Let's assume you have a 30 degree bag that 'just' keeps you warm at 30 degrees at sea level.

Now take that bag to 10,000 feet at 30 degrees, the only difference is the pressure difference, so it's about 70% that at sea level.

The question is, with the same thermometer registering the same 30 degrees both locations, would the bag seem warmer at altitude as there is less air to conduct heat out?

Thanks,
Mike

My unscientific answer is that I haven't noticed "altitude" making a differences. Humidity, wind, and how tired I am seem to have a much greater impact.

--mark

As an equally unscientific answer, space has absolutely no air in it, and it's colder than....oh I don't know what it's colder than. But realistically, the temperature gradient between you and the outside temperature (assuming your talking about the outside temperature being 30) should be the same as at sea level.

This is a great question and hopefully Roger Caffin or maybe Richard Nisley will chime in with some quantitative answers.

There are some effects that differ with altitude. I just don't know their magnitude, so I can't say overall whether you'd be colder or warmer at 30 degrees. But qualitatively, here are the major effects I can think of:

1) Likely higher winds at elevation would make it colder, but you could eliminate this variable like you did with temp.

2) The lower air pressure will have lower thermal conductivity, making the bag warmer at elevation. I think this will be a big factor. And, if you extrapolate it to a total vacuum (e.g. outer space) you'd eliminate conductivity entirely and be left with just radiation heat losses.

3) The lower air pressure at elevation will speed up evaporation, increasing evaporative cooling. The magnitude of this effect depends on how much moisture is present in the sleep system and how much is generated by the sleeper during the night. (This factor could be eliminated or greatly reduced with a vapor barrier at both elevations.)

4)The lower air pressure at elevation will likely affect convective heat transfer, but I'm not sure in which direction...The lower pressure will reduce the heat capacity of the air, reducing convective loss. But, the lower air pressure may permeate the insulation more quickly, increasing convective loss. Hmmm....

Cheers,

-Mike

Following on from Mike's comments:

> 1) Likely higher winds at elevation would make it colder
Possibly so, but let's assume you have a decent tent or a decent bivy bag.

> 2) The lower air pressure will have lower thermal conductivity, making the bag warmer at elevation.
True, but I am not sure the effect will be huge. What actually matters is not so much the thermal conductivity of the air so as the movement of the air inside the down.

As you may know, an empty air mat is quite cold compared to a foam mat or a down air mattress. This is because the air inside the empty tubes can circulate, exchanging heat from your body to the ground.

The down in a sleeping bag (or DAM) blocks the movement of air inside the shell, and this blocking action will not change a lot with the reduction in air pressure. This is THE key factor in how an SB works.

> If you extrapolate it to a total vacuum (e.g. outer space) you'd eliminate conductivity entirely and be left with just radiation heat losses.
Yep, that's how a 'thermos' works, but the living conditions ...

> 3) The lower air pressure at elevation will speed up evaporation, increasing evaporative cooling.
Um ... well, not precisely. What does matter is the relative humidity. This gets more complex as the humidity at high altitude may be lower due to various air movements, but I have been in a thick fog at 2,500 m (8,000'), and evaporation does not happen easily under those conditions.

> The magnitude of this effect depends on how much moisture is present in the sleep system
The moisture in the sleep system is going to have a far larger effect on how well the down fluffs up. THAT is probably a very significant factor imho. Putting your sleeping bag out in the sun for an hour or two to dry it out is very very smart.

> (This factor could be eliminated or greatly reduced with a vapor barrier at both elevations.)
Hum... I don't think I would want to be using a VB system above about -10 C. Far too much chance of swimming. ymmv.

> 4)The lower air pressure at elevation will likely affect convective heat transfer
Yes, but the way the down fluffs up and blocks circulation will dominate.

Cheers
Roger

High elevations will have minimal effect on insulation values. A backpacker’s trousers, merino woody hoody, and windshirt (~.6 clo) ensemble would have no change in insulation value between sea level and 15,000 feet. If a Cocoon hoody and pants (1.2 clo) plus a Thru-Hiker eVENT jacket were added there would still be less than a .2 clo reduction in the combined ensemble at 15,000 feet versus sea level. A 30 degree sleeping bag would have a clo value of about 5.88. It logically follows there would be less than a .5 clo reduction at 15,000 ft.

