The Science of Warmth

Introduction

There's a multitude of theories and personal preferences when it comes to staying warm. Possibly the only uniting factor amongst these hypotheses is that they share the same goal - fighting off the cold. This article will explain the basic principles of what creates and maintains warmth.

Facts

Average human body core temperature:

98.6°F or 37°C. On an individual basis this varies a few degrees Fahrenheit and about one degree Celsius.

Average human skin temperature:

about 92°F or 34°C. This varies greatly based on many factors.

Hypothermia:

a potentially fatal condition that occurs when body temperature falls below 95°F or 35°C

Heat Generation

Internal Sources:

Human produced heat is generated through "burning" calories. Essentially when your body uses energy heat is created. Higher levels of physical activity produce more heat.

Between the temperatures of 65°F and 130°F the body relatively easily maintains its normal core temperature at stasis (non-activity).

Core Warmth
When your core (torso) is warm it readily releases blood to the extremities. When your core gets cold your body's natural defense is stop sending blood to the extremities and keep it for vital organs and the brain. Therefore the best solution for cold hands and feet is often to heat up your core. Core warmth is most easily achieved through physical activity but increasing insulation is another option.

External Sources:

There are too many external heat sources to list but anything warmer than the ambient outside temperature or normal body temperature is a possible external heat source. Some common examples:

• Hot food or beverages
• The sun
• Fire
• heat packs

What It Means to Us:

Heat generation is based on activity level. Our clothing system should match our activity level in order to maintain desired body temperature. The fuel sources available to your body via diet also affect its ability to generate heat. The most important part of your body to keep warm is your core.

Heat Loss

Your body creates heat, but we can still get cold, how is that possible? Through heat loss. Heat is lost in four ways:

Conduction:

When one object touches another object the warmer object transfers heat to to the cooler, thereby losing heat.

Convection:

The transfer of heat by circulation or movement of heated parts of liquid or gas. Basically your body loses heat to the air via conduction > that air blows away > new air replaces it > grabs some heat > blows away - and so forth.

Radiation:

To make a long story short your body releases electromagnetic waves and in doing so loses heat. This happens mostly in your head, but also through your hands and neck.

Evaporative Cooling:

Your body transfers heat to perspiration > perspiration evaporates and in doing so takes heat with it, this is evaporative cooling and it is the human body's method cooling itself.

It should now make sense that if you lay on cold ground your body conducts heat to the ground and you get cooler. Meanwhile the cold breeze steals heat through convection. Your hatless head radiates heat away, and your wet clothes lose heat as they dry.

Insulation

Insulation is material surrounding a body to slow heat loss. The idea is to create a layer of trapped air called a "dead air space". Your body heats this air and it surrounds you like a warm blanket.

The dead air space still loses heat to the surrounding cold, yet still keeps you warm. This is possible because the cooling process is slowed by trapping air and moving the interface of warm-meeting-cool air away from the body. The air closest to the outside loses heat to outside elements, but your body is constantly heating the air behind it and giving it new heat, so you remain surrounded by warm air (insulation).

The degree of insulation depends on two factors: Loft and Fabric Breathability

Fabric Breathable

Fabric breathability refers to the degree to which a material allows moisture and air to pass through it.

Sealed fabrics are warmer than breathable fabrics - but countervailing factors often offset any benefit sealed fabrics offer. Non-permeable fabrics such as vinyl and rubber do not allow any air or moisture to pass through them. This traps air more effectively between your body and the fabric.

Sealed and low breathability fabrics have a serious caveat. Your body constantly generates moisture, especially when you work hard. Low breathability fabrics do not allow this moisture to effectively escape. This leads to moisture buildup between your body and the fabric, which is not only uncomfortable, it is potentially dangerous - being wet when it's cold is bad.

Breathable fabrics allow moisture to pass through them, the moisture generally evaporates on the outside of the fabric and becomes a non-issue. The moisture does cause cooling as it evaporates (evaporative cooling) but the interface is moved away from the body, which gives the body time to heat a protective layer of air behind it.

Loft

Loft refers to the distance between your body and the outside elements created with insulating materials (clothing). The distance creates a "dead air space". All other considerations held equal - the higher the loft the greater the insulating power > the warmer you stay.

