It's Not Just Altitude, Folks

When you live in the shadow of the Rocky Mountains, it's easy to assume that altitude is all you need to know to assess distance performance. But, there's much more to it than that. Colorado Track XC file photo.

Note: This article is a written summary of a clinic presentation I gave Tuesday, July 31, at the New Mexico High School Coaches Association Clinic in Albuquerque. There is some information included in this article that was not included in the clinic presentation, but the vast majority of the content was included in the clinic session. To all readers from Utah, although this article relies on examples from New Mexico, you should easily be able to draw parallels to Utah. Whether you accessed from the Utah or New Mexico site, please know that you're welcome to add your questions to the comments section of this article. I'll monitor that frequently and do my best to quickly answer any questions that arise.

For years, we've accepted the notion that running distance is more difficult at higher elevation than lower. The air is lense dense at higher elevation and, so, the oxygen molecules needed to make the human machine run are in shorter quantity. In fact, from a human performance standpoint, altitude is little more than differentiated density of oxygen molecules.

You can think of the atmosphere as something like the stack of bath towels in your closet. Once the towels are stacked, compression starts to take place. The weight of the stack compresses the towels in the stack. The towels at the bottom of the stack bear more weight and thus become more compressed than the towels higher in the stack.

The atmosphere is a lot like that. At sea level, the air you breathe is compressed by 5000 more vertical feet of atmosphere than the air you breathe in Albuquerque. Therefore, the packing of oxygen molecules is more dense at sea level than it is in Albuquerque, meaning that oxygen is more abundant at sea level than in Albuquerque.

But altitude is far from the only thing that affects air pressure and density. It is merely the most obvious and the easiest to quantify of the factors that affect air pressure and density. And, since altitude is so easy to quantify, the NCAA eventually created a table of conversions for adjusting distance performances at each of the member schools' tracks located above 3000 feet elevation.

A sampling of those adjustments would include:

  • A 9:30.00 3K run at the Great Friends of the University of New Mexico track is converted to a 9:15.21 equivalent at sea level.
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  • A 4:30.00 1500 run at Rex Field on the campus of Adams State College is converted to a 4:18.62 equivalent at sea level.
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  • A 30:00.00 10K at New Mexico State track in Las Cruces is converted to a 29:21.77 equivalent at sea level.
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What the NCAA conversion tables fail to take into account, however, are the other factors that influence air pressure and density--things like temperature, humidity, and movement of weather systems. In effect, the NCAA converstions proceed under the assumption that the abundance of oxygen molecules remains constant at each of the various competition venues located at elevations in excess of 3000 feet. To illustrate how misguided this assumption is, we need to introduce the ideas of density altitude and adjusted barometric pressure.

If you are a pilot, you are already familiar with the notion of density altitude. Since airplanes fly through the medium of air, it's important to know how dense the air is on any given day and for any given flight. Air density will affect the speed, performance, and climbing rate of the aircraft.

Altitude density is a way of expressing what your actual air density is in terms of the "standard" density at a given altitude. If you calculate an density altitude of 8000 feet, that means the air density at your location is the same as the "standard" density at an elevation of 8000. Your actual elevation could easily be as low as about 5000 feet when the density altitude is 8000 feet.

For simplicity's sake, humidity is often ignored in calculating density altitude, as it's impact on air density is relatively small. Furthermore, water vapor is lighter than air and, so, more humid air is less dense than less humid air (though not by much). The most frequently used density altitude formula incorporates an altitude, an adjusted barometric pressure, and a temperature. That formula is still more complex than most people care to set eyes on and I have omitted it here.

Since air density is also a function of barometric pressure, we need to discuss how that is reported as well. Any time you read or listen to a weather report that contains a barometric pressure, the reported pressure is invariably an adjusted (to sea level equivalent) barometric pressure. In most of New Mexico, the actual barometric pressure is always much lower than the adjusted barometric pressure. This is because most of New Mexico is well above sea level. Weather reporting services, however, use adjusted barometric pressures because their concerns have more to due with charting systems of high and low pressure than with performance of airplanes or distance runners. Pilots and coaches are left to figure things out on their own. An adjusted barometer reading of 29.75 means low pressure, no matter where you are. Similarly, an adjusted barometer reading of 30.25 means high pressure, no matter where you are.

With that background information in tow, let's take a few hypothetical examples of temperature and barometric pressure for Los Alamos (actual elevation 7230 feet above sea level) to illustrate how barometric pressure and temperature impact density altitude. Our calculations will ignore the relatively small impact of relative humidity.

On a day where the adjusted barometric pressure is 29.77 (inches of mercury) and the temperature is 79F at at Los Alamos High School, the density altitude is a whopping 10,371 feet. On a day where the adjusted barometric pressue is 30.02 and the temperature is 63F, the density altitude at Los Alamos High School is 9143 feet. On a day with an adjusted barometric pressure of 30.36 and the temperature is 46F, the density altitude is 7716 feet (note that density altitudes are almost always higher than actual altitudes except in periods of cold weather).

