Reporting sea level pressure from Lat & Long

[robertrau]

I need to get sea level pressure but I am starting with latitude and longitude location data. So far the only thing I have come up with is:
Using
api.weather.gov/points/{latitude},{longitude}/
and then parsing out the forcast office ID and grid points,
then using
api.weather.gov/gridpoints/{wfo}/{x},{y}/stations
to give me a station, and finally using
api.weather.gov/stations/{station}/observations/latest
to get sea level pressure.

I'm sure there is a better way.

I am grateful for any help.

[dwhitemv25]

You have the right order of operations. The observationStations property of the /points query gives you the /gridpoints/.../stations link so you don't have to build your own gridpoints URL. You can also cache the results of the earlier queries as they change very infrequently.

Be aware that not all stations report SLP or report it at every interval. ASOS stations (usually large airports) are your best bet for consistent SLP observations. Otherwise you may have to convert from station barometric pressure or altimeter setting, depending on what's available.

[robertrau]

Thank you. I see, several stations don't supply SLP. For converting from "altimeter setting" I also need current surface barametric pressure, don't I?
Can you point me to a link for this conversion?
Thank you for the help.

[dwhitemv25]

Reversing altimeter to station pressure is simple, but calculating sea level pressure is complicated. At a minimum you will need the station altitude. Ideally you will also want temperature and humidity (and/or dewpoint; with 2 of the values you can calculate the 3rd with psychometric formulas).

General meteorologic formulas can be found at:

• https://www.weather.gov/epz/wxcalc

A formula for getting station pressure from altimeter is here:

• https://www.weather.gov/media/epz/wxcalc/stationPressure.pdf

Converting station pressure to sea level pressure is more complex, depending on the level of accuracy you want. There are corrections for temperature and humidity (and air density, gravity, and the aspheric nature of the Earth if you want to get crazy), and some of the constants are from precomputed tables. The best sources I can find for this are:

• https://en.wikipedia.org/wiki/Barometric_formula
• https://integritext.net/DrKFS/correctiontosealevel.htm
• Davis technote on derived values in their weather station products

You may just have to choose or develop your own adjustment so your data is internally consistent.