Barometric Pressure

[bertilak]

I posted this as a suggestion earlier, but now think it is bug.

Barometric pressure is generally quoted as relative barometric pressure but YoWindow displays absolute barometric pressure.

It would be nice if YoWindow displayed the more standard relative number. There are two ways this might be accomplished:

1. Wherever or however YoWindow gets its info may actually provide both absolute and relative, giving you a choice of which to display.
2. You could allow the user to add in a correction value. This is what I have to do for the weather station I have at my house. Its sensor can only be aware of the absolute pressure so needs to be told what the "correct" (relative) pressure is. It then remembers the difference between the two (absolute and relative) and automatically compensates.

The reason I think it is a bug is that when I click through the "www" on YoWindow's location properties menu and follow through to the airport being used, the web page shows the relative barometric pressure at the airport. I believe it is how barometric pressure is generally quoted. YoWindow appears to be quoting the absolute, uncorrected for altitude, pressure.

I believe the first option in the quote above is the way to go.

[countryroads]

Hi Robert,

YoWindow is showing the barometric pressure that has been corrected for altitude.
Some of the others are showing the barometric pressure at sea level.

I hope this explains why YoWindow is showing a different number than the others.

Marty

[bertilak]

Hi Robert,

YoWindow is showing the barometric pressure that has been corrected for altitude.
Some of the others are showing the barometric pressure at sea level.

I hope this explains why YoWindow is showing a different number than the others.

Marty

As I write this reply ...
The Airport (http://en.allmetsat.com/metar-taf/south ... ?icao=KGTU) shows barometric pressure as 30.24 and YoWindow shows 29.09, using the same KGTU airport.

Please see http://www.ambientweather.com/sebaprco.html

Because the air pressure decreases as you rise in altitude, the sea-level corrected pressure (the pressure your location would be at if located at sea-level) is generally higher than your measured pressure. Thus, your pressure may read 28.62 inHg (969 mb) at an altitude of 1000 feet (305 m), but the corrected pressure is 30.00 inHg (1016 mb). The standard sea-level pressure is 29.92 in Hg (1013 mb)

(My underlines)

Since the KGTU airport is above sea level its, uncorrected (observed or absolute) barometric pressure will be lower than the relative, corrected (adjusted for altitude) pressure. YoWindow is showing the lower number (presumably the absolute pressure), not the higher (presumably the corrected) number.

YoWindow should show the corrected number as reported by the airport. At least that's the way it looks to me!

Some calculations as a sanity check:

Using the numbers in the above quote as examples we see that the (example) barometer's reading needed to be corrected upward by 1.38 inches (30.00-28,62). That's for 1000 feet, Since KGTU is at 780 feet its correction should be about 1.38 x (780/1000) = 1.08 inches. The difference between the numbers I get directly from the airport (30.24) and from YoWindow (29.09) is 1.15 inches. Now 1.15 and 1.08 are not the same but they are close. I don't know if the two readings were taken at exactly the same time so there might have been some drift. Also, the true altitude correction is not linear and depends on temperature. So things seem to be in the right ballpark if we assume YoWindow is showing the uncorrected number.