Clearly, not all days in Los Alamos are the same, even though the NCAA conversion formula, if one existed for Los Alamos, would treat all days the same. If you got to run a distance event in Los Alamos on a day when the density altitude is 7716 feet, you would be very likely to beat the NCAA conversion system. On the other hand, if you ran that same race on a day with a density altitude of 10,371 feet, you would likely go away thinking the conversion factor was a little miserly.

At this point, you may be wondering where density altitude and actual altitude converge. At the standard (adjusted) barometric pressure reading of 29.92 inches of mercury, the density altitude equals the actual altitude at about 40F for elevations of about 5000 meet. That suggests that, for most days when track meets are held, the effective altitude at any venue in New Mexico is considerably higher than the actual altitude. That does not mean that running distance in New Mexico is even more difficult than you thought it was. It only means that the density altitude scale is configured in such a way that it produces an reading that is typically higher than the actual altitude. Once you know and understand that, you will not tend to be as alarmed by the numbes. Knowledge is power.

Neither should this fact be construed to imply that the NCAA conversion tables are too conservative. The NCAA conversions are very good approximations based on data that took only race times and actual altitudes into consideration. It's probably more accurate to think of the altitude conversion for the UNM track facility as a conversion for a density altitude something on the order of 7000 or 8000 feet of density rather than as a conversion for 5260 feet of actual altitude. To restate the main point of this article, available oxygen is more properly thought of as a function of density altitude than as a function of actual altitude. Density altitude gives us means of describing how dense the air (and therefore the oxygen in the air) actually is. Altitude alone gives us only a crude approximation of how dense the air actually is.

Further muddying the picture is the fact that a track at a slightly lower actual altitude (say, for example, the track at Belen) may sometimes have a higher density altitude than the UNM track. If the air is enough warmer on a given day in Belen, the density altitude could actually be higher in Belen than Albuquerque. And, the barometric pressure could easily vary by a few hundredths between Albuquerque and Belen on any given day. The NCAA conversion tables do not take these facts into account.

My own coaching interest in this phenomenon was sparked by a very large cluster of outstanding performances at my school's cross country invitational in 2010. Although the course was no different than it had been for a few years running, a huge number of athletes, and athletes from almost every team assembled, recorded 5K PR times at that meet, many by more than 20 seconds. Digging further into the data for that meet, I discovered that a nearby weather reporting station had recorded a temperature of 55F at race time and an adjusted barometric pressure of 30.26. 

That prompted a theory. I pursued the theory further by digging out dates and places of all meets where our team had enjoyed near across-the-board outstanding performances. Armed with those dates, I went back and checked the weather records for those dates and places. Every last one of those dates revealed a high barometer (30.10 or higher) and relatively low temperatures (typically 60s or lower). I compared notes with Coach Chris Suppes at Fort Collins, whose team had also enjoyed a particularly special day at our invitational. He had already arrived at the same conclusion. If you can identify those same dates in your program's history, go and dig out the weather data for those dates.

But there are two sides to every coin. What about those frustrating days, days that every distance coach eventually experiences, where you feel you have the team completely prepped to go chase great times and, one after another, each athlete just seems to come up short? Both in track and field and cross country, when I checked out the weather conditions on those dates, the combination of temperature and barometric pressure turned out to be unfavorable. Often, though not always, it was the double whammy of high temperature and low barometric pressure.

The convergence of great peformances with favorable density altitude conditions and softer performances with unfavorable density altitude conditions is simply too striking and too consistent to dismiss.

As a result, I now make it a point to check out the density altitude before both key workouts and meets. I do not tell the runners in advance what the conditions are, not wanting excuses to emerge out of (literally) thin air before the race even begins. But, as I evaluate interval paces or evaluate race results (even as these events are going on), it is always with an eye toward the conditions. I've learned not to fret about sub-standard times or interval distances when the conditions are well short of ideal. Kids' confidence takes far less of a beating when they understand there's a good reason why their efforts fell short of expectations on a particular day.

Here are links to three resources I've found particularly useful:

Brunton Atmospheric Data Center Summit - This is a small, portable weather station. This device provides me the actual barometric pressure and, properly used, the actual temperature (don't expect to get an accurate temperature if you lay the device out on a rock in the open sunlight), wherever I am. Once I know what the "typical" actual barometric pressure is for where I am, this device tells me instantly if I'm dealing with barometric pressure that is higher or lower than usual. Couple that information with the temperature, and I have a quick seat-of-the-pants guide to the density altitude. This device can be purchased online through a variety of sources. Shop around for the best deal.

Online Density Altitude Calculator - If you have weather station readings from a nearby location, you can plug those values in (altimeter setting would be the adjusted barometric pressure) and get a fairly precise calculated density altitude from this site. Note that relative humidity has only a small impact on density altitude, and particularly so in drier conditions that are typical of New Mexico.

Weather Underground History Data - This link takes you to the wunderground.com page where you can look up historical data for weather at many, many sites across the country. 

