# Sea Level Air Pressure



## solidstate42 (Jun 30, 2007)

Can somebody point me to a website where I can find the sea level air pressure for Santa Clara, CA? I haven't a clue where to find it. Thanks.


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## sunwatcher (Feb 1, 2008)

One site here:

http://www.wrh.noaa.gov/cnrfc/getObs.php?sid=SJC&num=48


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## gus (Nov 25, 2007)

http://www.usairnet.com/weather/maps/current/barometric-pressure/


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## xraypilot (Feb 29, 2008)

Here's another:

http://www.weather.com/outlook/travel/businesstraveler/local/USCA1018?from=search_city

(Incidentally, sunwatcher, is that a Culgoora antenna in your icon? Or Learmonth?)


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## solidstate42 (Jun 30, 2007)

What's the importance of having an accurate sea level air pressure reference? Let's say I randomly pick 1020h hpa for the reference pressure even though it may be something else. If the pressure increases or decreases, the watch will graph it letting me know what the trend is. Isn't that what's important? Does it matter that it went to 1020 hpa to 1022 hpa, as opposed to 1018 hpa to 1020 hpa? Either way, I'm still being made aware that air pressure is increasing.


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## Jeff_C (Feb 11, 2006)

This has been my argument from the beginning. The trend is what is really important to real field use. IMHO.


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## sunwatcher (Feb 1, 2008)

xraypilot said:


> (Incidentally, sunwatcher, is that a Culgoora antenna in your icon? Or Learmonth?)


That's the 9 meter dish at Learmonth. Surprised to see that someone is familiar with the solar radio telescopes here! :-!


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## solidstate42 (Jun 30, 2007)

If I input a random sea level air pressure reference number, will that translate into an incorrect Altitude? This whole ABC deal is new to me and I want to take full advantage of it.


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## sunwatcher (Feb 1, 2008)

solidstate42 said:


> If I input a random sea level air pressure reference number, will that translate into an incorrect Altitude? This whole ABC deal is new to me and I want to take full advantage of it.


Yes, you will get an incorrect altitude. For seeing the weather pressure trends, it's not important to set the sea level pressure.

For a correct altitude reading, your instrument must be periodically updated with the current sea level pressure. The sea level pressure changes with changing weather patterns or even slightly with temperature, so it won't remain constant for long.

You can also set a known altitude and it will display any changes in elevation. Again, the known altitude must be adjusted periodically for a correct reading. As an example, along some mountain trails signs are posted with the known elevation which should be used to update your instrument.


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## xraypilot (Feb 29, 2008)

solidstate42 said:


> If I input a random sea level air pressure reference number, will that translate into an incorrect Altitude? This whole ABC deal is new to me and I want to take full advantage of it.


That was my understanding. Since the air pressure is harder to find than the elevation, I usually come at it from the opposite direction: From a topo map I know the elevation of my home/work/nearest airport, and I enter that as the reference and then let the ABC figure out the air pressure. Personally, I like to know the proper elevation and the *trend* in the pressure rather than the accurate pressure. I think some others have mentioned that too.

You can get free topo maps from http://www.topozone.com/ and find your elevation from that.

And if you fly across the country, it's easier to find the elevation of an airport from google, rather than the topo.


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## xraypilot (Feb 29, 2008)

sunwatcher said:


> That's the 9 meter dish at Learmonth. Surprised to see that someone is familiar with the solar radio telescopes here! :-!


I do solar too, but on the short-wavelength end of it. Mostly X-ray and EUV. b-)


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## paduncan (Sep 28, 2007)

solidstate42 said:


> What's the importance of having an accurate sea level air pressure reference? Let's say I randomly pick 1020h hpa for the reference pressure even though it may be something else. If the pressure increases or decreases, the watch will graph it letting me know what the trend is. Isn't that what's important? Does it matter that it went to 1020 hpa to 1022 hpa, as opposed to 1018 hpa to 1020 hpa? Either way, I'm still being made aware that air pressure is increasing.


If you don't choose an accurate sea level reference, your altititude will be off. Altitude and sea level reference are "hard coded" together on the Core.

Similarly, if you choose the wrong altitude, sea level reference will be off.


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