Dave Anderson
"Pressure, pressure, pressure gonna drop on you." The lyrics of the song "Pressure Drop," echo through my head. The song has been recorded by everyone from the reggae band Toots and the Mytalls to Guns and Roses guitarist Izzy Stradlin. But the lyrics could not be more appropriate for my present situation. The pressure drop—that is, the barometric pressure—has plummeted in the last 24 hours.
Barometric pressure is one of those meteorological terms that TV weather personalities spout off like they were some secret code that only the elite forecasters know and understand. The way it works is really pretty simple. The air that composes our atmosphere has mass (i.e. it weighs something). So, at sea level all of the air in the atmosphere is on top of you, thus there is more pressure. As you gain elevation there is less air above you so there is less pressure.
The TV weather people also talk about "high pressure systems" and "low pressure fronts." Without going into all the ways and whys, generally, high pressure means clear stable weather and a low pressure means poor unstable weather.
As a result, the barometric pressure not only changes when you go up or down in altitude, but a storm or sunny skies can also affect the pressure.
What does this have to do with climbing in Patagonia? The weather windows are extremely short down here and climbers use every available resource to predict/guess when the weather will improve. By following the trends in barometric pressure, Andrew and I hope to be positioned in our snow cave, not drinking beer in Chaltén, when the sun is shining and the wind is silent.
Barometers have been around for centuries, but in the last decade they have gone high tech and portable. Presently, they are the size of a wristwatch on steroids and can keep track of a myriad of data points. The instruction book that came with my watch has more pages than the one that came with my car.
Inside the watch there is a sensor that measures the change in pressure. The sensor works similar to the way a balloon behaves at various altitudes. If you blew up a balloon at 10,000 feet and brought it up to 20,000 feet, it would swell in size, because there is less pressure on it. Conversely, if you brought it down to sea level (0 feet), the increased pressure would cause it to shrink in size.
Now here is the really cool part. The barometric pressure and the elevation go up and down at a fairly similar rate. So, if you know the barometric pressure you can calculate (or the watch can do it for you), what elevation you are at.
Having a watch to track the change in pressure is a great tool, but it doesn't always predict the weather accurately. There is often a lag time between what the watch says and what is happening outside. Yesterday, I woke up to a substantial drop in pressure. The sun was out and there was a 10mph breeze from the north. I watched a group of Italians racing up the glacier to high camp, as I leisurely strolled around taking photos of the peaks pressed against the cloudless skies. As if on cue, at noon I felt some of the strongest winds I have felt this year and later the Italians staggered in soaked to the bone by the sideways rain.
The reverse also happens. Last year, all signs pointed to climbable weather, except the barometer had not shown a significant rise. My partner John and I gambled the pressure was slow to react and headed up the mountain, while the rest of the climbing contingent at Rio Blanco waited another day. The weather continued to improve and the barometer eventually went up, but our jump-start allowed us the time to climb another peak, before the weather turned. The moral of the story is use the barometer, but don't be ruled by it.
So, here is the final exam:
I go to sleep at the Rio Blanco Base Camp and the next morning when I wake up and check the elevation on my watch; it reads 300 feet higher than the night before. What happened?
A) I am a chronic sleepwalker
B) Extra terrestrials have beamed me 300 feet higher up the hill
C) A strong low-pressure system (i.e. a storm) came in over night and caused the pressure to drop and the corresponding elevation on my watch to go up.
If you picked A, it shows that you thought about the question, but skimmed the above information on barometric pressure. Go back to school.
If you picked B, it shows you fall into the same category of people down here who believe the full moon, or climbers arriving and or leaving base camp, will bring good weather. Please stay away from gambling casinos.
If you picked C, you are a smart person and will never go to Patagonia, because you value your time and actually like to climb on your climbing expeditions.
—
Dave Anderson, MountainZone.com Correspondent
