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contributed by gszlag

updated 15 March 2024

“…the atmospheric pressure measured with your own barometer remains the most important indication of weather changes at your location. To evaluate present weather or to forecast coming weather, we need accurate barometric pressure…” Dr. David Burch, author of The Barometer Handbook.

If we need accurate pressure, how do we obtain it? The following series of tutorials and guides are aimed at either a brand-new owner of a weather station or an existing owner that hasn’t properly calibrated their barometer because it is too complicated, the manual is unclear, barometers involve quantum mechanics to figure out, don’t have much time etc, etc. If so, these guides are for you.

Barometric sensors measure the ever-changing weight/pressure of the atmosphere around us. Measuring these changes is one of the most important aspects of meteorology, and for aviation it is safety-critical as it keeps aircraft away from the ground and from each other.

If you were like me, I skipped over the barometer section when I first purchased my weather station. Although I consider myself a reasonably technical person, the instructions in the manual were baffling. After several frustrating attempts to follow the scant barometer instructions, I gave up and went on to set up the rest of the station. I left the weather station uncalibrated for some months until I stumbled across wxforum.net.

However, a helpful wxforum.net forum member got me going and soon after, my barometer was finally set up and operational.

Time to pay it forward!

Things have changed a great deal since I purchased my weather station in 2019. Barometer set up instructions and calibration procedures that should increase accuracy are markedly different from just a few years ago. Hopefully, you will find this wiki as a useful resource for all things barometric.

Note: Although I may be referring to mainly Ecowitt weather stations, these tutorials also apply to the other Fine Offset manufactured brands like Ambient Weather and other re-branded (clone) equipment. Despite the different names, the hardware and barometer firmware is identical.

updated 15 July 2024

“Analysed pressure fields are a fundamental requirement of the science of meteorology. It is imperative that these pressure fields be defined as they form the basis for all subsequent predictions of the state of the atmosphere. Pressure measurements must be as accurate as technology will allow, within realistic financial constraints, and there must be uniformity in the measurement and calibration procedures across national boundaries.” World Meteorological Organization (WMO), Guide to Meteorological Instruments and Methods of Observation

There is no doubt that of all our weather station sensors, the barometer is one of the most important - if not the most important. The history of meteorology and weather forecasting, in particular, revolves around this instrument.

One of its most important uses was for sailing ships more than three hundred years ago. Sailers noted that changes in atmospheric pressure were related to wind and gale. Wind causes waves. Big waves sink ships and loses lives. Therefore, barometers became increasingly important. Even today, all ships still have them.

In modern times, we now have aircraft and they too, have barometers aka “altimeters”. And just like for ships, they can also save lives. Altimeters help maintain safe separation between aircraft and terrain.

Needless to say, in addition to weather forecasting purposes, barometers are safety-critical instruments for air, land and sea. Despite their importance, most people would have no idea (or care to know), how they work or how to use them.

But the barometer is different. Unlike most weather sensors, it has predictive qualities, which is a key element of weather forecasting.

Why are barometers important?

The atmosphere is important. Air is heavy - about 10 -11 metric tonnes per square meter on the Earth's surface. All that weight means that when the atmosphere is moving around, it can have considerable force as anyone who has experienced a hurricane, cyclone or tornado has found out.

Despite the fascinating qualities of the barometer, is predicting the weather the main reason for owning an amateur weather station? Probably not. However, if I see a very low pressure system say, 20 hPa lower than my current pressure and heading my way, this tells me to pay closer attention to the weather.

I think the main pleasure from our hardware forum (wxforum.net) is derived from measuring weather rather than predicting it. Although weather forecasting, in general, might best be left to the weather professionals, barometers play a critically important role in measuring and predicting the weather. No serious weather station could do without the essential barometer.

References and sources:

H.M.S Beagle, Captain Robert Fitzroy and Charles Darwin https://en.wikipedia.org/wiki/Robert_FitzRoy

A Weather Eye https://www.nzgeo.com/stories/a-weather-eye/

Weather Observer's Handbook - Stephen Burt 2nd edition 2024

WMO - Guide to Meteorological Instruments and Methods of Observation https://www.weather.gov/media/epz/mesonet/CWOP-WMO8.pdf

Why buy a barometer? - David Burch https://www.starpath.com/catalog/accessories/why_buy_a_barometer.htm

updated 16 February 2024

It is best to start with the sea. We need a starting point – a benchmark or datum to weigh the atmosphere. The average atmospheric pressure at the average sea level elevation (average because there’s tides!) is equal to 1013.25 mb. The world-wide sea also has an average temperature, and it has been set to be 15C.

So those two numbers are our baseline. Intuitively, we know that the higher we go in the atmosphere, atmospheric pressure becomes less, and the temperature becomes colder. After all, on Mt. Everest, the air is thin and cold. The question is: how thin? And how cold? We also know that the density of air changes with temperature – cold air is heavier (more dense) and warm air is lighter (less dense) – something that hot air balloons take advantage of.

Standard Atmosphere (model of the atmosphere):

1. Temperature: 15 °C @ 0 meters (sea level). The model assumes the temperature is always 15C at sea level elevation. Temperature declines linearly at the rate of 0.0065 °C per meter with altitude.
2. Pressure: 1013.25 mb @ 0 meters (sea level). The model assumes that the pressure at sea level always stays at 1013.25 mb. Pressure declines in a non-linear fashion (it’s on a curve) with altitude.

For what constitutes to be weather occurs in the lowest part of atmosphere called the troposphere. The height of this layer varies greatly, with latitude being much thicker at the equator and quite thin at the poles. Various sources claim that the height of the troposphere averages out to be about 10 km to 13 km high.

Note: There are other parameters in the Standard Atmosphere model but for our weather stations, pressure and temperature (and their relationships) are the most important ones.

updated 28 March 2024

If you have acquired an Ambient Weather/Ecowitt or clone weather station — new or used, you will have to set up your barometer before first use. Unlike a temperature sensor that just “works” as is, your barometric sensor requires additional steps to set up properly. The barometer, arguably, is the most important weather sensor of all.

