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Barometer Calibration 201 – How to use Windy.com for better calibration: ISOBARS

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galfert:
Barometer Calibration 201 – How to use Windy.com (isobars) for better calibration

Calibrating your station barometric pressure to match your local airport is a given that weather station owners know to do. But what happens when you don’t have a local airport? Or what happens if you are far enough away and between two or more airports? How can you properly calibrate your barometer and know which of the two or more airports to use? Being that these airports can show different pressure values at different times or at other times they seem to show the same pressure values, it can feel daunting to properly calibrate your station! Some will suggest you take your console to the airport and calibrate it there and then take it home. What happens when time goes by and you want to re-calibrate or to determine if your station is still working properly? Here is a better method that doesn’t require driving miles with your console to the airport.

Don't drive to the airport:
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How to use Windy.com to have it display barometric pressure isobars to then use to assist with calibration

Weather pressure systems are classified as Low and High pressure systems. These pressure systems move and change all the time. Wherever there is a Low or High pressure system from its center as you move away the pressure will gradually change. If you were able to look at a bunch of weather stations at the same time you might start to be able to see a trend and draw a picture of these pressure fronts. You often see the TV weatherpersons showing these blue and red lines with these weird looking triangles and half circles to indicate these pressure fronts in action. Well it doesn’t stop there. There is more data to visualize and much more fine detail to be had. A website like Windy.com is able to show these pressure systems at a more significant level using what is called isobars. In meteorology an isobar is a line of connecting locations that all have the same barometric pressure at any given time.

The way to use isobars to assist in calibrating your station is just a matter of choosing the right airport to calibrate with when conditions at your location match the conditions at the airport no matter how far away the airport is. So it is possible to calibrate with a far away airport if you calibrate when you fall in line with that airport, that is to say that the airport and your location both pass along the same isobar line. Differences in elevation do not matter because the comparative pressures are adjusted to Sea Level. It is still good practice to follow barometer calibration 101 recommended best practices – That is to calibrate when conditions are fair and steady and as close to 1013.25 hPa (29.92 inHg) as possible, and when the average temperature is 59 F, and get new METAR reading at the top of the hour. You can still calibrate when it isn’t optimal but you’ll then want to re-calibrate or fine tune when conditions are better.

Okay so now we need to learn how to use Windy.com to show these isobars. Windy.com can at first seem very complicated with many different controls and settings. You can create a free account to save some settings or you can use it without an account.

Follow these steps to get Windy.com to show you; isobars, cities with barometric pressure, and desired units (these steps work for PC browser – steps will be slightly different if using mobile device):

* Go to https://www.windy.com (log in if you like – optional)
* Zoom in to your state (not too zoomed in yet)
* Select More Layers from the top right menu. Do not use More Layers from the bottom menu which gives different options. [ You are not allowed to view attachments ]

* Click the On switch for Pressure to add to Quick Menu list, and also select Pressure after turning on Quick Menu option for pressure. (selecting Pressure is what really does it...enabling Quick Menu On Pressure just makes it an easier choice later as it pins it to the top menu visible choices) [ You are not allowed to view attachments ]

* Then select the Forecasted weather icon from the bottom menu (2nd icon from the left.) You should now see the cities with pressure values. [ You are not allowed to view attachments ]

* To change to desired units of measure click on the bottom bar till you get inHg or hPa. But it turns out that at least for now using inHg has more precision because they don't show any decimals when set to hPa. See follow-up posts in this thread for details. [ You are not allowed to view attachments ]

* Lastly to see the isobars on the map click on the Pressure indicator above the bottom menu bars. [ You are not allowed to view attachments ]

* Now make a mental note of what value indication the isobars have around your area. You may need to pan around or zoom out or zoom in. Follow the lines and then finally zoom in closer to your location. You can also look at the nearby cities and their reported pressures.
* Extrapolate the information you see to mentally draw in gradient isobars. Sure it is better to luck out and have one of the isobars pass right through your location and an airport, but you can approximate these lines. In the following illustration You represents your location as an example, CYZE represents your local airport that I've put on the map (I labeled it - but you can make the airport dot show... read next bullet point to see how.) Lastly the thicker pink lines are isobars that I drew for illustration purposes as an extrapolation. Notice the airport and You locations in this example lie on the same isobar. [ You are not allowed to view attachments ]

* You can get your airport to show up on the map if you do a search for it on Windy's search bar. All it does is paint a pulsing dot at that location on the map and it does give some METAR info on the left side but I prefer to look up METAR data elsewhere. Like https://aviationweather.gov/metar and be sure to select decoded and then enter and search for you airport. Here is Newark International Airport in New Jersey KEWR. [ You are not allowed to view attachments ]

* You can also now click on the map and drag a pin marker around your location to get a barometric pressure reading for that spot. - Thanks Gszlag for the tip!
That is pretty much it. Now you can be better informed of when your pressure should actually match that far away airport. Revisit this a few days at a time if isobars aren’t perfectly aligned. Eventually you’ll be in line with that airport. In the future when your pressure reads different than the airport you calibrated with if you check these isobars you’ll now know why you are different.