Mike and Roger’s correctly defined all of the major and minor variables at play. Published research papers that discussed the major variables are as follows:

The reduction in barometric pressure (Pb) at higher elevations alters the heat transfer mechanisms that affect clothing insulation. Pb has pronounced effects on air density and mass diffusivity, which in turn change the convective and evaporative heat transfer processes.

It is known that as Pb decreases, convective heat transfer diminishes [Chang et al. 1990].

Also, the evaporative transfer mechanism appears to be enhanced [Gonzalez et al. 1985].

While the evaporative heat transfer does not alter clothing insulation directly, evaporative heat loss does affect skin and clothing temperatures, thus indirectly influencing clothing insulation. The efficacy of evaporative heat transfer increases with elevations in altitude. Furthermore, insulation of air, trapped between clothing layers and at the clothing-skin boundary layer, increases with decreasing Pb [Gonzalez, 1987].

The combined or net effects on backpacker’s clothing insulation, by these Pb-mediated changes, are insignificant and are approximately what I stated in my initial paragraph.

Edited by richard295 on 02/07/2008 23:57:30 MST.

"The question is, with the same thermometer registering the same 30 degrees both locations, would the bag seem warmer at altitude as there is less air to conduct heat out?"

Not so much an answer but more of a question, and along the same lines as Mr. Verber. Perhaps the bag would "scientifically" be as warm, but at 10000 feet(3000 meters) would ones not be generally colder (less heat generated) due to the altitude? Therefore, if @ 30F, the bag keeps you just warm at sea level...this same bag would NOT seem as warm at altitude under the same temperature conditions? this is a question, not a statement. :)

Steve,

When you sleep you generate about .8x your Basal metabolic rate (BMR) heat output. BMR at altitude may initially be elevated 20 to 30 percent above those at sea level. After 2 to 3 days, BMR falls and may be maintained at 15 to 16 % above sea level values.

In summary the 9% reduction in the insulation effectiveness, previously explained, would be offset by the 15 to 16% BMR increase resulting in it feeling approximately 7-8% warmer at high elevations.

Edited by richard295 on 02/08/2008 19:51:17 MST.

Hi Richard-

Thanks! I knew I could count on you for an informative response. :)

I'm a bit puzzled by your last post, though. As expected, it looks like heat transfer via convection and conduction through the air both diminish with decreasing Pb. This should *increase* the overall insulation effectiveness.

What effects contribute to the 9% *decrease* in insulation effectiveness with lower Pb that you mentioned? Is the evaporative cooling effect dominant and swamps the others? Or, are there other (non-physiological) factors in play?

Cheers,

-Mike

Is it possible that it is more difficult for the body to oxidize food in a low-oxygen environment, and thus more difficult to generate heat, the higher the altitude? In the early 1980s, John Roskelly (if I remember) had to turn back on an Everest summit push because he refused to use bottled oxygen and became too cold to continue, while his rope partner, Phil Ershler, had bottled oxygen, unroped from Roskelly, and made the summit.

Mike,

I calculated .5 (~reduction in insulation) divided by 5.88 (~clo value for a sea level 30 degree rated bag) = .085 rounded to 9%. The skin evaporative increase is greater than the bags convection decrease. So yes, the evaporative cooling effect swamps the others.

This insulation evaporation swamping phenomenon, at high elevations, was first measured and documented by the US Army Research Institute Of Environmental Medicine in a 1995 research study.

Robert,

Just the opposite occurs during rest. Individuals native to high altitudes demonstrate elevated BMRs as compared to individuals at sea level of similar body size. Individuals who consume the energy at a level equivalent to their need will maintain an elevated BMR throughout altitude exposure.

Reference: page 37 of Nutritional Needs in Cold and in High-Altitude Environments: Applications ... By Bernadette M. Marriott, Sydne J. Carlson 568 pages

Thanks, Richard!

That's cool information. (pun intended)

I'd like to suggest that this makes a compelling argument in favor of vapor barriers at high elevation as it should mitigate the larger evaporative cooling effect. (Though, as Roger mentioned, 30 degrees (F) is a bit warm for a VB.)

Fascinating stuff!