The following guide covers everything you need to know to get started. Don't worry, many of these steps need to be done once only. If you run into a snag, post your calibration question on wxforum.net and a forum member will be sure to help you out.

Ecowitt defines elevation in terms of pressure. Instead of entering an elevation, you will need to enter a pressure in order to set up the elevation for your barometer. For your barometer to work properly, you have to tell your barometer how high it is above sea level. You do this by looking up the difference in pressure between your elevation and sea level. This pressure difference is called the Relative offset (REL offset).

The next step is to check if your barometer is accurate by adjusting your Absolute value against a calibrated reference – usually an official weather station at a close-by airport. We will be calibrating to the Altimeter reading at the airport.

This tutorial is aimed at brand-new station owners setting up their barometers for the first time or experienced owners that just need to do a quick calibration. Experienced owners can optionally skip some of the steps and refer directly to the Quickstart Method.

The process of calibrating your weather station barometer accomplishes two things:

1. It sets the elevation of your barometer.
2. It calibrates the barometer for accuracy.

The following tutorial can be used to set up and calibrate any Ambient/Ecowitt/clone display console or gateway device that contains a barometric sensor.

IMPORTANT: Do not assume that your barometer is calibrated at the factory. Ecowitt states: “This kind of correction [ABS correction] is entirely normal as during manufacturing small shifts in the pressure sensor readings can be introduced.”

Essential Terms & Definitions:

Absolute pressure(ABS) is the live pressure reading from your barometric sensor. Also known as station pressure.

ABS offset is used to adjust the absolute pressure reading (ABS) up or down for calibration purposes.

Relative pressure (REL) represents what the pressure would be at sea level elevation if our weather station’s barometer was located down there. REL can refer to Altimeter or SLP. The use of the term “relative pressure” means a pressure relative to sea level.

REL offset is the difference in pressure between your location's pressure and the pressure at sea level. You will need to calculate the REL offset and add this number to your ABS value (Absolute value). The REL offset can be calculated manually, or you can use an online calculator (see Step 10b), to calculate it out for you.

IMPORTANT: Before you start calibrating and comparing your pressure readings with an airport for reference purposes, it is critically important to do so only when your weather station is in the same pressure system as the airport!

Procedure:

a) To set your elevation, use a pressure difference calculator to give you the REL offset directly.  For instructions on how to use the calculator, see Step 10(b).

b) Calibrate the barometer by adjusting your ABS offset [enter a positive or negative number] until the REL value on your screen or display is equal to the Altimeter reading at the airport.

Steps:

1. There should be an official weather station at the airport sufficiently nearby to act as a reliable and accurate reference barometer.  To get the most precise results, change your settings to display pressure in hPa units. After you have calibrated your barometer, you may change the display back to your preferred units.

2. Determine the elevation of your barometric sensor above sea level. Depending on your Ecowitt weather station model, this sensor can be located in your display console, Wi-Fi gateway (GWxxxx) or in the temperature, humidity, and pressure device (WH32B).

Frequently, this question comes up. Can I use the GPS feature on my phone to determine my elevation? The accuracy of GPS can be highly variable depending on the quality of the GPS sensor chip used. GPS is accurate for lat/long measurements, but is generally not recommended for elevation measurements. This may change as satellite and GPS chip performance improves. Until then, it is best to use Google Earth or similar tools to determine the ground elevation above sea level of your location.

IMPORTANT: You want to determine the elevation above sea level of your barometric sensor, not the outside weather station array.

3. Once you have determined your ground elevation (above sea level), you must also add the extra height of your sensor above ground level. For instance, if you are on a ground floor and have the device containing the sensor on a desk, you’ll have to add the additional height (say it was 1 meter) to Google Earth’s ground elevation results.

4. Now that you have calculated your sensor elevation, you will need to determine the correct REL offset for your sensor’s elevation. This offset will be automatically added (by your station's firmware) to the absolute pressure reading in order to calculate your relative pressure.

5. In order to calculate the REL offset for your specific elevation, you need to look up the pressure difference [using an online calculator] for your elevation. See step 10 (b) for instructions.

6. In order to calibrate your barometer, you have to compare your readings with a calibrated reference. Go to https://metar-taf.com to find the ICAO code you will need to generate the latest METAR report. Make sure you choose a close-by METAR station (usually an airport). On the website, you will see a map. Hover your cursor over the closest airport (coloured dot) to your location. You will see the four-letter code for your airport in capital letters.

7. You can use a website like the Aviation Weather Center for METAR reports. For example, to retrieve the latest METAR report for Gore Bay Manitoulin airport.

To change to another airport, change the CYZE in the hyperlink above to the ICAO code for your airport. For instance, here is EDDB (Berlin Brandenburg airport). Just change CYZE to EDDB.

Bookmark your airport for future use!

In the METAR report, you might see one or two pressures; Altimeter and SLP. We are interested in Altimeter. Outside of North America, you might only see QNH, which is a close approximation of Altimeter.

8. After you have entered your REL offset, go to your device and observe the REL value. Compare your REL value with the Altimeter value from the airport. If, for example, the REL value on your display is 1022.9 but the airport Altimeter reading shows 1019.1 on the METAR report, that tells us that our sensor’s REL pressure is indicating too high: 1022.9 – 1019.1 = 3.8 hPa higher than the airport.

9. Because it is 3.8 too high, you will need to lower it by entering an ABS offset of -3.8 or if you have a display console, reduce the ABS value by 3.8.

Basically, we are re-calibrating and checking the accuracy of the barometer by adjusting the ABS offset due to possible shifts in the pressure sensor readings during the manufacturing process.

10(a) Alternative method (the best method!). Calculate your REL offset as in Step 10(b) below. Then use a precision calibrated barometer [make one or buy/rent one] that you can place right next to the console or device containing the barometric sensor. Adjust the ABS value or ABS offset until the ABS pressure reads exactly the same as the precision reference barometer.

10(b) Use the quick calculator. REL offset calculator: The REL offset is the pressure difference between your elevation and sea level elevation (0 meters). You will need to know the elevation of your barometric sensor to use the calculator. Don’t forget to click on the “Pressure Difference” button.

