Well I finally got to the bottom of a vexing question. I was trying to figure out how much less oxygen there is at the top of the trail I do each day. When I realized I had to know the barometric pressure the problems really began. Upon reflection, the pressure was not much different here as anywhere else I’ve lived. How could the pressure be the same as sea level if I live at altitude?
Well, it turns out that the barometric pressure given in the weather reports is in fact the ESLP or “equivalent sea level pressure.” The Banff weather data lists the pressure today at 102.2 kPa but upon performing the necessary calculations, it is actually 87.5 kPa Pressures are adjusted for altitude for ease of comparison I guess. This adjustment is useful for understanding weather but not very good if you’re trying to figure out how much O2 there is!
This discovery led to another thought. Based on the real air pressure changes, I can calculate what the perceived altitude is. The perceived altitude is the only one that matters if you are trail running. Here is an interesting example from a run I did today.
Under normal (ESLP) pressure of 101.0 kPa
*At my house at altitude 1320m there is 87.1% O2 than at sea level
*At the trail maximum of 1460m there is 85.7% of the O2 at sea level.
Today’s listed (ESLP) pressure was 102.2 kPa making real air pressure: 87.5 kPa
Because of higher pressure today, the perceived altitude of the high point was actually 1365m. This yields a true oxygen level of 86.6% of sea level. In other words, running in higher pressure today “lowered” my high point by over 100m of equivalent available oxygen. So I guess I work better under pressure!
This website has a good description of the relevant concepts and at the bottom of the page is a handy calculator for figuring out real air pressure, available O2, and perceived elevations. Perhaps other runners reading this might keep a pressure log and see if they notice a difference in performance between the max and min air pressures.