In spite of dismal current weather and worse predictions, on Saturday January 21 five of us—two Bobs, Luke, Jay, and Phillip—decided to give Telescope Peak a try. We left Ridgecrest at 0615 in the dark, with wind and rain, and expectations of more of the same but amplified where we were going. "Giving it a try" should rather read: We would start up and turn around when the conditions forced us to.
We made it to Mahogany Flat and I set my altimeter, 8133 feet. It was cold, windy, and spitting snow. As we started up the trail, my altimeter behaved relatively normally, primarily because we were on the lee side for the WSW wind. It had been predicted to be 40 to 45 mph with gusts to around 70. Every now and then we experienced those gusts and my altimeter jumped around a bit.
I noticed it more in the elevation rate display. On trails like this I usually manage around 15 feet per minute gain these days. But the display would bounce between more than plus 30, and negative numbers. I knew Bernoulli was at work.
We neared Arcane Meadows in good style considering the conditions, and my altimeter usually displayed close to the correct value of just under 9600 feet. At Arcane Meadow we were suddenly in a wind tunnel: We estimated the wind speed at a steady 50 mph. I had read my altimeter just a minute or so earlier to be 9990 feet, the elevation of nearby Rogers Peak—and 400 feet high!
At that point three turned back, while Bob H and Luke pushed on to Rogers. They later reported that they were on hands and knees for the last part. They hiked back on the road, and we all met up at Mahogany Flat at about the same time. Bob reported that his (accumulating) altimeter said he had gained 4000 feet for the day. But we knew from the map that the correct figure was less than 1900. Again, Bernoulli at work: The gusts had lowered the pressure temporarily and therefore increased the elevation. After the gust the elevation went back to normal but the elevation gain had been memorialized.
Pictures here:
Telescope Peak trip Back home, I decided to do some calculations. As is well known—all things being constant—as the wind speed increases, the local pressure goes down. And as the pressure goes down, the observed altimeter reading goes up. For example, as you are approaching a pass from the leeward side, when you enter a high wind zone the measured altimeter value will be too high. Experienced mountaineers are familiar with this effect, and know to adjust their altimeter readings for the local wind speed. It can be important when navigating in poor visibility.
Bernoulli's equation relates the pressure to the air density and the speed. And of course the pressure can be expressed as an altitude value. Without going into all the details, I calculated the pressure at Arcane Meadow as 69700 dynes/cm2 when it should have been 70500: 800 dynes/cm2 too low. With the air density of 0.00113 gm/cm3 Bernoulli says the wind speed was 1188 cm/sec. While this looks like a high number, it converts to only about 27 mph. And we had "guessed" almost twice that much. I think the explanation is that I read my altimeter as we were in the final approach to Arcane Meadow, not when we were actually bearing the full brunt of the wind. If I do this experiment in the future, I'll be more scientific about it.
The message: Navigation in the mountains sometimes needs to take into consideration the wind speed and how it is varying for your trip. And that altimeters that provide a readout of gross elevation gain can overestimate in gusty conditions.
Afterthought: My rule of thumb regarding an atmospheric low moving in--nothing to do with the Bernoulli effect--is as follows:
- If the elevation gain overnight is 100 feet or less, I don't worry about it.
- If it's 200 feet, I worry a little.
- If it's 400 feet or more, I get the hell out
That's what I do. Others may react differently.