REL offset calculator instructions:

• Click the “Pressure Difference” button.
• Enter your barometer sensor elevation in the Altitude 1 box.
• Enter the sea level elevation of 0 (zero) in the Altitude 2 box.
• Make sure you choose your preferred units for the output or the calculator will not work.
• The pressure difference should be automatically calculated. This is your REL offset.

11. Once you have entered the REL offset or adjusted the REL value, go back up to Step 8 and Step 9 (above) in order to calibrate your ABS value using the airport’s METAR Altimeter value. When you re-adjust the ABS value (up or down), the REL value changes by the same amount.

The REL offset calculator uses the International Standard Atmosphere model, which assumes the average pressure at sea level is 1013.25 hPa or 29.92 inHg.

12. Final step. For the best accuracy, one should check and re-adjust the ABS value next time when Altimeter = 1013.25 hPa or 29.92 inHg at the airport.

13. For examples how to enter in your pressure settings for your elevation, see Calibration - display console or Calibration - Wi-Fi gateway.

Congratulations! You are now calibrated.

updated 16 February 2024

a) Look up your REL offset for your elevation. You need to do this once only. Go to the sensorsone.com website and use their Pressure Difference calculator to obtain your REL offset. Make sure you enter your altitude as Altitude1 and 0 (zero) for Altitude2. Once the calculator has determined the pressure difference (REL offset), enter the REL offset into your display console or Wi-Fi gateway.

b) Adjust the ABS offset or adjust the ABS value (if you have an older style display console) until the REL value matches the current Altimeter reading at the airport.

c) For best accuracy, re-adjust the ABS value next time the Altimeter reading at the airport = 1013.25 hPa or 29.92 inHg.

TIP: You will notice that when you adjust the ABS value, the REL value changes by the same amount.

Congratulations! You are successfully calibrated.

updated 26 May 2024

“In order to make valid comparisons between two (or more) different weather stations that are at different elevations, all pressures must be reduced to sea level.”

You might see various versions of the above statement but the term, “pressure reduction” requires further discussion. Although the World Meteorological Organization (WMO) uses the term “pressure reduction” in its equations to calculate MSLP (mean sea level pressure) values, it is not a good description for what it really is.

Pressure is not reduced at sea level - pressure actually increases as we dive down through the atmosphere and approach sea level. The “reduction”, refers to the theoretical reduction in elevation from your location to sea level . The station pressure (ABS) at your weather station is mathematically adjusted (corrected or reduced) to sea level as if your barometer was located down there.

Alternatively, you could take an actual sea level pressure reading at sea level. Picture lowering your barometer down the side of a cliff with a rope down to sea level and reading the actual pressure at sea level. Or you can travel to the ocean and take a pressure reading there.

And yes, you can do that every time you want to take a sea level pressure reading but that would be inconvenient. Instead of measuring sea level pressure at sea level with a rope or travelling to the ocean’s edge, you can calculate the pressure at sea level directly from the comfort of your own home even though you may be 1000 meters above sea level.

When many weather stations (even though they might be all at different elevations) convert their pressure readings to the same elevation (sea level) using temperature, humidity and other factors, we can start to draw lines of equal mean sea level pressure (MSLP) called isobars.

Canada’s meteorological service describes isobars as “curving lines joining points of equal mean sea level (MSL) pressure.” On a weather forecast, these are the familiar lines of high and low pressure systems on a weather chart.

SUMMARY

For our Ambient/Ecowitt personal weather stations, sea level pressure is calculated versus something that is measured. Our REL value (sea level pressure) is derived/calculated from a measured ABS value.

updated 03 August 2024

The Fine Offset firmware algorithm for the barometer calculations is a very simple algorithm. It is implicitly represented to us as:

ABS + REL offset = REL (Absolute pressure + Relative pressure offset = Relative pressure).

Because the manufacturer of your weather station can’t possibly know everybody’s elevation, the REL offset is arbitrarily set to be zero at the factory. I guess they assume everyone lives at sea level (elevation = 0) unless we tell our barometer otherwise.

At sea level the REL offset = 0 (factory default setting):

ABS + 0 = REL therefore; ABS = REL (default factory setting)

Therefore, out-of-the-box, you will notice that The ABS value (ABS) is exactly the same as the REL value. Unless you live exactly at sea level, the REL offset should not be left at zero! It is left to us to figure out what our REL offset is for our own elevation.

The ABS value starts off as the raw, uncalibrated pressure from our barometric sensor. However, most of the time, the pressure sensor is not perfectly accurate from the factory and needs to be adjusted/calibrated by applying an ABS offset. Once it is calibrated, we can refer to ABS as our station pressure:

ABS (raw) + ABS offset = Station pressure

The Relative value (REL) can refer to Altimeter or SLP readings. I have been recommending setting the REL value to Altimeter so let’s assume that our target is to have the REL value in our display console or gateway = Altimeter reading at the airport.

Therefore:

Station pressure + REL offset = Altimeter or QNH

We know that station pressure is the atmospheric pressure at our sensor elevation, and that Altimeter is the theoretical pressure of our station pressure that has been converted down (mathematically reduced) to sea level elevation. We also know that atmospheric pressure at sea level is higher than our station pressure. So how do we calculate the REL offset?

If we live above sea level, REL offset = is the amount of pressure one must add to our station pressure in order to convert it to the equivalent sea level pressure.

Sounds a bit confusing? Here’s an example that should help us understand these concepts a bit better:

Imagine we are standing on a vertical cliff by the sea. What would happen if we could lower our barometer down to the base of the cliff, which is at sea level (0 meters). We know that it will be a higher pressure at sea level than the top of the cliff but by how much?

Assume the cliff is 300 meters high and that we recorded the pressure on top of the cliff before lowering the barometer down. Suppose this reading was 977.7 hPa. We then lower the barometer 300 meters down to sea level. A friend has been instructed to take a reading of the barometer when it reaches sea level. He calls back and tells us that the barometer pressure at sea level is now 1013.25 hPa. That’s a difference of 35.55 hPa higher than the top of the cliff.

We repeat the experiment the following day except the pressure has changed overnight. At the top of the cliff, the pressure has increased to 987.7 from yesterday’s reading of 977.7. You figure out that if there was a 35.55 difference yesterday, then the sea level pressure should be higher by the same amount the pressure has gone up.

