Pikes Peak O2 Numbers

Pikes Peak 02

by

Tony Eckel

for

www.skyrunner.com

Zebulon Pike

Introduction

This paper explains why and how atmospheric pressure decreases with altitude and discusses the variations in oxygen (O2) level that affect aerobic efficiency. It is written mainly for runners of the Pikes Peak Ascent and Marathon but it applies to most any altitude race. The perspective is that of a meteorologist so the physiological processes of respiration are not covered here. What is covered are the answers to two specific questions:

  1. As I run on Pikes Peak, what is the typical change in the amount of available O2?

  2. Since the pressure at the top can change, what is the range of equivalent altitudes I may encounter at the top? (Are there days that 14,110' feels like a lot more/less?)

Answer to Question #1

We are very lucky to have an atmosphere to breathe. That may seem like a silly thing to say because we would of course not even be here if it weren’t for the atmosphere. However, most people do not realize how tenuous the earth’s atmosphere really is. In the big picture, the gases that surround our planet are just a wispy little film. Given a model of the earth the size of a car, the atmosphere would be only ½ inch thick.

Another thing to consider is that since a vacuum surrounds our planet, why doesn’t the atmosphere just go fizzing out into space? The reason is that gas, even though we can’t see it, is matter just like Newton’s apple and is likewise pulled toward the earth. Unlike Newton’s apple, the gas is also very compressible. So why then doesn’t it squish down to a super dense layer at our feet? Balance is the key to this puzzle just like many things in nature. The gas is fighting to go fill the void in space but the earth says “I don’t think so” and thus a compromise is established, again lucky for us.

The technical term for this compromise is the hydrostatic balance, which is a simple little equation with critical consequences for high altitude runners:

Hydrostatic blance

which states that the rate of change of pressure (p) with respect to altitude (z) equals the negative of the density (r) times the gravitational constant (g). The negative sign tells us that the pressure decreases with altitude which means that the density must also decrease. Since the rate of pressure decrease is directly proportional to the density, there must be slower rate of decreasing pressure the higher up you go. This is known as an exponential decay. One curious result of this is that there is no real “top” to the atmosphere. It just slowly tapers off to infinity.

We can solve the hydrostatic balance equation for pressure as a function of altitude and plot it as in Figure 1. Notice that the curve starts off very steep near the surface (rapid rate of decreasing pressure with height) and steadily becomes more shallow (slow rate of decreasing pressure with height). The consequence of this for runners of Pikes Peak is that for running in the steep portion of this curve, the atmospheric pressure (and thus the amount of available oxygen) drops off rapidly with height.

Atmospheric pressure vs altitude
Figure 1. Diagram of atmospheric pressure vs altitude for the US standard atmosphere with markers for the elevations of Manitou Springs and Pikes Peak.

The lowest 60 miles of the atmosphere is very well mixed and thus maintains roughly the same proportions of the different gasses is consists of. O2 is actually only a bit more than one fifth (21% by volume, 23.14% by mass) of the atmospheric gas. (Most of what you breath, 78% by volume, 75.51% by mass, is nitrogen.) Since the O2 part of the atmospheric pressure is constant, we can simply equate the percent decrease in total pressure to the percent decrease in O2. For example, from 1000 mb to 500 mb the pressure decreases by ½. The O2 partial pressure (pressure exerted by O2 portion of the gas) decreases from 231.4 mb (23.14% of 1000 mb) to 115.7 mb (23.14% of 500 mb), a reduction of ½ just like the total pressure.

Runners like to know the percent decease in O2 which can get confusing because it is a relative thing. At any given altitude, all runners will of course experience the same absolute O2 partial pressure but the percent decrease is based on a reference altitude. The term reference altitude is defined here as the elevation of the place you live and spend the majority of your time. This is where the O2 level is at 100% for you since it is what your body is accustomed to. An increase in altitude from your reference altitude, decreases the O2 from that 100% and visa versa. Figure 2 can be used to figure percent O2 at another altitude for a given reference altitude.

