In this forum, the keisan sea level pressure calculator was used to calculate the elevation offset required to calibrate the barometer for Fine Offset manufactured weather stations (Ambient Weather, Ecowitt and clones).
The advice was to leave the default temperature set at 15°C at your location and calculate the difference between the pressure at your elevation (Absolute pressure) and the pressure at sea level (Relative pressure). The difference between the two pressures was supposed to be your elevation offset.
But one day, I was looking at the cartoon graphic of the mountaineer (see below) accompanying the calculator and saw that temperature =T at present location.
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A question came to mind. Why is the default temperature in the calculator said to be “fixed” at 15°C (at present location) up on the mountain?
In a standard atmosphere, the default temperature is supposed to be 15°C at sea level elevation and temperature declines at the standard lapse rate of 0.0065°C/meter.
Although it looks like the 15°C is a fixed temperature in the keisan calculator - it is not. The temperature can be changed in the calculator (it is a variable) so I suspect the keisan author used 15C just as an example or placeholder along with a sample altitude and a sample atmospheric pressure—not meaning for temperature to be fixed permanently at 15°C for all elevations.
Rather than assuming that local outside temperatures are fixed at 15C year-round, I would recommend using the current temperature at your elevation as the developer intended, For better results, you may want to use your 12 hour mean temperature at your location to calculate individual SLP values.
Fellow wxforum.net member, Dr. David Burch, author of the “Barometer Handbook” says the 12 hour temperature is very important for accuracy and recommends using the 12 hour average temperature rather than the current temperature for SLP calculations. I prefer using his starpath SLP calculator because it explicitly requires 12 hour mean temperatures as an input variable. Nice to see good notes and instructions to accompany his calculator.
Unfortunately, I suspect that the vast majority of keisan calculator users will leave the default 15°C as is - which unless you live at sea level, will produce an incorrect elevation offset. If you live at a relatively low elevation less than a few hundred meters, you may not notice (at least initially) that your calculations are out. Add in the effects of temperature and at higher elevations these errors become increasingly intolerable. But as they say, one should always use the proper tool for the job and that tool is a standard atmosphere calculator.
Do use the keisan calculator for what it was originally designed for – to do a manual SLP calculation based on the current temperature at your location. It is not meant to be an elevation offset calculator.
To calculate the fixed elevation offset for your specific location/elevation, use a standard atmosphere calculator. I like to use this one: digital dutch standard atmosphere calculator.
https://www.digitaldutch.com/atmoscalc/table.htmNeedless to say, weather software like WeeWX does a far better job at calculating SLP in real-time using the proper algorithms and 12 hour mean temperatures and humidity measured by your weather station. And it does all this automatically.
No manual calculations/re-calibrations required!
Related articles:
Tips and hints to calibrate your barometer:
https://www.wxforum.net/index.php?topic=44099.msg448794#msg448794Calibrate Relative pressure using a raspberry pi computer
https://www.wxforum.net/index.php?topic=41500.msg425120#msg425120