17.8%
30%	0.1975	7.9x	12.7%
40%	0.255	10.2x	9.8%
50%	0.3125	12.5x	8%
60%	0.37	14.8x	6.8%
70%	0.4275	17.1x	5.8%
80%	0.485	19.4x	5.1%
90%	0.5425	21.7x	4.6%
All water	0.6	24x	4.2%

This table shows that when just 10% of the dry air in your clothing is replaced by water, its insulation is less than 1/3rd! Introduce 50% water and the insulation value is only 8%!

This is because water conducts heat 24 times more than dry air!

Note: Evaporation has an even bigger effect, as it takes about 550 times more energy to evaporate it than raise its temperature by just 1'C!

Thermal Resistance

The Tog is a measure of thermal resistance developed in Britain by the Shirley Institute. It indicates how well clothing or bedding insulates (in the dry). FurTech garments provide about 2.5Tog (0.25K.m^2/W or 1.6Clo).

A Clo is the amount of insulation required to keep a resting person comfortable in a windless room at 21.1'C... a typical home or office environment.

Lab measurements conducted at the University of Manchester show GoreTex has no significant insulating ability and this is likely to be the same for most membrane hardshells. The warmth you get from such jackets is because windproof shells improve the insulating properties of under layers, even in still conditions, by reducing convection.

Table 4.12            Thermal Resistance of Fabrics

	AB 1	AB2	Mean AB	C1	C2	Mean C
Thermal Resistance of the test specimen (m².K/W)	0.23463203	0.25313283	0.24388243	-0.015873	-0.015873	-0.015873

(AB is FurTech and C is GoreTex - the slightly negative numbers indicate the error range at near zero measurements.)

FurTech offers an excellent warmth to weight ratio when compared to conventional hardshell layering systems!

Adapting to Stabalise Your Temperature

Venting before you get sweaty and adding insulation before you get cold, can make all the difference to your outdoor comfort, as increased humidity in your clothing dramatically reduces the effectiveness of insulation.

Remember, insulation doesn't add heat, it just slows heat loss.

The key elements in adapting your clothing are:

1. exposure to wind chill - are you about to hit a windswept ridge?

2. cooler temperatures at altitude (or lower, in a cloud inversion) - are you climbing or descending?

3. increased humidity or precipitation - is it about to rain, which tends to cool the air?

4. level of activity you are about to undertake - are you about to climb steeply or sit for the next hour?

5. adaptability of your clothing - can you vent your clothing, remove your hat and scarf etc?

Remember, it is far more effective to adapt your clothing before you feel the need to do it!

Thermal Innertia

When you add insulation, your body heat is first used to warm it up. The amount of heat to warm up an OverShell jacket very much depends on the density of its insulation - related to its thermal mass. Heavy linings take more energy to warm up before the insulation becomes effective.

Blizzard kit is very light so has a low thermal inertia (and reflects heat radiation).

By wrapping an OverShell jacket in your flask you have a jacket that is already pre-warmed, reducing the thermal inertia effect.

Adding Insulation

Insulation doesn't add heat, it simply conserves it, so donning an extra jacket or zipping up your vents when you are already cold is far less effective than acting as soon as you stop, or when you get exposed to that icy summit blast of wind. My advice is to be aware of when you are about to get cold and, even if you are still feeling warm, do something about it: close vents, add hats and gloves and add an OverShell or Shelter.

Energy Requirements

This link calculates the likely energy required for a whole range of activities, depending on your weight. Try it and see what the difference is between walking, running and climbing.

Remember that the heat you generate, how warm you get, how hard you breathe and how much food and drink you need, are all proportional to these figures.

(Please note that what are commonly referred to as calories are actually kcalories.)

Work, Heat and the Alpine Plod

The majority of the heat that we generate is provided by muscular activity and is the by product of work (force times distance). The more work we do the hotter we get. The fitter we are the more work and heat we can generate. During strenuous activity the heat output from our muscles can be 40 times that from all other tissue (please see this link from Cornell University). At maximum heart rate it may be 16 times the heat output at rest!

Why is this relevant to mountain sports? Well, just because we can power up that hill, at maximum respiratory rate, lungs bursting, doesn't mean that we should. If you want to be comfortable throughout a long day in the hills it may be prudent to leave the fastest ascents to the fell runners and adopt an alpine plod instead. This will result in greater endurance and a more stable temperature, requiring less clothing adjustment through the day, saving on the faff factor that slows many ascents.

