Cumulus Support

This is mainly geared towards USA and other users of the Imperial system of inches and feet and such.

I was having a heck of a time getting my barometer set on my Weatherwise WS-2080PC using inhg. I am between two small municipal airports with AWOS equipment reporting to NOAA. On a calm day where both would be at say 30.25", I would set mine at home to the same reading and after a few hours or a couple of days, mine would be higher or lower. Very frustrating.

So I came up with this system for setting the barometer. Instead of using inhg, use hpa (a.k.a. millibars) as the setting will be more precise...

29.95 inhg = 1014.2 hpa
29.96 inhg = 1014.6 hpa
29.97 inhg = 1014.9 hpa

As you can see, being able to enter hpa values of 1014.3, 1014.4, 1014.5, 1014.7, 1014.8, etc. allow for a more precise setting than inhg can offer.

You will need a nice, calm day and a reliable source that reports weather data to NOAA. The easiest way to find this would be to go to http://wunderground.com/ and enter your location. When the weather data loads, click on STATION SELECT and you will see airports in your area. Click on the closest one to your location. You will see the 4 letters identifying the airport. After selecting the airport you want to use, at the top right corner of the Weather Underground webpage is a small blue square. Click on that to change the data shown from °F to °C which will also change the barometric pressure readings from inhg to hpa.

For example, let's say you live in Macon, Missouri. Go to the weather page for that location on Weather Underground. Once it loads, click the STATION SELECT button. You will see there are 4 airports nearby. The closest two are KIRK and KVER. If the barometric pressure between both disagree, go with the closest airport where you will have your weather station installed. The elevation difference between the airports and the town are no big deal at all. For the following, we'll use KIRK as the main source. KVER can be used later to verify settings with.

Now we are ready to get started.

Close Cumulus. Also make sure the time and date on your display are synchronized to WWVB. If not, synchronize them manually.

On your display, hit MENU repeatedly until you get the barometer flashing. The first barometer option is to display absolute or relative. Make sure it is on RELATIVE. Press ENTER. Now you are on the barometer display option. Using the UP/DOWN keys, select hpa. Now hit the HISTORY button which will exit you from setup.

Now this is the tricky part. A lot of airports may or may not report at the same time. Some do it once an hour and others may do it multiple times an hour. KIRK reports three times an hour at :15, :35, and :55. These are the minutes you need to watch your display.

Pick an airport reporting minute coming soon. Looking at your display, watch the barometric pressure on it like an eagle during the entire minute you picked that corresponds to the airport reporting time minute. If it changes during that minute, wait for the next airport reporting minute. As long as the day is calm, you should not see it deviate. Let's say it shows 1020.2 hpa the entire time. The airport shows 1022.7 hpa for the corresponding minute after it reports to NOAA which may take a few minutes to be displayed on Weather Underground. The airport is at 1022.7 and you are at 1020.2 so you need to add 2.5 to be at the same reading.

Now open EasyWeather. After it reads the display's memory, click on SYSTEM > SETUP. At the bottom, you will see relative pressure of 1020.2 and whatever the absolute pressure is grayed out (we'll say the absolute pressure is 1001.2). Change the relative pressure from 1020.2 to 1022.7.

If by chance your hpa value of 1020.2 changed, add or subtract the difference to the airport's reading. So if you are showing 1020.5, you need to add 0.3 to the airport reading. You would enter 1023.0 instead of 1022.7.

Now do this...

RELATIVE - ABSOLUTE = DIFFERENCE

In this example...

1022.7 - 1001.2 = 21.5

WRITE THIS DIFFERENCE DOWN AND KEEP IT IN A SAFE PLACE! When you replace the batteries in the display, the barometer's relative difference from absolute will reset and you will have to correct the relative setting again in EasyWeather using hpa. It is so much easier to correct it when you already know what the difference is between relative and absolute pressure. When it is time to replace the batteries, make sure the USB cable is plugged in before taking the batteries out to avoid all of this.

Click SAVE.

You can now close EasyWeather. Watch for the next reading at the next airport reporting minute. Look at your display and make sure it stays steady throughout the entire minute then see what the airport reports in. If it is the same, you are done correcting.