Percent of oxygen at another altitude based on a reference altitude
Figure 2. Diagram to find the percent of oxygen at another altitude based on a reference altitude for an average summertime atmosphere. Find your reference altitude on th diagonal curves which are labeled in feet (dashed lines are odd thousand values). Trace this curve to the intersection of the new altitude (read from the bottom axis) then read across to the left axis to find the percent of O2. Since these curves are for average conditions, the percent O2 can fluctuate a few percent. (see Answer to Question #2)

For example, say your reference altitude is in Colorado Springs at 6,000'. To find the percent of this particular 100% O2 level at the top of Pikes Peak, first locate the diagonal curve for 6,000' in Figure 2. Now follow it down to the right to the intersection of the 14,110' altitude, then read off the percent O2 (about 74%) on the left. This result comes from the fact that the O2 partial pressure at the top of Pikes Peak is 141.6 mb, which is 74% of 190.1 mb (the O2 at the reference altitude). Now let’s say you are from Houston at sea level (0 ft). Figure 2 shows that Pikes Peak would have 61% of the O2 you are conditioned to. Do you see any advantage to living at a higher altitude?

Answer to Question #2

Now that you understand more about pressure variation with height, we can address the second focus question of this paper which deals with horizontal pressure variations. The atmosphere stays close to Figure 1 but sloshes all around creating horizontal pressure differences on big scales (HIGHS and LOWS you see on weather reports) and small scales (such as a thunderstorm). So as the pressure changes at any point, the amount of O2 is of course going to change. Normally, this fluctuation is imperceptible but if you are already in a decreased O2 environment, as up on Pikes Peak, you will feel the difference. In general during a high pressure it will feel like you are running at a lower altitude and during a low pressure it will feel like you are running at a higher altitude.

The average summertime pressure on top of Pikes Peak is 612.0 mb. On the large scale, the pressure typically varies about ±12 mb and for small scale about ±4 mb. Therefore the extreme range of possible pressures on the Peak during the summer is ±16 mb or 596 mb to 628 mb. Using the graph in Figure 3, the lower pressure gives an equivalent altitude of 14,810' and the higher pressure equates to 13,410'. This means that during your summer training on the Peak, it is possible to have a 1,400' swing in the equivalent altitude. Do you see why things can feel so much different depending on the day?

Equivalent altitudes for the extreme range of pressure fluctuations
Figure 3. Top graph is similar to zoomed region of Figure 1 but for an average summer atmosphere. The elevation for several landmarks on Barr Trail are marked. The bottom graph is a zoom in around the top of Pikes Peak to show the equivalent altitudes for the extreme range of pressure fluctuations. An equivalent altitude is found with respect to the average summer condition of 612.0 mb at the top of the Peak.

Forecast and Observation

The forecast variation of pressure on Pikes Peak is not something you are likely to get on a typical weather report. So how can you know in advance or during your run on the mountain if it is a high or low O2 day?

For forecasting the large scale pressure pattern, there is a wealth of information available on the world wide web. Unfortunately, most of it takes a meteorologist to digest. However, you can check out the latest upper atmospheric observation from a nearby location. Since the high altitude pressure pattern changes slowly, knowing the current pressure will normally give you a ballpark prediction for tomorrow.

Upper atmospheric observations are made with weather balloons in Denver and Grand Junction at 6 AM and 6 PM daily. One web site that displays the data is http://dweb.met.fsu.edu/index.pl/wxdata/textwx. For the Bulletin type select upperair decoded. In the Station box enter either DNR for Denver or GJT for Grand Junction. For the Time select latest or go back in multiples of 12 hours if needed. After hitting Submit you will get something like Figure 4.

Upper atmospheric observations made with weather balloons in Denver
Meters Feet
Figure 4. Sample upper atmospheric observation from Denver on 26 July at 6PM. The date/time of 26/00 is highlighted which is 26th of the month 6PM (00 is Greenwich Mean Time). The height (in meters, M) at which a given pressure (in millibars, MB) occurs is highlighted. Choosing the pressure reading of 610.1 mb (close to the average pressure at the top of Pikes Peak), we see that it occurred at 4267.2 m, or 13,996' using the conversion 3.28 ft / m. Referring back to the lower graph in Figure 3, a pressure of 610.1 mb normally occurs at about 14,190', so in terms of equivalent altitude you would be running almost 200' higher (14,190'-13,996') than normal.
During a run on the mountain, there are other ways to determine what is happening with the pressure. The best way is to carry along a well calibrated barometer, conveniently available as part of a watch. Comparing actual pressure readings to the expected pressure at particular landmarks (top graph in Figure 3) will reveal if you are working with more or less O2 than normal.

