Another webtag idea

[adam5]

Hi Steve,

Have another webtag idea for ya, if you're interested. A webtag that will display altimeter setting as opposed to sea level pressure...the value that gets reported to CWOP.

-Brian

[steve]

Yes, that's easy to do. I didn't really think about doing it as I thought it was only CWOP that was weird and wanted a completetely non-standard value

[callum91]

Just out of interest (or clarification for us non-Americans) is this altimeter value the same as QNH or is it QFE?

[steve]

Just out of interest (or clarification for us non-Americans) is this altimeter value the same as QNH or is it QFE?

It's QNH - station pressure adjusted to sea level using just altitude, which for users of non-Davis stations is what you get for sea-level pressure anyway (they use a fixed offset), which is why I've changed it back again to be the same as sea-level pressure for those stations.

[adam5]

Hi Steve,

Saw the updated version with the altimeter setting webtag. Thanks!

Actually, at least here in the U.S., altimeter setting is what gets reported as the "barometer reading" more often than not on local news weather reports...The Weather Channel...etc. I now have them both displayed on my webpage thanks to your new tag.

CWOP prefers altimeter setting simply as a means of standardizing their own sea level pressure calculations. Why? Sea level pressure calculations are less standardized, require temperature and humidity data for the last 12 hours (as you know), and is a value that many home weather stations simply do not compute (many report altimeter setting as the barometer reading). On the other hand, altimeter setting is a simple calculation and it's easy to work backward to compute raw station pressure given a sites station elevation.

If you ever wander over to the MADIS/mesowest data site where all the CWOP data ends up, you would see that they (mesowest) take the altimeter setting data from each site, back compute station pressure, then compute sea level pressure based on the temp/rh data and their own standardized SLP calculation. Check out the link below for an example. That's why CWOP prefers altimeter.

http://mesowest.utah.edu/cgi-bin/droman ... type=LOCAL


Thanks again!!

-Brian

[steve]

Why? Sea level pressure calculations are less standardized, require temperature and humidity data for the last 12 hours (as you know), and is a value that many home weather stations simply do not compute (many report altimeter setting as the barometer reading).

Yes, of course; I should have realised that, given that I've just changed the Cumulus code again for non-Davis stations as they are already supplying altimeter pressure.