* I'm not suggesting you use this method solely to calibrate your station. This is a crude method used just to assist with calibration. You still need to look up the airports METAR data and see what the reported pressure is. The purpose of this method is just to know when your pressure and the airport's pressure should match because you lie in the same isobar.

txweather.org:
Awesome write up and how too. Thanks!

platokidd:

--- Quote from: txweather.org on April 08, 2019, 02:59:13 PM ---Awesome write up and how too. Thanks!

--- End quote ---

X2 !

gszlag:

Many thanks for this as I am still fine tuning. Your tutorial is timely.
Since one should calibrate against a known good source, I am having a problem with CYZE in that it [barometer readings] are off almost 3 hPA according to MADIS. Then again MADIS QC seems too high compared to windy.com.

Other airports up here are few and far between. So currently, I am using windy.com to fiine tune.

Your recommendation to use windy.com was brilliant. It has definitely become my go-to weather website.

There is one trick one can try to fine tune the barometric pressure. Once you click on the three pressure switches you can get the barometric pressure for any location just by clicking on it. A flag pin marker will appear and you can drag it on either side of an isobar until the readings change. In windy.com the isobars at best resolve to 1 hPA. If I am lucky (or patient) sometimes I can see the readings change as I drag the flag pin around. For example if the flagpin reads 1020 hPA and I move it a hair, the reading changes to 1021. Aha! I am either in the high 1020's or at 1021 rather than between say, 1020.5.

All good stuff! Great tutorial!


--- Quote from: galfert on April 08, 2019, 02:58:01 PM ---Barometer Calibration 201 – How to use Windy.com (isobars) for better calibration

Calibrating your station barometric pressure to match your local airport is a given that weather station owners know to do. But what happens when you don’t have a local airport? Or what happens if you are far enough away and between two or more airports? How can you properly calibrate your barometer and know which of the two or more airports to use? Being that these airports can show different pressure values at different times or at other times they seem to show the same pressure values, it can feel daunting to properly calibrate your station! Some will suggest you take your console to the airport and calibrate it there and then take it home. What happens when time goes by and you want to re-calibrate or to determine if your station is still working properly? Here is a better method that doesn’t require driving miles with your console to the airport....


--- End quote ---

galfert:
Gszlag,
Thank you inspiring me to write this tutorial. Great tip about the pressure pin markers that you can drop in. I've updated the tutorial to show this.

I do see what you mean about CYZE seemingly not calibrated. You can try reaching out to the airport and finding out who is responsible and then ask about this.
https://weather.gladstonefamily.net/site/CYZE

Lots of people though say not to put much faith in analysis like these. They say to stop chasing lines that you know are wrong. It could be that CYZE is correct but there is no good corroborating nearby information to provide verification to yeild better analysis by MADIS. So what is the next closest airport? Did you try that? Or maybe in your case a trip to the airport maybe warranted to find the mystery. You'd have to go to two airports to compare.

In some cases there are factors that may also increase the discrepancies between the airport and your location. In your case though I do think something is up with CYZE. For others here are some of those reasons: When you compare Windy.com with your airport's METAR you are seeing some discrepancies. METAR data is usually only updated at close to the top of the hour unless there is special broadcast when fast changes are occurring. So depending on when you look at the METAR it might not be timely. Also Windy.com displays Sea Level Pressure and some METAR only show Altimeter Pressure which are slightly different things. Also Windy.com doesn't seem to be as Live as i would prefer. Then there is the lack in precision you mentioned of just displaying whole hPa numbers instead of showing tenths. Factor in all that and that is probably why you can't get it to match with what the airport is reporting. But that should not be the objective.

All of these reasons is why I mentioned to only use Windy.com as a guide and to assist in when you should be attempting to fine tune calibrate and when not to. If the isobars are going through you and the airport then you are good to go...otherwise (like you and the airport are on different sides of an isobar) wait for a better time.

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