(Thanks to Lonsdale Fell Runners for the picture)

Waterproof Membranes in Boots

Graham Thompson from Trail magazine, in conjunction with Leeds University, has extensively studied the build up of humidity in boots with membranes compared to those without. Some of the conclusions relate just as well to clothing:

1. People sweat to different degrees
2. Breathability depends on the temperature gradient
3. Membranes impede drying
4. Membranes reduce breathability and usually increase humidity and temperature inside

More commentary on this can be seen at Outdoors Magic.

Here's a link to Trail.

Dreich, Dreep or Dribble?

Why is it that a day spent out in light rain results in wetness inside most hardshells? Surely a bit of drizzle doesn't get through that fabric with the 20m hydrostatic head!

The rain probably doesn't get through the waterproof membrane, but it does chill it. Once cooled it can't breathe effectively and you get condensation inside.

FurTech type systems work exceptionally well in these conditions. The outer saturates with rain and condensation which actually helps suck water out of the garment. The outer looks sodden, but the inside is dry!

Click here for "Phases of Breathability".

Click here for more on waterproof standards.

Click here for more Scottish weather words.

Sentiment Change in Water Repellent Fabrics

Durable Water Repellent fabrics start life with a surface chemistry that spectacularly repels water. It beads up and rolls off, leaving not a trace of dampness on the material. However, this effect is usually fairly temporary due to a number of factors:

1. DWR can be worn off
2. DWR can be masked by dirt, detergents and washing powders
3. Water can be forced into the weave of the fabric, locking around the DWR fibres
4. Condensation can occur within the structure of the fabric, providing a surface that rain can stick to

In short, the fabric can go from water hating (hydrophobic) to water loving (hydrophilic): It has a sentiment change.

FurTech garments utilise this sentiment change because it tends to occur within the woven outer windproof fabric, which then draws moisture off the DWR inner material (this has a fur structure and pushes water outwards). In heavy weather you will see the outer wet out but the inner remains dry as condensation is extracted through the fabrics!

The outer fabric protects the DWR of the inner fabric so that it remains water repellent and the inside remains dry. However, even when saturated (falling into water) the system dries quickly because there is no waterproof membrane to trap the water inside.

Similarly, you may notice the outer fur of aquatic mammals (beavers, otters etc.) saturating, yet the inner fur remains dry!

Water Repellency: Cassie's Law

The effectiveness of water repellency isn't just because of the surface chemistry of the material: its texture can work with hydrophobic treatments to create a better effect. On the lotus leaf microscopic cones act to improve repellency. The contact points on the cones is at an increased angle to the leaf's surface, causing water droplets to become more spherical. A duck's feathers provide a similar textured surface for exemplary water repellency. This effect was first explained by Cassie's Law.

Breathability versus Venting

Breath ability is very important to clothing comfort but are the best fabrics breathable enough? According to the eVent web site we can produce 0.25 to 0.5 liters of sweat per hour just walking and as much as 1.5 liters per hour if working hard. Other sites indicate that much higher sweat rates can occur with the highest ever recorded being 3.7 liters per hour by Alberto Salazar in the 1984 Olympic Marathon! According to these independent lab results eVent breathes at about 5 liters per meter squared per 24 hours. If we generously assume that 4 meters of fabric are used and that the sweat is evenly distributed across that area (not true), then in ideal lab conditions eVent can cope with about 0.83 liters of sweat per hour, which may not be enough. This study shows that the back can generate approximately twice the sweat of the arms, overloading some areas of the fabric well before the previous figure indicates.

How much we sweat is a function of our core temperature and not just activity level. When we dress for cold conditions and then work even moderately hard our core temperature can shoot up dramatically. This is why adjustment of insulation is so important. Vents allow cool air into your clothing system without needing to take garments off, reducing the amount you sweat (as well as increasing breathability). FurTech garments use vents that allow air past the integral insulation directly to your base layer.

These results show how FurTech type fabrics breathe better than membranes and this shows how breathability changes in the rain.

Metabolism: Running Hot or Cold

There are many different factors effecting if you "run hot or cold" and it is an incomplete science. Here are some general factors:-

Heat is generated as a by product of activity (just like your car engine heats up when it's producing power). The more activity the more heat. But the body is designed to lose heat, even before sweating is needed. Just by breathing faster we take more cool air into our core and expel heat laden CO2. Moving rapidly adds a bit of wind chill (noticeable when running in hot, still conditions) and the large surfaces of the arms and legs move quicker still.