When you are done, change your display back to inhg if you want to. Don't do this in EasyWeather as the change will affect the relative pressure you corrected to. On your display, hit MENU repeatedly until you get the barometer flashing. The first barometer option is to display absolute or relative. Make sure it remains on RELATIVE. Press ENTER. Now you are on the barometer display option. Using the UP/DOWN keys, select inhg. Now hit the HISTORY button which will exit you from setup. You can now open Cumulus.

It is normal for your barometer reading to maybe be a few hundredths of an inch off from the airport you used to set it up with during barometric changes, but it will balance out during times of calm barometric pressure. If you feel you must adjust again, change your display to hpa, close Cumulus, and open EasyWeather. Add or subtract 0.1 from the relative reading. When done, click SAVE, close EasyWeather, change back to inhg on the display if you like, and open Cumulus. Be sure to update the difference number you wrote down earlier.

Sometimes I think our barometers on our Fine Offset stations may be more accurate than the airports' AWOS barometers. It seems to me the USA should go to the world standard of hpa/millibars as the readings are more precise than inhg.

So what do you do when your source is at a much higher or lower elevation than where your station is installed at? Easy. You go to the source! A barometer is also an altimeter so setting it at the same elevation as the source will make it adjust itself to a lower reading if you are at a higher altitude or a higher reading if at a lower altitude. To get the correct relative pressure, add 0.1 inHg (3 hPa) for every 100 feet above the source the barometer is or vice versa if below the source.

If you have a laptop with EasyWeather installed and know how to tether your cell phone to it so you have Internet access, grab your weather station display and USB cord and head to the source like an airport that reports its AWOS to NOAA. Follow the steps in the first post. Park nearby and watch airplanes take off and land to kill time.

You might also take a second or third barometer with you such as an aneroid (and a small flat head screw driver to adjust it with if needed) and/or a digital one so you have a backup reading. You can get really nice aneroid and little digital barometers on eBay cheaply.

Also keep in mind temperature, distance from the source, and current weather conditions will affect the barometer's reading once you get it back to its home location. After you add or subtract 0.1 inHg per 100 feet to make up for the difference in elevation from the source you used and the readings don't agree, these are the likely causes.

For UK and other places where density of reporting airports is low, select a website that interpolates official data to produce a forecast at your location (e.g. http://xcweather.co.uk/forecast/[your location]) and you will see the forecast weather for various times including the pressure in units that you choose.

ALTERNATIVELY: Given that the meterorological standard is to report pressure corrected to sea level, the simple way to set a Fine Offset station is to select a web site that shows isobars:
e.g. for UK http://www.meteorologica.info/UKisobars.htm
When the time of your observation matches the isobar update time, you can see for yourself if your setting needs changing. Obviously this works best when either the pressure variation is very small over a wide area (e.g. your site is in middle of a 'high') or the stations used to derive the isobars are such that the accuracy of drawing closely spaced lines is good for your location.

For frequent checking:
That particular updating image can be put on your website near to your current reading (if you normally report in inches of mercury, then javascript can convert value to millibars: where var bardata = "<#press>" i.e. latest pressure so that you can display in both units).

If you are skilled enough, superimpose your location onto the image using more script after the map has been loaded: Now you can see your station's position within the isobars and check your calibration when the map updates at a similar time to your reading.

The most 'Precise Way To Set The Barometer' is to use a 'precise barometer', one that can be set to 'null' or zeroed; in other words, one that can tell the QFE at the point of reference. I use a Greisinger GTD 1100 digital barometer. OK, it's not cheap, but it's accurate. Find your site QFE and adjust it for elevation : my site is 151' at ground level on both Google Earth and on UK OS but the Fine Offset sensor is at 163' (upstairs) amsl which divided by 27.76 (feet/milibar, 30'/mb is just a 'rule of thumb', by the way) requires a correction, added, of 5.9mb (rounded) to give QNH, eg QFE = 1000mb, QNH = 1005.9. BTW, I have no commercial interest in the barometer mentioned, it's just the one that I chose, any accurate barometer that can be similarly adjusted will do.