Now that we've made it this far, there are all sorts of questions rushing into the room, clamoring to be addressed. Let's take inventory of a few of those questions:

Q. Do differences in air pressure at lower elevations make any difference in distance performance?

The answer to this question is uncertain. Nobody, to my knowledge, has researched this question. It may well be that, up to a certain point, differences in air pressure and density are inconsequential to human performance. It may be, for example, that up to density altitudes of 3500 feet, there is more available oxygen than a distance runner can use. It's entirely plausible, as well, that this point oxygen availability starts to matter varies by individuals. VO2 Max calculations suggest rather strongly that the ability to use available oxygen varies from athlete to athlete, so it would not be surprising if the point at which reduced oxygen density begins making a difference would also vary from athlete to athlete.

Q. Does the increase in air density that accompanies lower temperatures help to explain why we frequently observe very nice distance performances at temperatures that seem too cold to be optimal for human exercise?

I strongly suspect the answer to this question is yes. Typically, my distance runners have performed very well at temperatures down to about 40F (below that, the meets have usually been canceled or postponed, with one exception where we all nearly froze to death in an ice storm and times were terrible). There is no way such temperatures are optimal for muscle elasticity and such. Enhanced oxygen supply seems like a compelling explanation for the phenomenon.

Q. Is it possible to quantify the impact of changes in density altitude on distance performances?

As long as you understand that any such quantifications are approximations and not neat-and-tidy formulas, the answer is probably yes. It will take a lot more data than I've yet been able to gather to produce reasonably reliable approximations, however.

The rest of this article is a discussion of how I approached my research and what I have learned to date.

Earlier, I mentioned that I purchased a meteorological measuring device and took it to each meet we attended this spring. The device I purchased was a Brunton Summit ADC (Atmosheric Data Center). While probably not the world's most precise instrument, it is simultaneously lightweight and very portable.

During every 3200 in the spring of 2011, I recorded a Beaufort Scale wind reading (thinking holistically of the wind conditions for the entire race), a temperature, and an actual barometric pressure reading (thus sparing myself the extra work of converting an adjusted barometric pressure back to an actual barometric pressure).

I compiled names and race times for every 3200 performance at every meet this spring. Eventually, I discarded state meet times due to the fact that I was concerned that the added adrenaline of the state meet made those data points fundamentally different in kind from the data points from other meets during the season. I separated male times from female times to reduce scatter.

Further, I eliminated all times from males who never dropped below 11:00 for a 3200 and from females who never dropped below 13:00 for a 3200. My rationale for discarding these data was the suspicion that these athletes likely have factors larger than the amount of available oxygen limiting their performances. If oxygen availability is not the main factor limiting their performances, then I end up measuring something other than what I think I am measuring.

Even after these deletions, I still had hundreds of data points for both males and females. At this point, I performed multiple regression analysis on the data. I used week of the season, temperature, wind, and density altitude as the explanatory variables and race time (measured in seconds) as the response variable.

The results I got were astonishing, on multiple fronts, and left me with the settled conviction that I don't yet have nearly enough data points.

For boys, I got a predicted increase of about 14.4 seconds in total 3200 race time for each 1000 feet of increase in density altitude. This increase was statistically significant (meaning we would be unlikely to see a slope this far from zero simply by chance variation). It should be noted that the data were somewhat non-linear, meaning that increases in race time tend to grow larger rather than remain constant with continued increases in density altitude. 

For girls, I calculated a predicted increase of only about 5.6 seconds in race time for each 1000 feet of increase in density altitude. This increase was not statistically significant (meaning that chance variation alone could have easily produced a slope this far from zero). As was the case with the boys, however, these data showed a somewhat non-linear pattern of race times increasing more rapidly as density altitude continues to increase.

While I will be the first to admit that there is more work to be done and more comprehensive sets of data points required, the differences between boys and girls cry out for attention. To be more specific, I am not at all prepared to suggest that the density altitude adjustments calculated and reported above represent the final word on density altitude adjustments. For the boys, in particular, I'm inclined to think that more data--and data that represents something closer to a true randon selection of data points--would ultimately reveal a small adjustment than 14.4 seconds of 3200 meter race time for each 1000 feet of increase in density altitude. Every fiber of my being believes that 14.4 is too high.

On the face of it, though, the differences above suggest differences in density altitude are felt more acutely by boys than by girls. And that is about as counter-intuitive as things get. We tend to want to think of differences in performance occasioned by increases in density altitude cashing out as a percentage of race time. If that were the case, the predicted increases would be larger for girls than for boys. Clearly, they were not in the data I collected.

At this point, the most plausible explanation I can conjure up is that more boys than girls are racing close to the edge in terms of using all of the available oxygen. Perhaps my 13-flat cut-off point for girls was not aggressive enough? Either way, we've certainly not eliminated the possibility that differences across gender might be due to physiological factors. And, it could still turn out that the observed differences disappear once we have more a more comprehensive set of data points drawn from a more diverse (and random) set of environmental conditions.

Collecting large sets of data points, which a study like this seems to demand, requires careful on-site recording of, minimally, temperature and barometric pressure for each race. At this point that's a lot to ask, so we remain--for the time being--in the realm of highly suggestive, but still inconclusive, evidence.

But there is no shortage of topics for further study.

 

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