Topic: Upload your Ambient Weather station data to CWOP - ambientcwop.com

[galfert]

My husband and I are working this out now, but I won’t be putting the info in until tomorrow because we’re in the middle of rain right now.

You don't have to wait for rain to make a preliminary adjustment. My instructions are broken down into two major steps. Step 1 has nothing to do with current conditions. Step 1 is only about giving your station your elevation by way of this offset that gets set. This will never change. From then on Step 2 is about fine tuning your calibration. You can do Step 2 today even with rain and then fine tune it further when weather conditions are more ideal.

1. My weather station is 9 ft above ground. So we’ve got a total elevation of 238.75, which comes to a difference off 28.21. This is the difference I need to change in the Absolute pressure, correct?

The elevation of your weather station outdoor sensors does not come into account when calibrating the barometer, because there is no barometer within the outside sensors. Your barometric sensor is actually inside of the display console. It is the total elevation of your display console that you need to take into account. Most importantly notice it will be considerably different if your display console is upstairs on a 2nd floor. You must take this total elevation into account.

When you registered your CWOP station you needed to provide them with your station's elevation. But again ...what they are asking is for the elevation of your barometric sensor! If you need to make a registration correction with CWOP you can email cwop-support <at> noaa.gov. After your station get ingested into MADIS you might want to also check out your station on MesoWest and see if they have the updated elevation...if not then send them an email too with the correction, atmos-mesowest <at> lists.utah.edu

But remember when you email cwop-support you reference your station by its FindU ID FWxxxx and when you email MesoWest you reference your station with Fxxxx for the ID (notice no W in the MADIS ID).

2. You say the METAR for the airport shows in hPa, but I’m only seeing Hg and mb for pressure. Am I using the Hg?

mb or millibar is the same thing as hPa (hectoPascal).

Another way to see this is that 1 millibar is 100 Pascal. In metric when you move the decimal place to the left two times from the base unit you end up with hecto (first deca then hecto....a third time would result in kilo). In Canada kPa are often the units used.

[CindiA]

Galfert, after a call to Ambient I was finally able to calibirate my 2902C. I think I calibrated in the right direction. Can you check it to make sure all is well with the absolute? I’m not going to do the relative until I know the absolute is correct. The help desk at Ambient is the BEST!

[galfert]

I can't see your Absolute pressure.

Just post both your Relative and your Absolute pressures that you see in your display console. It doesn't matter when you take this reading.

[galfert]

The key thing through this process is to understand the following.

Step 1: Sets your elevation. You do this by adjusting the Relative pressure only. You make the Relative pressure be greater than the Absolute by a given amount that corresponds to your elevation.*
Once you have entered in this offset, you never ever directly touch the Relative pressure again. Never. If your Relative pressure is wrong after having set this elevation offset in, then that is fixed in Step 2.

Step 2: Does the actual calibration of the barometer. You do this by only touching and adjusting the Absolute pressure.  But you all the while are looking at how it ends up affecting your Relative pressure. Move the Absolute pressure up and the Relative will go up by the same amount. Move the Absolute pressure down and the Relative will go down by the same amount. Therefore realize that the difference between Relative and Absolute will stay the same....because that was set in Step 1.

* Relative pressure will always be greater than the Absolute pressure for all locations above sea level. For the few locations on Earth that are below sea level then the opposite is true.

[galfert]

One other tip....Relative Pressure in your console is not the same as Altimeter pressure.

Some METAR report both Altimeter and Sea Level pressures. If your METAR only reports Altimeter then you are going to want to be sure that your resulting Altimeter calculation (what you see in CWOP) is what matches the METAR.

Therefore Step 2 gets a bit complicated until you realize that your console is doing one thing...but the CWOP software (AmbientCWOP.com in this case) is doing another thing.

But let's worry about Step 2 in a bit.

The first thing you need to do is confirm that you have completed Step 1. What is the difference between your Relative and your Absolute Pressure (on your display console)?
 

[CindiA]

OK, I’ve got:
990.1 - Absolute Pressure
1018.3 - Relative Pressure

I sincerely appreciate you being patient with me.

[galfert]

Okay all looks good. You got it.

[CindiA]

Glory Hallelujah! Once you said in your first reply to me today that I had to set the Relative Pressure first it all came together and made sense! That just wasn’t clear to me in the first set of instructions you sent, lol. Now I’m off to write out the instructions in “Cindi-speak” so I can do this all over again when we move in a few years...

Again, forever grateful for your instructions, knowledge and patience with me.