This time you call your friend who is waiting patiently at the base of the cliff. You tell him before the barometer reaches the bottom of the cliff that the new reading is going to be 1023.25. Your friend calls back amazed. He reports that the barometer reads 1023.25! He asks; “How did you know…you can’t see the barometer from way up there?” “Magic?” he asks.

Hardly. Calculating sea level pressure using our very simple algorithm was just a simple addition of 35.55 to our pressure at the top of the cliff in order to estimate the sea level pressure at the base of the cliff 300 meters below. We just realized that the 35.55 difference in pressure from the top of the cliff down to sea level elevation is our REL offset. To calculate sea level pressure, all we have to do is add the REL offset of 35.55 to whatever the current pressure is at our weather station. Now that we know our REL offset, we can continue calibrating our barometer.

However, it's not very practical to lower down your barometer down to sea level to get your REL offset for your elevation or lower the barometer to sea level every time you want to take a reading of the sea level pressure. There must be an easier way.

Actually, someone has already done the math for you and made a model of the atmosphere (ISA) that tells us what the difference in pressure between your elevation and sea level should be. There’s an online calculator that will figure out the REL offset for you. To use the calculator, all you need to know is your elevation.

Although the REL calculator provides us with our required Relative offset (REL offset) number, what is the calculator actually calculating?

REL offset = (ISA pressure at sea level) minus (ISA pressure at your elevation)

The ISA (International Standard Atmosphere) standard pressure at sea level is held as a constant. It is = 1013.25:

Therefore, the equation becomes:

REL offset = 1013.25(ISA pressure at sea level) – ISA pressure at your elevation

We now need to calculate the ISA pressure at your elevation. As this involves a rather complex equation, we need a calculator to figure this out.

The calculator tells us that the pressure of a 300-meter altitude/elevation should be 977.7 hPa when the sea level pressure is 1013.25 hPa.

EXAMPLE

1. Altitude = 300 meters
2. ISA pressure at sea level (@ 0 meters) = 1013.25
3. ISA pressure @ 300 meters = 977.70

then:

REL offset = 1013.25 (ISA pressure at sea level) – 977.7 (ISA pressure @ 300 meters) = 35.55 hPa

How we enter this REL offset = 35.5 depends on the type of Ecowitt device(s) that you have. If you have an Ecowitt GW series gateway, it is easy. All you have to do is enter the 35.5 directly in the calibration screen as your Relative offset.

If you have a display console, it is not as convenient. To enter these numbers, you have to cursor around using the physical buttons located beneath the display screen and change the existing values (one button press at a time). In a display console, you will only see a ABS value and a REL value. There are no fields to enter any offsets. You will have to change the REL value by the REL offset amount we just calculated.

Note: Newer display consoles or consoles that have had upgraded firmware don't require you to use the push buttons. You can now change pressure settings either using an app or using your web browser to configure settings.

For examples of how to adjust these values, see the “Calibration - display console” or “Calibration - Wi-Fi gateway” articles (see next two articles below.

updated 27 July 2024

Let’s assume that you have a brand new weather station. As you are going through the manual’s setup instructions, you run into a snag. The barometer calibration instructions are very short and somewhat cryptic.

Note: In many cases, there are no barometer calibration instructions in the manual. You might find a few sentences in a support document at the manufacturer's website, but information there can be sparse.

Rather than trying to figure out the manual, let’s see how the calibration process works by using an example:

The calibration/settings screen in the console indicates the current pressure measured by your sensor. This will be inches mercury (inHg) by default.

Initially, you will see two values; ABS and REL (Absolute pressure and Relative pressure).

IMPORTANT: Some older display consoles have newer (upgraded) hardware with new firmware versions (the built-in software that operates your console). The new firmware will allow you to change barometer settings using an app or web browser. No more button pushing required! Adjusting barometer settings will be much easier with the new firmware's addition of absolute and relative offsets. As the procedures are quite a bit different, please skip to the Calibration - Wi-Fi gateways section at the end of this article below.

You will also notice that out-of-the-box, the initial pressure values will be the same (ABS = REL) because your barometer hasn’t been set up yet and the factory default settings assumes your elevation is zero. These settings must be changed to your elevation!

We know that the pressure declines as we go higher into the atmosphere. Think Mt. Everest. The atmosphere is a lot thinner way up there than at sea level.

Assume the barometer elevation is 300 meters. Therefore, we know that at a 300-meter altitude, our atmospheric pressure should always be thinner than the pressure way down at sea level:. For the mathematically inclined : ABS < REL (Absolute value is less than Relative value).

We now have to figure out how much less our pressure will be at 300 meters compared to sea level (0 meters).

There’s an online calculator for that. Press the “Pressure difference” button and put in your preferred units as hPa. Don’t forget to choose your units by the answer box, otherwise the calculator won’t work.

The Pressure difference calculator can be found here: https://www.sensorsone.com/icao-standard-atmosphere-altitude-pressure-calculator/

The calculator will give an answer of about 35.5 (rounded). This number represent the pressure drop between sea level and your barometer’s altitude of 300 meters. Or you can think of it the other way – that pressure will increase by 35.5 from your barometer’s altitude of 300 meters down to sea level (altitude = 0)

But, there’s a question mark. How do we know if our barometer is accurate or not?. The manufacturer tells us that there could be shifts in accuracy due to the manufacturing process, so chances are that your barometer is not accurate out-of-the-box and must be calibrated before first use. Next question. How do we know what the true pressure is?

For that, we need a second barometer as a reference. This barometer has to be calibrated to a high standard. Where are we going to find such a barometer? We can buy one or perhaps make our own. The cheapest (but not the best) option is to use a close-by airport’s barometer as a reference tool to calibrate your barometer.

This is what your calibration/setting screen might look like with the factory default in Imperial units. Let’s use 28.53 inHg as a random example:

At elevation = 0 (default setting)

• ABS = 28.53 inHg
• REL = 28.53 inHg

Note: On our display consoles, the manufacturer expresses elevation in terms of pressure only. There will be no fields to enter an elevation.

