 # Sea Level Pressure Calculation

On the derived metrics web page (https://weatherflow.github.io/SmartWeather/api/derived-metric-formulas.html#sea-level-pressure), the adjustment from station pressure to sea level pressure is given as a function of both ground elevation in meters and station height above ground in meters.

In the API, there is a field for elevation under station_meta:

``````<station_meta>
<share_with_wf>true</share_with_wf>
<share_with_wu>false</share_with_wu>
<elevation>21.75008773803711</elevation>
``````

Does anyone know if this is a sum of the ground elevation and the station elevation above the ground, or just the ground elevation? If it is just the ground elevation, is any assumption made about the elevation of the station above the ground when calculating sea level pressure?

The reason I ask is that I am getting a 0.2 mb offset between the sea level pressure I am calculating, and that displayed on the pressure card for the same station. Mine is lower. I am pretty darn sure I’ve got the equation and constants correct, so I can only think the difference comes down to the height value. The equation I am using in Python is:

``````Psta * (1 + ((P0/Psta)**((Rd*GammaS)/g)) * ((GammaS*Elev)/T0))**(g/(Rd*GammaS))
``````

I am assuming the elevation value in the API refers to height above ground, and I am assuming the height of the station above the ground is zero.

Hope this makes sense!

The sea level pressure shown on the card takes into account the station elevation and the height above ground level that is specified in stations/devices/AIR configuration. 0.2mb difference is about 2m elevation difference.

Hope this helps.

Dan.

1 Like

Perfect! It took me a while to realise that ‘agl’ in the API must mean ‘above ground level’. Thanks for the help.