Heat is also generated by metabolising food and has a bearing on our core temperature. Eating hot or cold food adds or subtracts heat to our core. Drugs may also effect heat generation. This study shows that ephedrine-caffeine increased carbohydrate burning and metabolic temperature and it's long been known that alcohol decreases vasorestriction and feelings of cold (and increases susceptibility to hypothermia). Coffee increases circulation to the extremities. More on food here.

Body shape has a big impact on how we lose heat. The bigger your surface area compared to body weight, the quicker you cool, expressed here in Bergmann's and Allen's Rules. Stockier individuals have far greater survival times in water than thinner types.

Fat is an excellent insulator, used by most marine mammals, and carrying it around means that you burn more energy and generate more heat. Le Blanc demonstrated fat added a significant ability to resist cold.

Age and sex also have a bearing. Apart from the obvious differences between the sexes, studies show that older men typically appear to have a deteriorating ability to maintain their core temperature.

The human body can adapt to conditions over time. In his excellent book "Survival of the Fittest" Dr Mike Stroud (Sir Renaulph Fiennes' "partner in crime") comments on how the body can adapt to hot climates with a faster sweat response and how some fishermen have developed a tolerance to immersing their hands in icy water - conditions that would cause a surprising amount of pain for most of us (try it at home)! Some studies also suggest that fitness has a bearing on your ability to maintain core temperature.

Psychology also has an effect. Stress, for example, causes a sweat response which may run counter to staying warm. This article even suggests that extrovert personalities are less susceptible to the cold. Lawrence Irving recorded that his cold acclimatised subjects were very aware of sensations in their chilled extremities, hinting at a conscious link. Certainly your attitude to comfort can have a bearing, but beware Stoicism slipping into hypothermia.

You may also be interested in this link about the "Ice Man", who has apparently trained himself to cope with extreme cold without clothes! Is it April 1st?

Layers and Breathability 2

It is clear that the breathability of your layering system is reduced when you add more layers of less breathable material. However, if you need to keep warm then you will be generating less sweat, so adding windproof insulating layers isn't necessarily a problem - just take them off when you get too warm. This is a system used to good effect by Mark Twight, amongst others, and described in his excellent book "Extreme Alpinism".

Wearing FurTech over a baselayer gives maximum temperature flexibility for variable conditions and heat output, but it can be worn in other ways: wear it over a WindShell (or other shell) as conditions worsen or add an OverShell to it when you stop. FurTech is remarkably effective at drying inner layers, even in the rain, so long as you remain active.

Why does it feel colder when it rains?

According to the UK Met Office winter rain normally falls as conditions become warmer and in summer, when conditions become cooler. This is probably due to warm and cold fronts moving across the land mass.

Rain Is Usually Cold: As rain falls from a higher and colder altitude it is usually cooler than ambient conditions at ground level. The very act of falling may further reduce the droplets temperature as it falls through air with less than 100% humidity, where wind chill increases evaporation from its surface, cooling it quickly by removing the latent heat of evaporation.

Rain Cools Us: When the droplet reaches us it cools its surroundings. Water absorbs a vast amount of heat as it has a high specific heat capacity (about 3 times that of iron!) drawing heat out of the atmosphere and anything else it contacts.

Increased Humidity Can Make The Air Feel Colder: As the rain water warms it begins to evaporate, increasing the humidity of the air which correspondingly loses its ability to insulate - the air its self begins to feel cooler.

All this without your clothing actually getting wet!

(100% humidity can prevent sweat from evaporating, so you feel clammy when working hard or if ambient temperatures are tropical: Humidex Table)

Water Chill

Water chills rapidly because:

1. it conducts 24 times faster than dry air (click here for some values for common materials)
2. when it evaporates it absorbs approximately 550 times more energy than just raising its temperature by 1'C - the latent heat of evaporation
3. flowing water removes heat by convection, as cold water replaces the water next to your body which you have just warmed (the reverse of this can be experienced when you feel the temperature increase as you move in a hot bath - if you stay still the water is more bearable)

Humidity in the air has a big impact on how hot or cold we feel. Cold damp air feels colder than dry air because it insulates less (conducts more). Hot, humid conditions feel hotter than hot, dry conditions because our sweat can't evaporate as effectively.