A barometer watch is an expensive toy so there are some rules of thumb you can use as well. In a mountainous environment, the small scale pressure patterns (as well as the weather in general) can vary rapidly. Heating and wind patterns create all sorts of small low and high pressure anomalies. High winds is therefore one clue that something abnormal is occurring. In general, a windy day means below normal pressure since a large scale storm is likely nearby plus small scale eddies with even lower pressure may form on the Peak.

Another rule of thumb deals with clouds. Very high, smooth clouds above the Peak will generaly mean above normal pressure since the atmosphere is very stable. Where clouds form on and against the Peak, you can generally expect pressure lower then normal. This can happen in a number of ways. As just one example, Figure 5 shows what happens when a strong, uniform wind blows up against a peak. A “banner cloud” forms in the downstream eddy where the pressure drops several mb. This type of effect, as well as more complex ones, are often observed on Pikes Peak.

Banner cloud on the Matterhorn
Schematic of wind flow of a banner cloud
Banner cloud on the Matterhorn, Switzerland.
Schematic of wind flow and pressure pattern (L-low pressure,
H-high pressure) that produces a banner cloud.
Figure 5. Photo courtesy of J. F. P. Galvin. Diagram adapted from Houze's Cloud Atlas.

Conclusions

A mountain trail runner should understand the normal rate of O2 decrease with altitude and that this rate can vary. A slow day at high altitude can often be contributed to a lower then normal pressure. Being aware of how much O2 is available should help proper pacing.

Acknowledgments

I would like to thank Nolan Duskin from the Colorado Climate Center for providing historical data on atmospheric pressure on top of Pikes Peak. Thanks also to Matt Carpenter for suggesting this paper and for all the support, advice, and encouragement he has provided to aspiring trail runners.


Numbers, Tools and Links

The concepts above and the calculators below are based on the hydrostatic equation using a “standard atmosphere” designed for average summer conditions at Pikes Peak:
  • Sea level pressure (SLP): 1011.0 mb (Note: General standard SLP is 1013.25 mb)
  • Temperature lapse rate (decrease in temp with height): 6.5C per kilometer
  • Sea level temperature: 34C (93.2F) which means 21.6C (71F) in Manitou Springs
When using the calculators the station pressure and station altitude must be accurate for the results to be useful! If you don’t have a barometer to give you a station pressure, you need to get the information from a source like in Figure 4 or some other weather report.

(Note: If you are not interested in knowing the exact percent O2 and perceived altitude for the current conditions, you can simply use the standard values of 1011.0 mb for station pressure and 0' for station altitude. The resulting percent O2 will be for the average conditions and the perceived altitude will of course match the true altitude.)

If you go with the weather report option, it is important to know that the reported barometric pressure typically is not station pressure but actually the equivalent sea level pressure (ESLP). This is a trick forecasters do to aid in surface weather analysis. ESLP is the station pressure increased (by an amount based on the principles discussed above) to a value that equates it to a common reference altitude, chosen to be sea level. That is why the Colorado Springs weather report might say the barometric pressure is 29.85 "hg which sounds like a number you would get no matter where you live — which is exactly the point. If you are ever confused whether a given pressure is station pressure or ESLP, refer to Figure 1 to get the ballpark pressure for the report location. Example: Reported Denver pressure is 998 mb. That has to be ESLP since the typical station pressure at 5,000' is around 850 mb, 20 mb.

(Note: To use an ESLP in the tools, there is no need to convert it to a station pressure. Think of it as a station pressure at sea level. So enter the ESLP as the station pressure and enter 0' for the station altitude.)

The ideal way to use these tools is to have a pressure observation as close as possible to the altitude of interest. This is because the deviation from average conditions can change with altitude. For example, it is possible to have a slightly lower than normal pressure in Manitou but a higher than normal pressure at the top of Pikes Peak. So using info like in Figure 4 is the better way to go. If you plan to use your own barometer (like on a watch), make sure you know your exact altitude when you note the station pressure. Also, you should ensure that the barometer is well calibrated against another accurate barometer. This can be accomplished by using the ESLP in a nearby weather report. Set your barometer to the station pressure computed using the ESLP to SP calculator below. You should repeat this every so often since your barometer can get out whack with all the up and down it must endure.