For better accuracy, change the inHg units in the console to hPa (hectopascals). Don’t worry, you can always switch back to your preferred units after. Look in the manual for instructions to change units. 28.53 inHg is equivalent to 1000 hpa.

From the calculator we just used, we found out there should be a 35.5 hpa pressure difference between ABS and REL. We also know that ABS should be less than REL (ABS < REL).

Therefore, calculating what the REL should be very simple – we just add 35.5 to our ABS value to get the REL value.

Therefore, 1000 hpa(ABS) + 35.5 hpa (pressure difference)= 1035.5 hpa(REL) but how do you change the REL value on the display from 1000 hpa to 1035.5 hpa?

CAUTION: For the older style display consoles that require you to use push buttons, changing the REL and ABS value can be a bit tricky, as you have to change the REL values in the correct sequence and save their values while maintaining the “spread” of 35.5 between ABS and REL. You have to go to the console calibration screen and change the REL value from 1000 to 1035.5 by pressing the buttons below the display screen, which will change the digits one at a time. Change the REL value first. We will also need to change the ABS value as part of the calibration process.

Note: newer display consoles can be accessed by the Ecowitt app or by your web browser, making configuration much easier as it's the same procedure as configuring a gateway. If your display console has an Absolute offset and a Relative offset, skip to the “Calibration - Wi-Fi gateway” section at the end of this article.

For older display consoles, the barometer firmware works in a rather non-intuitive fashion. You can change the REL value directly, or you can also change the REL value by changing the ABS value.

After changing the REL value from 1000 to 1035.5, our console now shows these ABS and REL values for a 300-meter elevation:

At elevation = 300 meters

• ABS = 1000 hpa
• REL = 1035.5 hpa

The ABS = 1000 hpa is an actual measurement from our barometric sensor. We don’t know if it is an accurate number, so we are going to use the airport’s pressure reading as our calibrated reference.

We need to do the following to see if our ABS reading of 1000 hpa is accurate.

If our barometric sensor is perfectly accurate, the airport should have the same reading as our REL reading on the console. The reading at the airport is called the Altimeter or Altimeter (setting). Outside of North America, where Altimeter setting is not used, you will be comparing to QNH.

Note: Unlike Altimeter (setting) which uses decimal values, QNH values will be in whole integer units.

Suppose the current Altimeter reading at the airport is 1036.5 mb. However, our barometer REL shows 1035.5.

This means that our REL reading is 1.0 hpa too low, and we have to increase our REL by 1.0 hpa to match the airport reading of 1036.5.

Instead of changing the REL value again, we can move the REL up by 1.0 by increasing the ABS value by 1.0. The barometer firmware is designed so that the REL will move with ABS lock in step. If you move the ABS value, REL moves by the same amount.

Let’s increase the ABS value by 1.0 hpa (increasing ABS from 1000 to 1001). The display now shows:

• ABS = 1001
• REL = 1036.5 (REL automatically increases/decreases when ABS goes up or down)

Important: Even though both ABS and REL have changed values, you will notice that the pressure difference of 35.5 stays intact, i.e., REL - ABS = 35.5. After changing the numbers on the console, make sure that the “spread” between REL and ABS stays the same when you save the settings.

SUMMARY

To calibrate a display console only requires a short number of steps.

1. Calculate the pressure difference between sea level elevation and your altitude. 2. Add the pressure difference to your current ABS value to get your REL value. 3. Change ABS up or down until the console REL = Altimeter reading at the airport. 4. Double check your readings!. Next time when Altimeter = 1013.2 at the airport, repeat Step 3 if required.

You are now calibrated!

updated 27 July 2024

EXAMPLE

Calibrate barometer if you have a Wi-Fi gateway

Let’s assume that you have a brand new weather station. As you are going through the manual’s barometer setup instructions, you run into a snag. The barometer calibration instructions are very short and somewhat cryptic.

Rather than trying to figure out the manual, let’s see how the calibration process works by example:

The calibration/settings screen in the gateway indicates the current pressure (ABS value) measured by your sensor. This will be inches mercury (inHg) by default.

Initially, you will see two values; ABS and REL (Absolute pressure and Relative pressure).

You will also notice that the numbers will be the same (ABS = REL) because your barometer hasn’t been set up yet and assumes your barometer’s elevation is zero (sea level). If you don't live exactly at sea level, we have to calibrate it to your specific altitude.

We know that the pressure declines as we go higher into the atmosphere. Think Mt. Everest. The atmosphere is a lot thinner way up there than at sea level.

Assume the barometer altitude is 300 meters. Therefore, we know that at a 300-meter altitude, our atmospheric pressure should be less than the pressure way down at sea level. For the mathematically inclined : ABS < REL.

We now have to figure out how much less our pressure will be at 300 meters compared to sea level.

There’s an online calculator for that. Press the “Pressure difference” button and put in your preferred units as hPa. Don’t forget to choose your units by the answer box, otherwise the calculator won’t work.

The Pressure difference calculator can be found here: https://www.sensorsone.com/icao-standard-atmosphere-altitude-pressure-calculator/

The calculator will give an answer of about 35.5 (rounded). This number represent the pressure drop between sea level and your barometer’s altitude of 300 meters. Or you can think of it the other way – that pressure will increase by 35.5 from your barometer’s altitude of 300 meters down to sea level (altitude = 0)

But, there’s a question mark remaining. How do we know if our barometer is accurate or not?. The manufacturer tells us that there could be shifts in accuracy due to the manufacturing process, so chances are that your barometer must be calibrated before first use. Next question. How do we know what the true pressure actually is?

For that, we need a second barometer as a reference. This barometer has to be calibrated to a high standard. Where are we going to find such a barometer? We can buy one or perhaps make our own. The cheapest option is to use a close-by airport’s barometer as a convenient tool to calibrate your barometer.

This is what your calibration/setting screen might look like with the factory default in Imperial units. Let’s use 28.53 inHg as a random pressure value:

EXAMPLE factory default readings. The factory setting assumes everyone's elevation is 0 (sea level). This must be changed!