Loft is only effective if the air space it creates is "trapped". If air passes straight through the loft with little or no resistance, it escapes too quickly to insulate effectively. If wind passes through the loft easily, warm air is washed away. On the other hand if the outer fabric is too sealed water buildup becomes an issue.

In order to trap the air that loft creates outer layers should be wind resistant or proof. Breathability should remain a concern because moisture must be allowed to escape. There are many modern materials that fall along a continuum of trying to achieve wind resistance vs breathability.

If an outer layer that traps air, such as a hardshell, windshell, or softshell, is worn - then under layers do not need to be weather resistant. Materials such as waterproof, windproof, or resistant membranes and/or facing are common on fleece and pile jackets. These materials improve the performance of garment when it is used as the outermost layer. These materials do not add appreciable loft (the fleece, pile, or other faced fabric does that), so when they are used beneath an air trapping layer they add extra weight and inhibit moisture transfer without adding significant warmth. Once an outer air trapper is established, in the interest of warmth, the only important thing to create underneath it is loft. Standard no frills no "weather resistant" materials added fleece or pile breathes dramatically better, is lighter, costs less and dries faster - making a more suitable under layer.

Many climbers still use weather resistant mid layers, and there are good reasons to do so, but it should be noted that you can save a lot of weight and money by using basic breathable layers and throwing a super light wind shell over the top to provide weather resistance.

Diet

What you eat and your level of hydration strongly affect your warmth.

Food

Your body creates heat by burning calories. If you don't have adequate calories available to burn it's harder to stay warm.

Fat contains more calories per gram than carbohydrates or protein - so given the same weight of food - a diet higher in fat will be warmer than one low in fat (assuming the fat will actually be burned).

Spicy foods naturally raise the metabolism and hence speed up the heat making process. Ginger has long been known to help keep you warm.

If the amount of Calories is held equal, food that is warm won't take heat away from you, and hence keep you warmer.

Hydration

Adequate hydration is critical to staying warm. Water is amazing at retaining heat, so keeping your body properly hydrated means that there's more material in your body to retain warmth with.

-much more to be added-

Staying Cool

Climbers generally prefer to not stay sweaty, so they pick fabrics that "breathe" and wick well, which is to say they don't absorb much water and air passes through them relatively easily.

Cotton may kill, but it actually keeps you cooler than wicking fabrics by retaining more water and prolonging evaporative cooling. Your body actually needs to perspire more in wicking fabrics to maintain an effective evaporative cooling rate.

Wicking fabrics are still preferred by most climbers, myself included, because of increased comfort (no wet feeling), you are more dry when activity stops, and there's less risk of becoming to cold if the temperature suddenly drops.

Water evaporates fastest off of bare skin. Sun burns are another possible issue with uncovered skin. It may seem counter intuitive to wear long sleeves and pants when it's very hot, but it can actually keep you much cooler. Check out the people that live in the desert, whay are they in cotton from head to toe?

Utilizing External Heat

Black and darker colors absorb more heat that white and lighter colors. Hence most belay layers are dark in color. Wearing darker colors will keep you warmer in the daylight hours. Darker colors of the same fabric also dry faster.

Shared tents are warmer than solo shelters or bivies. Smaller shared tents are warmer than large ones. Snuggling up to your partner has obvious benefits... and obvious issues :) In extreme situations sharing a sleeping bag or bivy sac can save both of your lives. Many a partners toes have been saved through contact with a warm partners skin.

Heat packs are lightweight and usually work well. Some don't work at altitude.

Lukewarm and warm liquids don't require you body to heat them - they'll keep you warmer.

Boiling water is a climbers best friend. Keeping it next to your body will not only give you it's heat, it will slow it's cooling - meaning you won't be drinking cold water later.

Fumaroles stink - but they're warm. Hot springs are awesome.

Fire is a great heat source, just make sure it's safe, non-damaging, and respectful.

Angry people are warmer than happy ones - so if you're partner's being an asshole maybe he's just trying to keep you warm :)

Natural wind blocks greatly reduce convective heat loss.

General Clothing System and Names

Clothing Systems for climbing are generally built around trapping heat or letting it escape. Let's assume we're in a cold environment.

        Layer 1:         Next to skin        (base layer)

When you're working hard you tend to sweat. Even if you're cool while working your body still creates localized perspiration and general moisture. For this reason most climber choose a moisture "wicking" fabric for their first layer. These fabrics move moisture away from the skin quickly and allow moisture to evaporate rapidly - this minimize the evaporative cooling.

