With that background information in tow, let's go back to the example of Potts Field in Boulder (actual elevation 5260 feet above sea level) to illustrate how barometric pressure and temperature impact density altitude. Our calculations will ignore the small impact of relative humidity.
On a day where the adjusted barometric pressure is 29.71 (inches of mercury) and the temperature is 75F at Potts Field, the density altitude is 7703 feet. On a day where the adjusted barometric pressue is 30.02 and the temperature is 63F, the density altitude at Potts Field is 6628 feet. On a day with an adjusted barometric pressure of 30.36 and the temperature is 49F, the density altitude at Potts Field is 5369 feet.
It's a good article, but density altitude is not a useful concept for interpreting distance running performance. That's because it is not the decrease in atmospheric density that primarily affects human respiration at altitude, but the lower partial pressure of oxygen (PPO2).
At a fixed temperature, these two variables would move together (see the ideal gas law) and density altitude would just depend on barometric pressure. Indeed, as long as you stick to the ISA standard temperature used as a benchmark in the density altitude formula (15C), there could be some validity to using density altitude as a scale or intuition for the effect of barometric pressure (and the resulting differences in PPO2) on distance running performance.
However, very little of the variation in density altitudes discussed in the MileSplit article, or that Bosshard talks about, are driven by differences in barometric pressure. Outside of extreme weather scenarios, barometric pressure typically varies by only 1-2% (though it could drop by as much as 20% in the center of a tornado - don't try to set a PR in the center of a tornado!) Rather these differences are overwhelmingly driven by temperature.
Obviously, temperature does affect running performance. But it does so through different mechanisms than altitude, limiting our ability to dissipate heat rather than to get oxygen. So thinking of this in terms of density altitude - the hypothetical altitude that would have the same atmospheric density at 15C and 29.92 Hg as your actual temperature/pressure combination - just isn't useful.
Just to further clarify this last point, since I think it's the main point of confusion in the linked MileSplit article (and possibly with Bosshard): Hotter air is less dense, but this in itself does not affect our ability to extract oxygen from the air. Instead it's the partial pressure of oxygen that matters, via the rate of gaseous diffusion across membranes in the lungs, and this is independent of temperature. For example, see this article: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1114067/ Key quote: "At high altitude [...] the alveolar-arterial difference for oxygen is higher than would be predicted from the measured ventilation-perfusion inequality. This is because the decreased driving pressure for oxygen from alveolar gas into arterial blood is insufficient to fully oxygenate the blood as it passes through the pulmonary capillaries. This is more evident on exercise as cardiac output increases and blood spends less time at the gas exchanging surface (diffusion limitation)."
In conclusion (thank you for coming to my Ted Talk) - the concept of density altitude is misleading when applied in this way, because it conflates changes in atmospheric density, which don't affect oxygen exchange, with changes in the partial pressure of oxygen, which do.
Final sidenote - density altitude could actually be a useful concept for sprints! That's because it is indeed the atmospheric density which is relevant to drag forces.
I’ve heard it mentioned that physiological benefits of training at altitude might be triggered by “pressure differences.” Would this be the barometric pressure or partial pressure of O2?
I’ve heard it mentioned that physiological benefits of training at altitude might be triggered by “pressure differences.” Would this be the barometric pressure or partial pressure of O2?
Good question - the tl;dr is probably PPO2, but also these are kind of the same.
Specifically, the partial pressure of oxygen within your lungs is probably what matters physiologically, because this is what is driving differing rates of oxygen uptake.
But these are "kind of the same" because the partial pressure of oxygen in dry air is fixed at about 20.95% of the atmospheric pressure. This is just determined by the mole fraction, i.e. the fraction of dry air molecules that are oxygen. This quantity is more or less uniform throughout Earth's atmosphere, at least until altitudes of ~100 km (the uniformity is due to mixing caused by solar heating at the surface). So basically that means these two things change together - for example if barometric pressure goes up by 2%, the partial pressure of oxygen also goes up by 2%. So you could think of it like the differences in atmospheric pressure are driving differences in the partial pressure of oxygen within your lungs which are (probably) driving whatever physiological changes occur.
Two clarifications - first, I mentioned that the partial pressure of oxygen in dry air is pretty fixed (typically to within 0.01% or less). But variations in humidity can throw this off by up to a percentage point or so, which would seem to be a large difference. However, it doesn't matter, because by the time inspired air reaches your lungs it has been humidified to full saturation with water vapor from the mucosal linings of your upper airways. So PPO2 within your lungs is unaffected. (Of course humidity affects running in other ways, namely limiting evaporative cooling for heat dissipation.)
And second, just to clarify atmospheric vs barometric pressure. At sea level these are synonyms. But at elevation, barometric pressure sometimes refers to an adjusted value that has been normalized to the mean sea level. Of course it's the unadjusted value - the literal, observed, physical pressure in terms of force per unit area, sometimes called station pressure or true barometric pressure - which is physiologically relevant.