• ABS = 28.53 inHg
• REL = 28.53 inHg
• ABS offset = 0
• REL offset = 0

Note: For the gateway devices (and display consoles), the manufacturer expresses elevation in terms of pressure only. There is nowhere to enter an elevation/altitude directly.

For better accuracy, change the inHg units in the gateway to hPa (hectopascals). Don’t worry, you can always switch back to your preferred units after. Look in the manual for instructions to change units.

28.53 inHg is equivalent to 1000 hpa.

We are going to use 300 meters as an example. From the calculator we just used, we found out there should be a 35.5 hpa pressure difference between ABS and REL for our 300-meter elevation. We also know that ABS should be less than REL (ABS < REL).

Therefore, calculating what the REL should be very simple – we just add 35.5 to our current ABS value to get the REL value.

Therefore, 1000 hpa(ABS) + 35.5 hpa (pressure difference)= 1035.5 hpa(REL) but how do you change the REL value on the display from 1000 hpa to 1035.5 hpa.

The gateway firmware works very differently than the display consoles. It is far easier to work with offsets (adjustments)rather than have to change ABS and REL values on a push button display console.

For the gateway, you only have to enter the adjustments (offsets) to increase or decrease ABS and REL values.

Back to our example. To change the REL value from 1000 to 1035.5 go to the calibration/setting screen in the gateway and enter the pressure difference (the one we just got from the calculator) of 35.5 into the REL offset field.

EXAMPLE: Entering elevation by using a REL offset — what the readings should look like for a 300-meter elevation:

• ABS = 1000 hpa (current reading)
• REL = 1035.5 hpa
• ABS offset = 0
• REL offset = pressure difference = 35.5 (this number sets an elevation of 300 m)

Note: To change the ABS value, you enter a ABS offset. To change the REL value, you enter a REL offset.

In the example used here, ABS = 1000 hpa is the live current reading from our barometric sensor. We have no idea if it is an accurate number or not, so we are going to use the airport’s pressure reading as our reference true value.

We need to do the following to see if our ABS reading of 1000 hpa is accurate.

If our barometric sensor is perfectly accurate, the airport should have the same reading as our REL reading on the console. The reading at the airport is called the Altimeter (setting) or Altimeter.

Suppose the current Altimeter reading at the airport is 1036.5 mb. However, our barometer REL shows 1035.5. They do not match.

This means that our REL reading is 1.0 hpa too low, and we have to increase our REL by 1.0 hpa to match the airport reading of 1036.5.

To move the REL up by 1.0, all we have to do is enter 1.0 into the ABS offset field

Let’s increase the ABS value by 1.0 hpa (increasing ABS from 1000 to 1001).

EXAMPLE: Adjusting the barometer for accuracy:

• ABS = 1001 (ABS changes by the ABS offset amount)
• REL = 1036.5 (REL automatically changes value when ABS changes value)
• ABS offset = 1.0
• REL offset = 35.5

IMPORTANT: Notice that two things happened when you entered the ABS of 1.0: The ABS value increased by 1.0 and the REL automatically increased by 1.0. Even though both ABS and REL have changed values, you will also notice that the pressure difference (REL offset) did not increase by 1.0. It stays at the desired 35.5 and stays intact, i.e., REL - ABS = 35.5 (1036.5 minus 1001 = 35.5). We want to keep the 35.5 reading intact because the REL offset number tells the barometer that we are at a 300 meter elevation)

SUMMARY

To calibrate a barometer in an Ecowitt GWxxxx gateway requires a short number of steps.

1. Calculate the pressure difference between sea level elevation and your altitude. 2. Enter the pressure difference as a REL offset. 3. Change the ABS offset values up or down until the console REL = Altimeter reading at the airport. 4. Double-check your readings. The next time, when Altimeter = 1013.2 at the airport, repeat Step 3 if required.

You are now calibrated!

updated 30 July 2024

Although using a local airport's METAR report comes in handy as a free barometer calibration tool — it is not the best way to calibrate your weather station barometer(s).

Just about every weather station manual or book suggests using a nearby airport as a calibration reference in order to set up your barometer.

Even if you live at the airport, comparing your barometer with the airport's barometer is not so easy. The airport uses different air pressure calculations and algorithms than our equipment. Station pressure (QFE) is a calculated value because airports use runway elevation as the reference point for station elevation.

It is important to note that our Fine Offset manufactured weather stations doesn't do and can't do SLP calculations. Our weather stations lack the ability to incorporate the required temperature compensations needed for SLP calculations.

If you live far away from the airport, then this introduces even more complications. Most of the time, you are never really sure if you are in the same pressure system as the airport.

And lastly, experts tell us that calibrating using one airport alone is not sufficient and that a minimum of four or five airports should be used. Ideally, your weather station should be somewhere in the centre of multiple airports, no farther than 10–15 miles away (max 25 km).

All of these things make it difficult - sometimes impossible, to set up your barometer properly by comparing readings with an airport.

What is the best way to set up and calibrate a weather station barometer?

The best way is to adjust your ABS value (station pressure) to be as accurate as you can. All the other pressure values depend on it. Other than sending your pressure sensor to an expensive calibration lab, the best way is to accomplish this goal is to compare your sensor pressure side-by-side with a calibrated reference barometer.

Since most of us don't have pressure chambers or expensive calibration lab equipment, our calibrated reference should be another barometer that has higher specifications than yours. Ideally, the reference barometer should have some evidence of an accuracy certification, i.e. NIST traceable or equivalent.

The calibration process is dead simple. All you have to do is place the reference barometer side-by-side at the same elevation as your barometric sensor. The reference barometer should be displaying absolute pressure (station pressure) or QFE. Just adjust your barometer's absolute value (ABS) to be the same as the reference barometer's value, and you are done!

No need for an airport to calibrate (or try to calibrate) your barometer!

For an example of a low cost, accurate barometer that can be used for calibration purposes, see our review: http://meshka.eu/Ecowitt/dokuwiki/doku.php?id=reviews#starpath_usb_baro_barometer_review

updated 16 February 2024

The advice, just a short time ago, was to calibrate our Fine Offset barometers to the SLP reading at a local airport. Although this advice is valid for very low altitude weather stations that are below 50 meters, most of us live at much higher elevations. If you live greater than 50 meters, the use of SLP is not recommended.

