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Air pressure at altitude
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My general sense is that air pressure should be reduced significantly at altitude; so that if one's 6000 ft above sea level, the pressure should be about 80% of what it is at sea level. Sea level pressure is generally about 30 in Hg, yet when i look at weather reports, the pressure for places at altitude is still in the 27-28 range. What am I missing here?
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Re: Air pressure at altitude [echappist] [ In reply to ]
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deleted because i cant read so good...
Last edited by: tfleeger: May 24, 17 11:33
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Re: Air pressure at altitude [echappist] [ In reply to ]
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It's not linear with altitude
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Re: Air pressure at altitude [tfleeger] [ In reply to ]
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tfleeger wrote:
80%...That just sounds silly. The PSI difference at even 10,000ft is only approx 5 psi less than sea level. If anything, I would think you would want to increase your tire pressure a few PSI, assuming you pump up your tires at altitude, although personally I wouldn't sweat a couple PSI either way.

He wasn't talking about tires.

Considering that the atmo pressure at sea level is 15 psi, reduction by 5 psi means 66%. What sound silly? That it's 80% at 6000 feet? Because it IS right around 80% at 5000 feet:

Edit changed this to a link it was messing up the formatting:

https://upload.wikimedia.org/...ure_vs._Altitude.png


Now, why weather forecasts do not reflect that I don't know either. Right now in Boulder CO (at 5600 feet) the forecast says 29.76in or 1002.1mb which is very nearly sea level pressures! The pilots correct for altitude and temperature (google density altitude). Maybe weather people are doing something similar? My bro is in NWS. I can ask him.
Last edited by: Dilbert: May 24, 17 11:29
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Re: Air pressure at altitude [Dilbert] [ In reply to ]
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um.. brain fart.. yup you're right.. i had tires on my mind from something else....
Last edited by: tfleeger: May 24, 17 11:35
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Re: Air pressure at altitude [Dilbert] [ In reply to ]
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Dilbert wrote:
tfleeger wrote:
80%...That just sounds silly. The PSI difference at even 10,000ft is only approx 5 psi less than sea level. If anything, I would think you would want to increase your tire pressure a few PSI, assuming you pump up your tires at altitude, although personally I wouldn't sweat a couple PSI either way.

He wasn't talking about tires.

Considering that the atmo pressure at sea level is 15 psi, reduction by 5 psi means 66%. What sound silly? That it's 80% at 6000 feet? Because it IS right around 80% at 5000 feet:

Edit changed this to a link it was messing up the formatting:

https://upload.wikimedia.org/...ure_vs._Altitude.png


Now, why weather forecasts do not reflect that I don't know either. Right now in Boulder CO (at 5600 feet) the forecast says 29.76in or 1002.1mb which is very nearly sea level pressures! The pilots correct for altitude and temperature (google density altitude). Maybe weather people are doing something similar?
yep, and that's exactly what got me wondering

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My bro is in NWS. I can ask him.


that'd be greatly appreciated!

i was looking at an event in Colorado (6000 ft elevation) where people were going very fast at relatively lower powers (as is expected). I wanted to get a sense of the CdA of the people at the event but realized that it would be impossible when the air pressure is all wrong. One of the sites I used (wunderground) has the words "at sea level" next to the pressure reading. Seems a bit strange to have to apply an artificial correction (which i'd assume not to be constant) whereas they could just report the actual pressure values.
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Re: Air pressure at altitude [Dilbert] [ In reply to ]
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Dilbert wrote:


Now, why weather forecasts do not reflect that I don't know either. Right now in Boulder CO (at 5600 feet) the forecast says 29.76in or 1002.1mb which is very nearly sea level pressures! The pilots correct for altitude and temperature (google density altitude). Maybe weather people are doing something similar?


Yes, weather barometers are corrected for altitude. The reason is that the value of barometric pressure as it relates to weather is not the absolute number, but rather to know if it's above or below "normal," as well as which way it's trending. Weather barometers are therefore calibrated with an altitude adjustment so that they all read the same at "normal" pressure as they would at sea level (29.921 inHg). That way, any meteorologist can see the pressure reading from any station and instantly know whether it's above or below normal without having to know the elevation of the reporting station, then calculating what normal should be for that elevation.

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Last edited by: gary p: May 24, 17 14:24
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Re: Air pressure at altitude [echappist] [ In reply to ]
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echappist wrote:
My general sense is that air pressure should be reduced significantly at altitude; so that if one's 6000 ft above sea level, the pressure should be about 80% of what it is at sea level. Sea level pressure is generally about 30 in Hg, yet when i look at weather reports, the pressure for places at altitude is still in the 27-28 range. What am I missing here?
Air pressure is normally reported by weather bureaus as values normalised to sea level. Those isobar charts you see on TV etc are all normalised to sea level.

If you want to know the actual air pressure, you need to either measure it directly or apply an altitude correction to the reported (sea level) value.

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Re: Air pressure at altitude [AlexS] [ In reply to ]
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gary p wrote:
Dilbert wrote:


Now, why weather forecasts do not reflect that I don't know either. Right now in Boulder CO (at 5600 feet) the forecast says 29.76in or 1002.1mb which is very nearly sea level pressures! The pilots correct for altitude and temperature (google density altitude). Maybe weather people are doing something similar?


Yes, weather barometers are corrected for altitude. The reason is that the value of barometric pressure as it relates to weather is not the absolute number, but rather to know if it's above or below "normal," as well as which way it's trending. Weather barometers are therefore calibrated with an altitude adjustment so that they all read the same at "normal" pressure as they would at sea level (29.921 inHg). That way, any meteorologist can see the pressure reading from any station and instantly know whether it's above or below normal without having to know the elevation of the reporting station, then calculating what normal should be for that elevation.

AlexS wrote:
echappist wrote:
My general sense is that air pressure should be reduced significantly at altitude; so that if one's 6000 ft above sea level, the pressure should be about 80% of what it is at sea level. Sea level pressure is generally about 30 in Hg, yet when i look at weather reports, the pressure for places at altitude is still in the 27-28 range. What am I missing here?

Air pressure is normally reported by weather bureaus as values normalised to sea level. Those isobar charts you see on TV etc are all normalised to sea level.

If you want to know the actual air pressure, you need to either measure it directly or apply an altitude correction to the reported (sea level) value.

thanks for the explanations. learn something new every day
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Re: Air pressure at altitude [echappist] [ In reply to ]
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I use this resource that gives half decent numbers going all the way from sea level up to Everest if you want to. Just plug in your altitude and you'll be close

http://www.altitude.org/air_pressure.php
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