DCS QNH appears to be QFF

[MARLAN_]

In the mission editor / brief it indicates a QNH setting, but this is actually QFF.

QFF being the true / actual altimeter setting as if it were measured at sea level. It could also be referred to as QNH when measured at sea level, but it's important to make the distinction that QNH would normally be measured at a station based on local pressure & temperature and corrected with ISA. QFF is using true values as if you dug through the ground and measured at sea level.

Understandable since DCS is currently setting pressure globally across the map, would be neat if updates to DCS weather allowed local settings, but even currently it is pretty confusing.

For example, real world Nellis AFB is at this time reporting a QNH of 29.95 with a temperature of 38 C.

To have this match correctly in DCS you would need to set the mission value to ~29.779* inHg with a temperature of 42 C (corrected for temperature lapse rate for sea level)

If you want to check it yourself, try the linked equation (*still in WIP, only tested at Nellis, but consistently precise for DCS, just inaccurate by about 0.01-0.02 inHg, I'm guessing I'm missing a step, thinking DCS doesn't actually measure QFF from sea level but something like 15 meters.)

Put together by referencing:

https://en.wikipedia.org/wiki/Atmospheric_pressure

https://en.wikipedia.org/wiki/Pressure_altitude

https://www.metpod.co.uk/calculators/pressure/

I am not a mathematician, meteorologist, or aviator of any sort, just a nerd learning how to program and noticed these issues while trying to create a program that sets real world weather in DCS.

Edited July 25, 2022 by MARLAN_

[Scaley]

Yeah, this has been true forever in DCS but I've never seen anyone from ED discuss it. Two bits of note:

1) The bottom of the DCS atmosphere is 10m, so surface wind is at that altitude (AMSL). I'm guessing that's the offset you are seeing

2) All AI altitudes set in the mission editor are in vertical straight line distance from this plane, regardless of altimeter setting. Some aircraft avionics also use this magical altitude rather than the altitude set by the pilot. 

[Goggles]

How does this discrepancy affect practical gameplay?

[MARLAN_]

16 hours ago, Goggles said:

How does this discrepancy affect practical gameplay?

Field Altitude (MSL) won't match when setting what DCS incorrectly indicates is QNH.

e.g. at Nellis, you could have an altitude that is hundreds of feet inaccurate.

Edited August 22, 2022 by MARLAN_

[Frederf]

It's just an inaccurate label. They should change "NH" to "FF" in the briefing template and be done. It's impossible to report QNH as a single number because there are an infinite number of QNHs depending on where you mean. If they wanted to report a particular number it would have to be calculated based on single player spawn location or something. In multiplayer everyone would need a different calculated number.

[Syd]

It was one of the things I mentioned some time ago in this topic:

 

Unfortunately I didn't get much response.