View Full Version : What wind speed will blow someone off his/her feet?
11-28-2008, 09:39 AM
I'm sure you folks at the observatory have a good feel for this! My guess is the variables are:
-- Person's weight including gear
-- The ground surface, especially whether smooth or slick
-- Whether the wind comes in a sudden gust
-- (Related to the gust factor) Whether the person is braced and/or balanced on their feet
Personal observations: One year, hiking up the cone of Washington the last fall weekend before the summit visitor center closed, I had trouble staying on my feet. When I went into the visitor center, the anemometer showed 70-75 mph. (Variables: I weigh 125 and was carrying 12-15 lbs. of gear. Was hopping between boulders with no ice.) On another occasion, hiking in March between Bondcliff and Bond, I was knocked off my feet twice. (Variables: gear was up to 20+ pounds, surface was smooth water ice, but I was wearing crampons; wind came in gusts.)
11-28-2008, 01:41 PM
You pretty much answered your question in more ways than one. There really isn't a proven formula that if you plug "x" amount of vairables you will get "y" as your threshold wind speed to remain standing in. I am not saying that you can't calculate a speed, because you can. You can use PV=nRT, F=MA, horizontal wind velocity vectors in in relation to pressure, the horizontal equation of motion, and so on and so forth. But, I always tell the people that stay overnight that the ability to stand on ones feet comes via trial and error more than anything. And the two things I always say matter more than anything are center of gravity and weather conditions. I can't tell you how many overly confident people I have seen getted humbled by some of the weakest winds. Just because you weight 300 lbs at 6 feet tall and used to play football does not mean that you can tolerate higher winds than someone that weights 90 lbs and is only four feet tall and is a gymnyst. But when hiking, you want your center of gravity to be just that and located as close to your center as possible. One of my rescues was on a woman that overpacked her bag above her center of mass and when a gust of around 50 mph came up by the Great Gulf, she was knocked down into the gulf. But on the other hand, I have seen plenty of people ascend in higher winds with lesser trouble. But then there is weather to consider. Ice versus wet versus dry are like night vs twilight vs day. This can be said about anywhere. As a kid, you may have walked to school and found that dry gound was a lot easier than having the sidewalks glazed over with snow on top. The other thing is variability of the wind or "gustiness". Walking in winds that are constant are a lot easier than winds that are dying to 30 than instantaneously going to 70 mph. I always like to use the example from when I was younger and local fire departments used to compete. They would have a oil drum suspended and two rival departments spraying at it. The group that would have an easier time was the one that could maintain constant contact with the can rather and not the one with the stronger stream of water that kept missing. But it isn't just gustiness of winds, temperature and its relation to air density are big as well. The colder the air is, the more dense it is and therefore is usually more forceful. So a 100 mph gust in 90 F would feel different than in 20 below F. And lastly would be what trail you take in relation to where the winds are blowing. If you are taking Lions Head (most common route) and winds are NW, the strongest winds are at Lions Head and on the summit itself, the rest of the time, you are in the "shadow" of the summit and winds will feel and act lighter than what is being recorded at the summit. But in contrast, if winds are from the southeast, pretty much once you get above treeline, winds will get considerably higher the higher you go with wind to your back up and then front down. Location is everything with winds.
So, I hope this kind of helps. If you are wondering what my threshold speed is, it is about 112 mph. I am 5'6" and weight just below 140 lbs. I say that 112 mph is my threshold speed but that is working on the summit without a pack on. Backpacking, I find my threshold speed to usually be around 90 mph with a full pack on and even then, I am not strolling very well. I always get a kick out of youtube vids with the claim of hiking in 100 mph winds. After seeing many people ascend in 100 mph winds, there is a look and a way of walking that is very distinct and noticable that I can easily see a vid and be like "No, it is not 100 mph." But who am I to rain on their claim.
11-28-2008, 04:46 PM
Thanks so much for your detailed response! What you say about the center of gravity makes a lot of sense. It applies to the ability to resist winds and also things like balance in skiing. On winter trips when I've skied in for a certain distance and then switched over to snowshoes, I've found that the top-heavy feeling on the skis, carrying a fairly heavy winter pack, really makes me a bit unsteady. Body shape (as opposed to weight or muscle in themselves) seems to make a difference, too. Someone with short thick legs is going to be able to keep balance better than someone with long skinny legs (like me) whether it's on skis or trying to stay upright in strong winds.
Whenever I watch the Olympics, I'm always struck by how each event has its own ideal body type. It would be fun to have a "Stay Upright in Powerful Winds" event (with a mechanical wind-creating device) and see what the ultimate ideal body type would be:). Wouldn't be too much sillier than some of the stuff they have now!
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