Cheers,

-Mike

Richard, I am no expert, but if you're saying the body produces more heat at alititude, that doesn't feel right to my subjective gut feelings. "After mixing with water vapour and expired CO2 in the lungs, oxygen diffuses down a pressure gradient to enter arterial blood around where its partial pressure is 100mmHg (13.3kPa).[2] Arterial blood flow delivers oxygen to the peripheral tissues, where it again diffuses down a pressure gradient into the cells and into their mitochondria. These bacteria-like cytoplasmic structures strip hydrogen from fuels (glucose, fats and some amino acids) to burn with oxygen to form water. Released energy (originally from the sun and photosynthesis) is stored as ATP, to be later used for energy requiring metabolism. The fuel's carbon is oxidized to CO2, which diffuses down its partial pressure gradient out of the cells into venous blood to finally be exhaled by the lungs. Experimentally, oxygen diffusion becomes rate limiting (and lethal) when arterial oxygen partial pressure falls to 40mmHg or below.
If oxygen delivery to cells is insufficient for the demand (hypoxia), hydrogen will be shifted to pyruvic acid converting it to lactic acid. This temporary measure (anaerobic metabolism) allows small amounts of energy to be produced. Lactic acid build up in tissues and blood is a sign of inadequate mitochondrial oxygenation, which may be due to hypoxemia, poor blood flow (e.g. shock) or a combination of both.[4] If severe or prolonged it could lead to cell death….
To counter the effects of high-altitude diseases, the body must return arterial P02 toward normal. Acclimatization, the means by which the body adapts to higher altitudes, only partially restores P02 to standard levels. Hyperventilation, the body’s most common response to high-altitude conditions, increases alveolar P02 by raising the depth and rate of breathing. However, while P02 does improve with hyperventilation, it does not return to normal. Studies of miners and astronomers working at 3000 meters and above show improved aveolar P02 with full acclimatization, yet the P02 level remains equal to or even below the threshold for continuous oxygen therapy for patients with chronic obstructive pulmonary disease (COPD).[5] In addition, there are complications involved with acclimatization. Polycythemia, in which the body increases the number of red blood cells in circulation, thickens the blood, raising the danger that the heart can’t pump it.

In high-altitude conditions, only oxygen enrichment can counteract the effects of hypoxia. By increasing the concentration of oxygen in the air, the effects of lower barometric pressure are countered and the level of arterial P02 is restored toward normal capacity. A small amount of supplemental oxygen reduces the equivalent altitude in climate-controlled rooms. At 4000 m, raising the oxygen concentration level by 5 percent via an oxygen concentrator and an existing ventilation system provides an altitude of 3000 m, which is much more tolerable for the increasing number of low-landers who work in high altitude.[6] In a study of astronomers working in Chile at 5050 m, oxygen concentrators increased the level of oxygen concentration by 6 percent (that is, from 21 percent to 27 percent). The result was increased worker productivity, less fatigue, and improved sleep." (Wikipedia.)

Robert,

I agree that it doesn't feel right intuitively. I suspect that the confusion lies in the fact that I was addressing .8*BMR (sleep in a 30F rated bag), not activity. The maximum activity level at high altitude will be constrained by the availability of oxygen and hence less heat can be generated at maximum output.

Edited by richard295 on 02/09/2008 16:27:41 MST.

Richard,
Another factor affecting the ability to do work at altitude is the type of substrate supplied by the diet. For any given amount of O2, more carbohydrate will be oxidized than either fat or protein. I can't remember the exact ratios for fats or amino acids, but it is considerably greater than the basic C6H12O6 + 6 O2 = 6 CO2 + 6 H2O. It is a function of the proportionally greater amount of O2 embedded in a glucose molecule than in either a fatty acid or amino acid. I have found this to be true personally at altitudes up to 20,000', buttressed by the experiences of my climbing mates. Moral of the story: the higher you go the more carbs in your diet.

Gentlemen,
Late reply - I missed the posts but would like to thank you for the information. The answers are actually the exact opposite then what i thought. Thanks for the clarification.
Steve

The energy expenditure of a specific physical task at high altitude is likely to be greater because the lungs and diaphragm of even an acclimatized person must work harder to get the oxygen in to do the task. Apart from likely differences in temperature and wind chill etc.