Humans and Density Altitude

[AggieMike88]

common knowledge to us pilots that density altitude affects performance of engines and airfoils.

But does it have an affect on human performance? Other than us low landers feeling the effects of altitude sickness due to the lower partial pressure of oxygen.

Curiosity came from my father vacationing in Colorado this week and sending me a photo of him standing next to the sign at Independence Pass marking the Continental Divide. Elevation shown on the sign was 12,100 MSL.

Toss in 29.92 inches Hg and a temp of 75°F, and you get density altitude of over 16,000.

Being over 80, he did say he "felt" the effects of the higher altitude.

But for the general population, does current barometer and temp cause a big play in how you feel at higher altitudes?

 

[X3 Skier]

Been skiing and living at Steamboat during ski season since I retired 21 years ago and at Altitudes up to 10500’ (Temps-10 to +20F) with the Base at 7000’ (Temps -20 to +40) I’ve never noticed any effect.

Cheers

 

[FastEddieB]

Recently discussed in another thread.

Apparently, Pressure Altitude is what matters, with temperature having no effect.

 

[azure]

The main point being, as I recall, that the OAT is irrelevant, only the temperature at the gas exchange surface in the alveoli of the lung matters, and that is always basically body temperature.

 

[benyflyguy]

The main point being, as I recall, that the OAT is irrelevant, only the temperature at the gas exchange surface in the alveoli of the lung matters, and that is always basically body temperature.

Yupper. The percentage of oxygen varies little as you go higher. But the capillaries in our lungs require a certain amount of pressure to “push” the oxygen in. The weight of a column of air 20,000 feet is substantial enough to make that kind of difference. Kinda cool when you think about it.

 

[sarangan]

Yupper. The percentage of oxygen varies little as you go higher. But the capillaries in our lungs require a certain amount of pressure to “push” the oxygen in. The weight of a column of air 20,000 feet is substantial enough to make that kind of difference. Kinda cool when you think about it.

I believe it is diffusion, not pressure, that moves the oxygen across the cells in the lung. Rate of diffusion is directly proportional to density gradient. Strictly speaking it should be density altitude that determines oxygen diffusion; however, since the temperature of the air inside the lung is always at 98F, pressure becomes the only variable that determines oxygen density in the lung.

 

[Rushie]

It's just volume. Less dense air at higher altitude has fewer oxygen molecules per unit of volume. Your lung, or one of its alveoli, has more or less a fixed volume. That is, of course, the definition of less pressure, assuming the temperature is constant. The transfer process is indeed diffusion. But the higher the pressure (with fixed temp and volume) the more O2 molecules are present and the more diffusion can occur.

 

[azure]

It's just volume. Less dense air at higher altitude has fewer oxygen molecules per unit of volume. Your lung, or one of its alveoli, has more or less a fixed volume. That is, of course, the definition of less pressure, assuming the temperature is constant. The transfer process is indeed diffusion. But the higher the pressure (with fixed temp and volume) the more O2 molecules are present and the more diffusion can occur.

Minor nit: it's actually the equation of state, to which for air at STP, the ideal gas law is a very good approximation.

 

[Rushie]

Minor nit: it's actually the equation of state, to which for air at STP, the ideal gas law is a very good approximation.

You're right, I wasn't happy with how I phrased that but was too lazy to work on it anymore.

 

[denverpilot]

As mentioned, temperature doesn’t matter.

Still fun to take sea level folks up to Pike’s Peak summit and watch them pass out on the way back down as soon as they relax a bit in the vehicle.

 

[AggieMike88]

As mentioned, temperature doesn’t matter.

Still fun to take sea level folks up to Pike’s Peak summit and watch them pass out on the way back down as soon as they relax a bit in the vehicle.

Did that on the cog railway

 

[denverpilot]

Did that on the cog railway

Taking them up on the road is more fun, especially with people who’ve never seen a road like that with no guardrails and many thousand foot drops a couple of feet away.

They scream sometimes and probably pee their pants a little. LOL.

Cog railway is wicked cool for rail buffs though. Fun ride. Been a long while since we’ve done it.

The really crazy bastards are the guys who drive the Pike Peak Hill Climb race cars... this year’s record setter went up the mountain at some unholy speed I can’t even fathom driving that road at. They’re completely nuts.

 

[azure]

I never did Pike's, but in 1968 on a family trip to Colorado we drove up to the top of Mt. Evans. I remember being very winded very quickly up there, but I didn't pass out. Of course, I was only 13.

 

[chemgeek]

It's the partial pressure of O2 that determines efficiency hemoglobin loading. Nothing else, so pressure altitude would be the best proxy for biological effects of altitude. It is a nonlinear function, BTW.