Hi All: Here are responses to various comments:
Scott: The pressure differences that I refer to are typically between the interior and exterior of the windshirt. I have discussed vapor pressure differential in a bunch of these articles. If you want to figure it out for your conditions, use this online calculator.
If you want to know about how air pressure differences that create wind come about, use this link. There are more relevant articles on this site. Regarding air pressure inside a jacket, I would generally assume the pressure to be around zero in any units you choose. If you know wind speed in meters/sec, you can convert to approximate pressure with this formula: Velocity Pressure=(WS/1.29)**2. WS (Windspeed) is in units meters/second, VP (Velocity Pressure) is in units of Pascals. This formula misses the impact of static pressure, which will generally be minimal for this use. It will get you in the ballpark.
Vapor pressure is related to temperature and humidity. Air pressure may be impacted by humidity but mostly from the forces described in the link above.
Jerry and others-Pit Zips: Don’t expect miracles from pit zips. If you want a cooling breeze and you are walking at 2-3 mph, in calm winds, you won’t get much benefit. If you are above the tree line and the wind is blowing at 20 mph, you will get plenty of benefits. With pit zips open, the garment MVTR can increase dramatically, depending (as always) on vapor pressure differential and what you are wearing between your skin and the jacket. If you are wearing a backpack that restricts air movement or vapor movement toward the pit zips, then you will get less impact. Of course, if you are wearing a jacket when it is too warm, or you are working too hard, take it off.
Baja: Skiing or boarding in spring conditions is tough when you start in cold temps and then it becomes warm enough to make corn. At some point, you need to get rid of your wind layer, or you will sweat too much. Unfortunately, at the resort, you may have no place to stow it, so you will sweat while wearing it, pit zips or not. Remember: Be Bold, Start Cold. And stow a Montbell Tachyon in your pocket.
Thom: I think everyone who describes the Ferrosi remarks on the weight. It is a beast for a wind shell. Nevertheless, for aerobic activities where you need some wind protection, it works very well.
David: The tested MVTR allows us to compare performance amongst garments. It is much harder to apply these numbers to field use because there are many variables. There is no drawback to a high MVTR if it is achieved with a reasonable Air Permeability rating. This way, you can enjoy wind protection in high winds but still have the best chance of eliminating vapor. For me skinning up mountains in the winter is about the most aerobic thing I do. I found that my base layers remained dryer using the Ferrosi instead of my Neoshell jacket, as described in the article. If I expect to get rained on, I will wear my now-discontinued Montbell Shakedry or my Neoshell jacket. A word on MVTR. The number from a test depends on the specific test. For my test method, 3000 is pretty good. 4500 is great. Most of the published data use JIS L1099, B2. Here, 60000 is good. 80000 is excellent. Often, 20000 is a result from this test. That level of performance is pretty bad. Not sure I can actually do lab testing of pit zips. It is something for me to think about. It would be a pretty complex thing to measure.
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