Ok so what type of heights do weather providers request or specify, relative to what?
Hi Matt,
That's what I've been wondering too. Starting this thread is a first blush at trying to understand this subject. With the now 2-week long shutdown of about 1/4 of the federal government, this may not have been the best time to try digging into what standards various entities, including NOAA, have pertaining to height related metadata on their equipment. My inquiries to NWS were responded to with a muted, "
Due to the government shut-down we are only allowed to answer mission critical questions. Please get back to us once the shut-down has ended.". Writing to the FAA concerning our local AWOS so far has only yielded a null response.
In a response to my writing to a friend of a friend, and who is retired meteorologist, Rick wrote back in part:
"
To get halfway decent barometric pressure measurements, one needs to know the altitude of the pressure sensor. Probably to the nearest 5 ft to 10 ft depending on the quality of the weather station. I generally use a USGS topo map to determine that. Some libraries have topo maps, but suspect many don’t. One can also get a fair idea using a hand held GPS, but the vertical component of GPS isn’t great unless the device is of very high quality therefore one reason I rely on the topo maps. There are other sources besides the topo maps. For weather stations I generally want the pressure to be reduced to mean sea level and I use millibars or hecto pascals for units. Depending how on far the station is going to be from Trenton Airport, you can use the latest pressure reported by the Bar Harbor FAA weather station in either inches of HG or millibars. It is better to use that if it isn’t windy – slacker pressure gradient. It can be found at:
https://w1.weather.gov/data/obhistory/KBHB.html
You sound like a scientist or engineer with your comments/map about the orthometric height change. In all honesty, for these type of simple weather stations, I wouldn’t worry about that level accuracy. It is an interesting point however and probably I should be a bit more rigorous and take into consideration such changes for my network of weather stations.
I think you need to decide which parameters are the most important for you to obtain good measurements from and perhaps what the data is going to be used for. For example, if you want temperature data which can be compared to other stations including those operated by the NOAA or the FAA, then you need to try to follow the exposure standards they use. That isn’t too easy to find in this heavily forested state. For e.g., you want the sensors out in the open and not under a tree or next to a building or pavement. The same applies for wind and precipitation, but if one’s instruments are pretty simple and the data requirements aren’t too rigorous, then sensor exposure isn’t too critical. I know quite a lot of sensor exposure and can share some of the standards if it is needed."
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The
NWS shows the local AWOS at our county airport displaying its elevation as 82 ft. The
airnav listing shows it to be 83.2 ft. Both of these values; i.e., 82 and 83.2, 1) don't identify to which vertical datum these elevations relate; 2) and are spatially higher than the little building housing the AWOS. Grade near the building is about 71.5 ft NAVD88. Given that wind speed and direction are critical to pilots, these heights might therefore pertain more likely to the anemometer than the barometer.
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A couple of days ago, I brought the Ambient Weather WS-2902A display console down to the airport to check its barometric reading against the nearby AWOS located about 3200' to the north of the public air terminal.
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The day and time turned out to be perfect with a large steady mass of high pressure which only rose 0.01" over the course of the 90 minutes of observations.
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