Barometric pressure
-
Hi, I’ve set my altitude manually to 120m above sea level and Suunto Race shows 1019 hPa. The weather forecasts show 1023 hPa and temperature of 23°C. I tried to convert the pressure from absolute to relative using calculators and it doesn’t match. What am I missing or misunderstanding here?
-
@tdominiak first thoughts are
1 that’s a very small error of 5/1000
And
2 weather forecasts will have some wiggle room as there will be slight local variation -
@Desdinova I get it but doesn’t Suunto show absolute pressure? Forecasts show relative one so 5hPa is a huge difference. It translates to over 1033hPa in relative pressure hence my question. And no, forecasts show 1024hPa in the whole region.
-
@tdominiak
Isn’t suunto displaying pressure at sea level? -
-
@tdominiak Suunto shows pressure at sea level not absolute. From my years of tracking barometer readings, it is really best to watch barometer trends on the graph vs single at a glance data point comparison against other weather sources. Learn how barometer trend would be helpful when you dont have access to the internet, and what other weather signs to additional look at. The barometer function is meant to help make decisions on breaking camp for example, and between the trend and sunset make informed choices.
-
@tdominiak good question. To reiterate and add to what others have said, adding some more thoughts. Your situation is likely just fine.
- The watch measures absolute pressure with the onboard sensor but only displays the barometric pressure (QNH). Not sure if the new UI watches have a service menu but back in the Ambits you could enter the service menu and see the absolute measured pressure as well. This is something we briefly touched on before. https://forum.suunto.com/topic/2718/ambit3-peak-ambient-pressure-calibrating-question
- The discrepancy may not be as large as it seems, if 1019 = 1019.4 and 1023 = 1022.5, then that’s a 3.1 hPa difference. So it’s tough to compare the values as-is. The watch resolution is only 1 hPa.
- Check the weather map (Windy etc) for isobars between your weather station and your location, you may already have some of that difference accounted in there.
- The forecast data may be outdated (if you could check the timestamp), or your elevation may not be 120 mASL. I assume you’ve already checked this, so I’m mentioning this last.
If you appreciate weather forecasting in general, you might find this interesting for some peace of mind:
I recommend going to a local airport and checking the METAR (http://metarreader.ca/) for the QNH value and the runway elevation closest to you. You can quickly verify your measurements with more accurate to compare against. The watch is not a certified pressure gauge and so expect that there will be some minor differences. I’m not expecting to land an aircraft in foggy ILS conditions with the GPS watch altimeter.
I agree with Mike that the important tool here is the trend, but I’d also love the watch to be accurate in all realms.
-
@Umer-Javed this is the kind of post that keeps me coming back to the forum. Excellent. On iOS there’s a really good METAR app called Station WX.
-
@Umer-Javed, maybe air temperature must also be taken into account, my SV show +4 hPa against airport METAR 10 km from me now when is +24°C, and when was cold weather 0 or below 0 my SV was on spot as METAR data, strange thing is that my wife´s Srace are spot on now but in cold weather were around -3 hPa against METAR data
-
@Umer-Javed just to clarify, during LVO (Low Visibility Operations) approach and landing we don’t use GPS alt, rather radio altimeter for the DH/DA
For a standard ILS minimas are based on barometric reference instead, ciao