(Note: It is possible to set your barometer to read ESLP but then its reading will only make sense when used at the altitude where it was set. So this is OK for a barometer you hang on your wall but it is not recommended for a watch barometer.)

Percent O2 calculator

This calculator uses 4 values:
Alt of interest: Altitude where you want to know the percent O2.
Reference altitude: Altitude where you spend most of your time.
Station pressure: True pressure. See note if using an ESLP.
Station altitude: Altitude of the pressure reading.

Notes: ESLP is the barometric pressure most often given in weather reports. If using an ESLP, enter 0' for station altitude. If interested only in average percent O2, set station pressure to 1011 Mb and station altitude to 0'.
Example: You live at 2,050' and you want to know the percent O2 on the summit of Pikes Peak using average conditions


Alt of interest: Feet Meters
Reference altitude: Feet Meters
Station pressure: Inches of mercury Millibars
Station altitude: Feet Meters
Percent O2: %

Perceived Altitude calculator

This calculator uses 3 values:
Alt of interest: Altitude where you want to know the perceived altitude.
Station pressure: True pressure. See note if using an ESLP.
Station altitude: Altitude of the pressure reading.

Notes: ESLP is the barometric pressure most often given in weather reports. If using an ESLP, enter 0' for station altitude.
Example: Before heading out for a run up to Barr Camp (10,200') on Pikes Peak you check the weather in Manitou Springs and get an ESLP of 30.19 "Hg.


Alt of interest: Feet Meters
Station pressure: Inches of mercury Millibars
Station elevation: Feet Meters
Perceived Elevation: Feet; Meters
This is feet or meters than normal.

ESLP to SP calculator

This calculator uses 2 values:
ESLP: Equivalent sea level pressure.
Altitude: Altitude of barometer being calibrated.

Note: ESLP is the barometric pressure most often given in weather reports. The closer you are to the station reporting the better.
Example: You want to recalibrate your watch/barometer at the Barr Trail trailhead (6,670'). Before heading to the trailhead you check the weather in Manitou Springs and get an ESLP of 29.85 "Hg and use the calculator to find the value you will enter once you get to the trailhead.


ESLP: Inches of mercury Millibars
Altitude: Feet Meters
Station Pressure: Inches of mercury; Millibars
To learn more about some of what was covered in this paper check out:
Atmospheric pressure and its various units of measurement.
Why we are lucky Mt Everest (or Pikes Peak) is not in Alaska.
Understanding the hydrostatic equation.


About the Author

By Matt Carpenter

Tony is a US Air Force meteorologist who started running with the Incline Club in 1999 and soon became the official Incline Club weather forecaster. I remember one time it was very foggy and Tony said (before we started the run from Manitou Springs) we would break out of the fog about 3 switch-backs before Barr Camp. Sure enough about 2 switch-backs from Barr we were running under clear skies. Often he would give the club 4-5 days advance notice of bad weather when most news reports never saw it coming until it was too late if at all. In 2000 the club also adopted the top mile of the Barr Trail. This is dangerous work because of the weather (one hiker was killed by lightning that year) and Tony made us all feel more comfortable by keeping an eye to the sky. His love of mountain weather was contagious and it was an awesome experience to hear him say “see that cloud, in a few minutes it is going to shoot straight up several hundred feet and then we are going to get some snow” and then just sit back and watch it all happen. He moved away from us to pursue a Ph.D. in atmospheric sciences at the University of Washington in Seattle, but hopes to return to CO someday. We all miss him and we will also miss his PPA/M forecasts that gave remarkably accurate temperature predictions at different points along the course depending on the pace you were running. In 1999, after running a 3:19:21 Ascent during a 5:25:12 Marathon Tony set his sights on a sub 5 hour marathon. In 2000 Tony came to 23 of the last 27 Sunday Incline Club long runs and the hard work paid off when he did a 3:06:10 Ascent during a 4:55:21 Marathon!


Back to Pikes Peak Central