pressure and altitude dependencies

Thread: pressure and altitude dependencies

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  1. #1
    ntp
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    Question pressure and altitude dependencies

    Hello.
    Let's suppose I'm high in the mountains, it's a little cloudy, and I want to go back down. The pressure would rise as the altitude decreases. Let's say more clouds gather up for a storm (but I can't notice them because the sky was already cloudy), hence the pressure decreases. What would a Pathfinder show me? Is there any chance it might show me a constant or slightly increasing pressure (leading me to believe that the weather could actually improve)? I don't think the temperature could be such a big deal maybe I'm literally chilling out as I go downslope. So could this happen?

    Or would the air pressure actually go down because of the bad weather? Are clouds a much more powerful pressure modifier than the descent speed I could achieve on foot? But what if I was on my bike, or in a car and I would have descended more rapidly? The actual formula on which those calculations are made would be useful.

    Sorry for not beeing more explicit, hope you understood my question and I'm looking forward on your answers. Some sort of mathematic understanding of those relationships would be useful.

  2. #2
    Member Gawain's Avatar
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    Re: pressure and altitude dependencies

    Wow....no idea... sorry
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  3. #3
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    Re: pressure and altitude dependencies

    Quote Originally Posted by ntp View Post
    Hello.
    Let's suppose I'm high in the mountains, it's a little cloudy, and I want to go back down. The pressure would rise as the altitude decreases. Let's say more clouds gather up for a storm (but I can't notice them because the sky was already cloudy), hence the pressure decreases. What would a Pathfinder show me? Is there any chance it might show me a constant or slightly increasing pressure (leading me to believe that the weather could actually improve)? I don't think the temperature could be such a big deal maybe I'm literally chilling out as I go downslope. So could this happen?

    Or would the air pressure actually go down because of the bad weather? Are clouds a much more powerful pressure modifier than the descent speed I could achieve on foot? But what if I was on my bike, or in a car and I would have descended more rapidly? The actual formula on which those calculations are made would be useful.

    Sorry for not beeing more explicit, hope you understood my question and I'm looking forward on your answers. Some sort of mathematic understanding of those relationships would be useful.
    My understanding is that pressure varies inversely with altitude at a rate of roughly 1 millibar per 10 metres or 1 inch mercury per 1000 feet and that even at walking pace your ascent/descent will for practical purposes vastly outweigh the meteorological element of the pressure change. I posted about this a bit ago because I thought the baro chart on the watch had stopped working. What actually happens is that the watch knows that the pressure change is too fast to be weather related and therefore ignores it.

    Hope that helps.

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  5. #4
    ntp
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    Re: pressure and altitude dependencies

    So after further thoughts on the subject:
    The lowest atmospheric pressure at sea level was about 870 hPa or roughly 0.87 atmospheres / bars during a typhoon. That means a pressure variation of about 0.13 bars, the same variation given by a 1.3 km descend. So let's say I descend 1.3 km in about 3 hours and a half or more (sorry if my approximations are way off, I don't live in a mountain area). If the pressure graph were to remain constant, that sort of a "weather change" (a big bad typhoon) would have to take place in 3 and a half hours. Not likely, especially in mountain areas.

    Anyway thanks for the reply englishperson; if the watch acts that way it's truly awesome, really. So in everyday worsening weather conditions and during a descend, the watch would actually show a negative pressure tendency graph, right? So I could actually take notice that the weather gets worse. Cool.

    Can't wait to buy my PAW1400T to try out all these things.
    Last edited by ntp; September 24th, 2007 at 01:34.

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