        Layer 2:         The intermediates         (layers)

Intermediate layers create loft and may or may not trap air for use as an outer layer. Common Fabrics: non-lined fleece, lined fleece, softshells, pile, and wool.

        Layer 3:         Wind and Rain         (shells)

This layer block the wind, and the rain (or other external moisture) if desired. Many climbers opt out of this layer by combining a mid layer with this layer (this is called a "softshell"). The desired effect is to block the elements and trap heated air.

        Layer 4:         The Belay layer         ("down jackets" "belay jacket")

Most climbers don't wear large down or synthetic jackets or pants under other layers, although thinner versions are commonly used as mid layers. Instead, when it's really cold, most climbers add a down or synthetic jacket and/or pants over their existing layers.

Common Fabrics

Base Layers:
          • Polyester - wicking, low water absorption, soft feel
          • Nylon - wicking, low water absorption
          • Lycra and Spandex - added to the above to create stretch

Mid Layers:
          • Fleece (non-lined) - lightweight, almost no water absorption, high loft, wicks rain
          • Fleece (lined) - same as above but with a bonded nylon layer that blocks wind and rain
          • Fleece (stretch) - fleece bonded to a nylon/spandex layer. Higher water absorption, better movement
          • Pile - mid weight, low water absorption, sheds light rain, good wind resistance
          • Wool - mid weight, low water absorption, sheds light rain
          • Schoeller soft shell fabrics
          • Polartec Powershield soft shell fabric

Shells:
          • Gore-tex - standard waterproof breathable membrane
          • Gore XCR - same waterproofness as standard g-tex, lighter and more breathable
          • eVent - same waterproofness as standard g-tex, lighter and more breathable
          • SilNylon - silicon impregnated nylon, blocks wind and rain, low breathability
          • DWR - durable water resistant coated fabrics, shed light rain, usually windproof
          • Company Specific - most companies have their own g-tex knock off it usually functions about the same.

Belay:
          • Goose Down - the "warmest" material known to man, almost useless when wet.
    Synthetic Insulations:
          • Polarguard
          • Primaloft

Common Theories: Action Suit vs The Layering System

The Layering System camp uses more intermediate layers to achieve the desired temperature. The belay layer is often omitted in favor or more "static" layers. Layers are added and removed to maintain temperature.

The Action Suit camp (coined and championed by Mark Twight) believes in wearing only a few layers that are rarely or never taken off during high activity levels. During colder periods and/or low activity a belay layer is donned.

The main difference is what is frequently removed and added - intermediate layers vs a belay layer.

Tips

-Utilize natural shelters
-If you carry the layers, why not sleep in them too?
-Boiling nalgene in the sleeping bag... aahhhhh... so nice... check the seal!
-Eat fat before bed
-Start cold, work cold, chill warm (the puns I tell ya!), camp warm
-You can put boiling water into platypuses, other bladders warp the seal
-Don't wait to add more layers when stopping for prolonged periods
-Use heat packs
-Understand your layers, why will they keep you warm?

Conclusion

Warmth is generated internally and externally.

Creating trapped dead air space insulates the body. The amount of trapped dead air space is determined by loft. The "trap" is determined by the breathability of the outer layer of fabric over the loft.

Most of the marketing directed at consumer is bogus - the concepts that keep you warm are simple and basic.

Balancing what happens to your sweat makes a big difference in your temperature control.

Your diet affects your warmth.

Hydration affects your warmth.

Physical activity is the most influential factor in your warmth.


-This article mostly done, but I'm tweaking it, if you feel compelled to vote negatively please PM me first so I can address you concerns. I love feedback.

Image Citations

The images used in this article are provided courtesy of these sites, listed in order:

http://www.holidaycityflash.com/article%20pix/frostbite.jpg
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/images/irbody.jpg
http://www.rspb.org.uk/Images/canada%20goose_300_tcm9-139738.jpg
http://www.rtctextiles.co.uk/images/photos/barrier.gif
http://www.needlesports.com/acatalog/rabexpedition_downsuit.gif
http://www.earthfoot.org/places/ly001z.jpg
http://cires.colorado.edu/images/people/t/twightM-a140.jpg

Images

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