#### Re: APPM Models

Dale Ghent

On Nov 29, 2020, at 12:26, Lee Dodge <ldodge12@gmail.com> wrote:

I am at a loss to understand the details of the pressure required for the APPM model, and how some folks are getting that from the local weather report. The APPM model asks for "pressure" in mbar. My local weather station reports barometric pressure in inches of mercury (Hg). So, for today, the barometric pressure 30.52 in. Hg., and using a conversion factor of 33.8639 mbar/in. Hg, I would get an absolute pressure of 1033.5 mbar, which sounds reasonable for a nice clear day. However, my observatory is at 8565 ft. above sea level, or 2611 m. Using this barometric pressure at my altitude would be nonsense, as I compute the absolute pressure at my altitude and a temperature of -11 C (today's temp.) to be 725.3 mbar. The difference between the barometric pressure and the actual absolute pressure at my location is that the barometric pressure is always converted to some sea level equivalent, and regardless of how high you go, the barometric pressure is always around 1013 mbar. If the model is looking for barometric pressure, then it must be labeled barometric pressure rather than pressure, and if it is really looking for pressure, then how are folks getting that from the local weather report. Which pressure is it looking for?
I think it's pretty obvious and well-understood that the local air pressure is what goes there. How one gets that number is up to their discretion and judgement. For some, the reports given by a weather service are sufficient as those are collected from both official and private weather stations and from local airport weather equipment can be quite accurate enough. Others use local devices. I have a small Arduino-based project the size of a stick of gum that includes a BMP80 air pressure chip and an ASCOM driver that can read it. Yet others have their own weather stations and enter the data either manually or via ASCOM driver, if their weather station has an ASCOM driver that can talk to it. If you find that you need to take a local reading and modify it for your altitude, and it works for you, then it works for you.

mbar is equivalent to the SI unit for air pressure, hectopascals (hPa). The ASCOM ObservingConditions driver specification says that all its various units are SI units, and the astronomy world in general deals in SI units for all measurements. I'm not sure why there needs to be any consternation here.

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