Note: The term, Altimeter (setting) is often shortened down to just “Altimeter”, not to be confused with an “altimeter” – the instrument. Outside of North America, QNH would be used instead of Altimeter.

We could go into a long discussion about how to calculate Altimeter, but the short explanation of Altimeter is that it is a stripped down version of SLP (sea level pressure). Strip away the effects of temperature, humidity, adjustments for high altitude stations, you are left with Altimeter. Like SLP, Altimeter is also a sea level pressure.

When I purchased my first weather station (Ambient WS-2000) back in 2019, I followed the standard barometer calibration advice to use SLP as a reference. Here was my experience using SLP to calibrate:

“Although the SLP calibration approach/method seemed to work initially, I noticed my SLP values starting to drift (compared to the airport’s SLP) especially in colder weather. Winter was the worst. I needed to constantly re-calibrate in order to keep up with the airport SLP readings. I found myself re-calibrating twice a day, only to find myself having to start all over the very next morning. Then again, on some days, the readings would come back tantalizingly close. The next day, no – it is drifting again. Very frustrating. As winter set in, these errors and discrepancies became large enough to make my barometer readings unusable. Something was not working – but what?”

I stumbled across a very old wxforum post from around 2008 which provided me with a clue. The original post mentioned that you can force a Davis VP2 LCD console (not the Vue console) to calculate an Altimeter value by putting the console into a fixed offset mode. Unfortunately, this also disabled the console’s ability to calculate SLP.

Note:Davis weather equipment is a competitor to Ambient and Ecowitt equipment.

As soon as I saw the words “fixed offset”, “disabled SLP” and “Altimeter”, I instantly recognized what the solution had to be. Definitely a eureka moment.

The problem? We had been trying to calibrate to airport SLP readings using a fixed offset. Unfortunately, SLP does not use a fixed offset – it uses variable offsets. Since our consoles and gateways only use a fixed offset, we should be doing the same as our fellow Davis colleagues had been doing for years. The solution? Use Altimeter to calibrate, not SLP!

Now, here’s the thing – just because we have conveniently used an airport’s Altimeter (setting) as a handy tool for calibration purposes, does not mean you have to actually use Altimeter. After all, meteorologists use SLP, not Altimeter, for their surface analysis charts and isobar forecasts.

The lack of a SLP calculation, poses a bit of an issue for weather enthusiasts that need SLP for meteorological purposes. Our weather stations can do Altimeter, but not SLP values. So, how can we get SLP?

You can obtain, log and graph more accurate SLP and Altimeter values with a modest investment in additional hardware and software. Capable 24/7 hardware (raspberry pi microcomputer) currently costs less than $30 CAD or so. To save even more money, you can recycle an old computer and run it 24/7. It should be Linux capable, but I believe a macbook would work as well.

However, nothing beats the efficiency and power savings of a tiny raspberry pi microcomputer.. You can buy a complete ready-to-go raspberry pi kit for not much money. The software is free.

updated 16 February 2024

It all started with a simple question: How do I get the elevation into my Ecowitt weather station? There's no place to put it! The reason why is that Ecowitt and other Fine Offset brands use pressure as a substitute for elevation. If you know the pressure — you know the elevation. Pilots know this relationship very well because they do the same. They adjust their pressure on their altimeters (the instrument) to read altitude.

If you purchased an Ecowitt weather station, your system will either have a display console or have a displayless Wi-Fi gateway, like the Ecowitt GWxxxx series or equivalent. There are differences between the two devices.

For setting your elevation and calibrating your barometer, the display console can use either the ABS/REL fixed offset system or the Ecowitt GW series gateway ABS offset/REL offset system.

If you have an earlier model of display console with the older firmware, you will have to enter an ABS value and a REL value. You will have to change these values manually one digit at a time. There will be a whole lot of button pushing involved. Check to see if there is newer firmware available.

Note: The newer display consoles can be configured using an app or a web browser. These newer consoles use the ABS/REL offset system just like the gateway devices. You will be able to enter the ABS offset and REL offset settings directly. This system is far simpler, quicker to input and more intuitive.

SUMMARY

Your new weather station can’t calculate sea level pressure properly if it does not know your elevation above sea level. For Fine Offset manufactured weather stations, there is no way to directly enter an elevation to do the initial set up. Our barometer firmware can only understand pressure. If you understand pressure, then you know elevation and vice versa. Once you set the correct elevation for your barometer, you will have to check its calibration.

We should expect our barometer sensors will drift every year, especially during their first year of operation. They need to be checked and recalibrated at least once a year (minimum).

updated 04 April 2024

For amateur weathers observers, they may choose to upload their weather station data to CWOP. CWOP is one of many weather services that will accept data from personal weather stations.

Note: this is only one method of many methods to calibrate a barometer. It is not a recommendation. It is not specific to Ecowitt (or equivalent) weather stations but I have included it here as a reference only. I have made some edits for readability purposes.

According to Wikipedia:

“CWOP allows volunteers with computerized weather stations to send automated surface weather observations to the National Weather Service (NWS) by way of the Meteorological Assimilation Data Ingest System (MADIS). This data is then used by the Rapid Refresh (RAP) forecast model to produce short term forecasts (3 to 12 hours into the future) of conditions across the contiguous United States. Observations are also redistributed to the public.”

CWOP has specific barometric calibration instructions for their members:

CWOP strongly encourages our members to use altimeter pressure from a nearby airport to calibrate the pressure being sent to CWOP.

The following calibration procedure is recommended:

Select a nearby (within 20 miles or 32 km) airport weather station (regional or larger) to provide your reference or calibrated pressure.

Wait for optimal weather conditions to conduct a series of comparisons; these conditions are:

• High pressure is nearly overhead
• Wind is less than 5 mph (3 m/s), preferably calm
• Outside air temperature should relatively stable or slowly changing
• Best time to conduct pressure comparisons is in the early afternoon; if the winds are light, then you are reasonably certain high pressure is in the area.

Edit: Unless you live very close to the airport, make sure that you and the airport are in the same pressure system/zone before doing the comparisons.