Pilots have known this for decades. You need a longer runway to take off on “high altitude days”. Most coaches at altitude are aware of this. Joe doesn’t ever come up with anything new.
Oh and to directly answer Rojo's initial question from the thread title - how much of a low altitude day can you have in Boulder? Excluding extreme weather events, and basing the adjustment off of partial pressures, I think the answer for most locations should be a few hundred feet in either direction from the average.
To test this for Boulder in particular, I picked a weather station in Boulder at random and looked for the highest and lowest station pressures over the past year. These implied a "maximum altitude day" of about +600 ft, and a "minimum altitude day" of about -400 ft, using pressure altitude for the adjustment. (This asymmetry in deviations from the average is typical, and based on storm systems occasionally bringing in very low pressures.) I think this is a bit more variation than you would see in most cities, due to the mountains nearby.
If I could give one piece of advice to Bosshard's group, it would be this: The atmospheric variable relevant to oxygen uptake is not density, but pressure. Variations in density altitude due to temperature are thus spurious and do not reflect differences in respiratory oxygen exchange. Changes in atmospheric pressure may indeed be meaningful for distance running performance, especially at altitude, but the proper way to account for this is not density altitude, but pressure altitude. So track pressure altitude instead, and consider the effect of temperature on performance via limiting heat dissipation separately. Here is a calculator for pressure altitude: https://www.weather.gov/epz/wxcalc_pressurealtitude.
Of course, unless the point is some sort of mental game to manage athlete psychology. For that maybe density altitude is better since the changes are more dramatic. But it really isn't how it works in terms of physics or physiology.
Yes, really interesting info on this concept from an aviation standpoint.
In terms of running, it seems like an overcomplicated way of saying he adjusts training paces due to weather extremes and fluctuations. Something every coach does already.
most people think altitude is slower because there is less oxygen in the air. That is WRONG
There is more than enough oxygen in the high altitude air to fully oxygenate the blood but the issue is that the lower atmospheric pressure causes lower partial pressure of oxygen which means less oxygen transfers from the lungs to the bloodstream.
you're wrong. There is less oxygen at altitude. Fact.
He's confusing barometric pressure (which varies with the weather) with partial pressure of oxygen. Given a constant altitude, the partial pressure of O2 should stay about the same.
With that background information in tow, let's go back to the example of Potts Field in Boulder (actual elevation 5260 feet above sea level) to illustrate how barometric pressure and temperature impact density altitude. Our calculations will ignore the small impact of relative humidity.
On a day where the adjusted barometric pressure is 29.71 (inches of mercury) and the temperature is 75F at Potts Field, the density altitude is 7703 feet. On a day where the adjusted barometric pressue is 30.02 and the temperature is 63F, the density altitude at Potts Field is 6628 feet. On a day with an adjusted barometric pressure of 30.36 and the temperature is 49F, the density altitude at Potts Field is 5369 feet.
It's a good article, but density altitude is not a useful concept for interpreting distance running performance. That's because it is not the decrease in atmospheric density that primarily affects human respiration at altitude, but the lower partial pressure of oxygen (PPO2).
At a fixed temperature, these two variables would move together (see the ideal gas law) and density altitude would just depend on barometric pressure. Indeed, as long as you stick to the ISA standard temperature used as a benchmark in the density altitude formula (15C), there could be some validity to using density altitude as a scale or intuition for the effect of barometric pressure (and the resulting differences in PPO2) on distance running performance.
However, very little of the variation in density altitudes discussed in the MileSplit article, or that Bosshard talks about, are driven by differences in barometric pressure. Outside of extreme weather scenarios, barometric pressure typically varies by only 1-2% (though it could drop by as much as 20% in the center of a tornado - don't try to set a PR in the center of a tornado!) Rather these differences are overwhelmingly driven by temperature.
Obviously, temperature does affect running performance. But it does so through different mechanisms than altitude, limiting our ability to dissipate heat rather than to get oxygen. So thinking of this in terms of density altitude - the hypothetical altitude that would have the same atmospheric density at 15C and 29.92 Hg as your actual temperature/pressure combination - just isn't useful.
Just to further clarify this last point, since I think it's the main point of confusion in the linked MileSplit article (and possibly with Bosshard): Hotter air is less dense, but this in itself does not affect our ability to extract oxygen from the air. Instead it's the partial pressure of oxygen that matters, via the rate of gaseous diffusion across membranes in the lungs, and this is independent of temperature. For example, see this article: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1114067/ Key quote: "At high altitude [...] the alveolar-arterial difference for oxygen is higher than would be predicted from the measured ventilation-perfusion inequality. This is because the decreased driving pressure for oxygen from alveolar gas into arterial blood is insufficient to fully oxygenate the blood as it passes through the pulmonary capillaries. This is more evident on exercise as cardiac output increases and blood spends less time at the gas exchanging surface (diffusion limitation)."