Water Chill is one of the reasons that it usually feels colder when it rains but warm, muggy conditions can cause over heating because sweat can't evaporate.

Details of the science of water can be found in Philip Ball's book "H2O".

Wind Chill

Wind has an enormous impact on cooling because of:

1. increased evaporation (water absorbs vast amounts of heat when it evaporates - latent heat of evaporation)
2. replacing warm air with colder air

In scientific language it can be referred to as forced convection, explained by Newton.

Exactly what temperature is felt at different wind speeds has been debated in scientific circles. Here is a wind chill table.

Interestingly, hot winds (when the air temperature is greater than your body temperature) will make you hotter than still air of the same temperature. This is why ovens equipped with fans cook food quicker.

The flight feathers of birds offer far greater wind resistance than fur and down. They are analogous to the woven outer fabric used in FurTech garments. Click here for more.

Leeds Survival Conference 2007

The following report was written by Charles Ross, who attended the conference.

"Leeds Survival 2007 conference organised by Dave Brook, Dept of Materials, University of Leeds

The most well established academic conference relating to the Outdoor Industry was staged with a range of papers reflecting some of the diverse projects that this regarded authority on materials testings has become involved with. Two of these really stood out for the outdoor gear freaks:

"Comfort Performance in soft shell clothing" by Katy Stevens &

"Highly water resistant footwear using plasma enhanced technology" by Dr Stephen Coulson - a guy from P2i.

To add to this in his own idiosyncratic way the conference was wrapped up by Andy Kirkpatrick (of psychovertical.com) whose tale of experiences whilst crossing Greenland was enhanced by Anna McCormack (the face of Event fabrics) being in the audience: Anna was on that same expedition.

Although the conference was titled Molecular Engineering for Performance Textiles it contained some very factual information & made the audience aware of new technologies being developed

Katy Stevens' paper was based on investigation into the whole concept of soft shell and produced answers on whether the traditional way of creating the system a la Buffalo (materials only joined together at their edges) or the current method of bonding the layers together produced the better results. The key thing about performance clothing is that it should be able to deal with the changes of body temperature, without resulting in the wearer either overheating or getting too cold. Skin naturally produces perspiration to help it cool down (epidermis); the skill of the clothing is to move this moisture away from contact with the skin to avoid the chilling effect when the wearer stops producing the heat & then to get rid of this moisture before it condenses into liquid. In a perfect world:

Heat Balance = Heat Production +/- Exchange with Environment +/- Heat loss via Evaporation= 0

Soft shell was defined as being a replacement for two layers of garments: the insulation layer and the shell layer. In its history it was Hamish Hamilton (of the original Force 10 tent design) who first offered the Buffalo system during the 1970s by replacing the Ventile cotton with Pertex fabric; supported by Montane's establishment in 1995 before Patagonia popularised the name & concept in 1997; to then be copied by all the major outdoor brands over the last decade. The key areas of qualifying for the test were that the garment must have good breathability, some thermal resistance & water repellence ability. 9 garments were then tested & results compared. Of these 4 garments had a "free" liner, whilst the remaining 5 had "fixed" or bonded liners.

The results of the lab testing gave a clear divide between the free & the fixed liner garments. Essentially the membrane garments (fixed liner) felt clammy, chilled, damp & cold, sticky and full of moisture; whilst having little air permeability; and their tighter fit controlled the chimney effect and the tests demonstrated this. The membrane garments seemed to place more importance on the benefits of waterproofness over breathability, which led to an increase of condensation accumulation, which results in greater heat losses to the wearer. A more closely fitted garment had a greater thermal heat loss through conduction, plus limited the air flow (cutting down on the breathability) thus cutting down on the air space for holding warmth and a sort of waiting space for moisture vapour & hence had quicker condensation rates which led to a greater clamminess

Opposed to this were the results of the free liner construction methodology. The naturally occurring extra space gave a greater thermal insulation area, held more moisture vapour thus preventing it from condensing so fast; any eventual wetting out tended to stay in the area between the layers, rather than next to the wearer's skin

Interestly Katy demonstrated how a real person test resulted in less rain penetration as the movement of the garment aided its ability to shed water droplets on its surface."

Notes highlighted by Andy Davison. Thanks again to Charles Ross

FurTech garments are constructed with a "free" non-bonded liner and do not use membranes.