In conclusion (thank you for coming to my Ted Talk) - the concept of density altitude is misleading when applied in this way, because it conflates changes in atmospheric density, which don't affect oxygen exchange, with changes in the partial pressure of oxygen, which do.
Final sidenote - density altitude could actually be a useful concept for sprints! That's because it is indeed the atmospheric density which is relevant to drag forces.
Oh and to directly answer Rojo's initial question from the thread title - how much of a low altitude day can you have in Boulder? Excluding extreme weather events, and basing the adjustment off of partial pressures, I think the answer for most locations should be a few hundred feet in either direction from the average.
To test this for Boulder in particular, I picked a weather station in Boulder at random and looked for the highest and lowest station pressures over the past year. These implied a "maximum altitude day" of about +600 ft, and a "minimum altitude day" of about -400 ft, using pressure altitude for the adjustment. (This asymmetry in deviations from the average is typical, and based on storm systems occasionally bringing in very low pressures.) I think this is a bit more variation than you would see in most cities, due to the mountains nearby.
If I could give one piece of advice to Bosshard's group, it would be this: The atmospheric variable relevant to oxygen uptake is not density, but pressure. Variations in density altitude due to temperature are thus spurious and do not reflect differences in respiratory oxygen exchange. Changes in atmospheric pressure may indeed be meaningful for distance running performance, especially at altitude, but the proper way to account for this is not density altitude, but pressure altitude. So track pressure altitude instead, and consider the effect of temperature on performance via limiting heat dissipation separately. Here is a calculator for pressure altitude: https://www.weather.gov/epz/wxcalc_pressurealtitude.
Of course, unless the point is some sort of mental game to manage athlete psychology. For that maybe density altitude is better since the changes are more dramatic. But it really isn't how it works in terms of physics or physiology.
Spot on. One more piece of advice for the Boss Babes: Kipyegon, Muir, Kipchoge, Jakob etc don’t care about your “altitude density.” Neither should you. Just get your arse out there and keep working hard — rain, cold, snow or sunny and warm “happy” weather.
most people think altitude is slower because there is less oxygen in the air. That is WRONG
There is more than enough oxygen in the high altitude air to fully oxygenate the blood but the issue is that the lower atmospheric pressure causes lower partial pressure of oxygen which means less oxygen transfers from the lungs to the bloodstream.
you're wrong. There is less oxygen at altitude. Fact.
Correct. The reduction in barometric pressure at altitude makes the oxygen molecules bigger relative to lower altitude, so there are less of them per unit volume.
Final sidenote - density altitude could actually be a useful concept for sprints! That's because it is indeed the atmospheric density which is relevant to drag forces.
As well as jumps and throws for the same reason. Everyone remembers Beamon's big jump from the Mexico City Games. At the same meet the women's LJ record went from 6.76 to 6.82. Men's TJ WR was surpassed 5 times ending at 17.39 (from 17.03). Women's shot WR also fell 18.87 to 19.61. OR in most other throws were established as well.
There may be high and low altitude days in Boulder, but wouldn't the same principle apply in Flagstaff? Flagstaff would still be a better place to work at altitude?
Just to further clarify this last point, since I think it's the main point of confusion in the linked MileSplit article (and possibly with Bosshard): Hotter air is less dense, but this in itself does not affect our ability to extract oxygen from the air. Instead it's the partial pressure of oxygen that matters, via the rate of gaseous diffusion across membranes in the lungs, and this is independent of temperature. For example, see this article: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1114067/ Key quote: "At high altitude [...] the alveolar-arterial difference for oxygen is higher than would be predicted from the measured ventilation-perfusion inequality. This is because the decreased driving pressure for oxygen from alveolar gas into arterial blood is insufficient to fully oxygenate the blood as it passes through the pulmonary capillaries. This is more evident on exercise as cardiac output increases and blood spends less time at the gas exchanging surface (diffusion limitation)."
In conclusion (thank you for coming to my Ted Talk) - the concept of density altitude is misleading when applied in this way, because it conflates changes in atmospheric density, which don't affect oxygen exchange, with changes in the partial pressure of oxygen, which do.
Here's an interesting recent article regarding oxygen availability on Everest.. it seems like weather very well may affect the partial pressure of oxygen. At truly high altitude, such as Everest, this may make a difference, but no one trains for running in the death zone. The difference in partial pressure of oxygen from one day to the next in Boulder is likely to make no true physiological difference for training, whereas it can be the difference between a successful or unsuccessful ascent of K2 or Everest.
Makes sense, but it seems more like the concept of dew point and humidity and their effects on daily performance; like it's good to know that on a "more dense" day at altitude you can expect tougher conditions and plan accordingly, but it doesn't seem like it can be used as a training method, like the idea going up to altitude.