3. Take a series of four simultaneous pressure measurements using the altimeter pressure from the airport “METAR” report and your barometer:

a) Each comparison should be at least be 15 minutes apart or 1 hr apart for airports that report only hourly.

b) After completing the four comparisons, noting your altimeter and the reference airport pressure [differences]; sum the differences between the comparisons and divide by 4 (the number of comparisons) to get a mean difference.

4. If the mean difference between your station and the reference station is more than +/ 00.03 inches for altimeter comparisons, or +/ 1.0 mb; add (or subtract) the difference to correct your altimeter. Repeat the procedure until you achieve the goal of a pressure difference of less than +/ 00.03 inHg or +/ 1.0 mb.

5. Barometers will “drift” requiring re-calibration. Therefore, barometer comparisons (with the Altimeter setting at the airport) should be done at least annually.

SUMMARY

• Calibrate to the airport’s Altimeter.
• Wait for optimum weather conditions.
• Compare your barometer’s Altimeter readings with the airport’s Altimeter (setting) at least four (4) times.
• Calculate a mean (average) difference: take the four differences and divide by four. Apply the average difference to your barometer. Repeat until your readings are within +/- .03 inHg or +/- 1.0 hPa to the airport.
• Re-calibrate annually.

Edit: CWOP requires you to only upload Altimeter to them, not to upload SLP. If you have followed the calibration offset method, you already have a calculated your Altimeter value and may upload this value to CWOP. Before uploading Altimeter values to any other weather service, ensure that the weather service can differentiate between Altimeter and SLP.

updated 16 February 2024

In any discussion regarding weather sensors, the subject of accuracy comes up. Sensors have specifications (specs). Barometric sensors are no exception. Let's take a look at a barometer accuracy “spec” of +/- 0.7 hPa.

To make things more interesting, we shall use an analogue (it has a dial) aneroid barometer as an example:

Let's assume that someone made their first weather related purchase.

You found a remarkable eBay find. Somebody was selling a used aneroid barometer – a Fischer precision model 103 at a great price. When you received it, it was in mint condition and clearly, it was still working. You have no idea if it was calibrated or calibrated badly by a previous owner.

Note: Whether this barometer is calibrated or not (new or used), is not relevant. You will still need to check, verify and calibrate as necessary.

Like most analogue dial barometers, in order to calibrate, you have to move the needle by turning a set screw on the back. You might have to move the set screw a little or a lot, depending on how much it is “out”. But how do you know if your barometer is accurate or not?

Although the Fischer precision model 103 aneroid barometer is reported to be a very precise instrument, it still could be inaccurate if it is not set properly to your altitude.

The new owner was initially very puzzled; “What do you mean by inaccurate according to altitude?”

Accuracy is not the same as precision. The owner knew that the accuracy as specified by the manufacturer is +/- 0.7 hPa. Not only that, the accuracy was also specified as full scale, meaning that any measurement through the whole range of readings on the dial would be within +/- 0.7 hPa.

To calibrate a barometer, you need a reference calibrated barometer to compare readings. As it turns out, a friend of a friend had a very expensive digital portable barometer that had just been re-calibrated at a certified lab. He was willing to come over and lend a hand to calibrate the Fischer barometer.

To the owner's considerable horror, his Fischer barometer was 3 hPa “out” compared to the reference barometer. He was prepared to try to return his purchase back to the eBay seller because he was not getting his promised 0.7 hPa accuracy. His barometer was off by 3 hPa! Obviously, it was broken!

The guy with the reference digital barometer interceded. He said; “Hang on for a second, don't get too excited. All we have to do is match your barometer's reading with mine. That's why they have an adjustment screw in the back.” And that's what he did. He turned the set screw until the two barometers had the same reading, and carefully explained that all he did was set the barometer to the correct pressure reading for his altitude. He assured the new owner that from now on, all your readings will be accurate to +/- 0.7 hPa.

The new owner looked a bit happier. “You mean that was all we had to do?” “We're changing the starting point?”

The friend of the friend replied. “Well, I would have used the term true value, but yeah, we re-calibrated your barometer to the correct starting point. You can't adjust the precision of the instrument, but you can adjust it (as close as possible) to true value.”

With our Ecowitt digital barometers, we are doing the same thing. Instead of a set screw, we adjust accuracy using a ABS offset in order to move/adjust our ABS readings to “true value”. True value is established by a reference calibrated barometer.

Let’s get a bit statistical for a moment.

Accuracy is how close the set of sensor measurements are to a “true” value. Precision is about consistency and repeatability. Think bell curves. You are basically comparing the true value to the mean of a set of measurements.

Analyzing the set of measurements tells us how precise the sensor is. It tells us nothing about how accurate the sensor is. For that, we need a reference. We will call this reference the “true” value. True value can be established by another sensor that has already been calibrated to a high standard. The higher the reference standard – the better. Once we have established a true value, we can make valid comparisons and calibrate our barometer.

Accuracy is the distance from true value to the mean of the distribution curve of the measurements. The distance of the data points from the mean of the distribution curve establishes the level of precision of the sensor, i.e. low precision vs high precision. In other words, you want a narrow distribution curve with steep slopes versus a flattened, wide one. Assuming we have a normal distribution curve, the process of calibration is to shift the distribution curve and centre it on true value (zeroing).

updated 16 May 2024

The study of meteorology overlaps with aviation. For obvious reasons, weather is vitally important for all aircraft. The science of measuring altitude is called Altimetry. The altitude of an aircraft can be measured by a number of different technologies, but most small aircraft rely on the use of a calibrated aneroid barometer (pressure altimeter) to measure altitude using atmospheric pressure or differences in pressure. The aviation view of atmospheric pressure, and particularly the study of air density, will give additional insights into how atmospheric pressure is measured with a weather station and how temperature affects pressure.

For weather station owners, we should be familiar with the meteorological definitions of station pressure, Altimeter and SLP. For Ecowitt personal weather stations, ABS = station pressure and REL = Altimeter or SLP. However, the use of SLP is only recommended for very low altitudes close to sea level.

Aviators use slightly different nomenclature:

• QFE = Station pressure
• QNH = Altimeter(setting)
• QFF = SLP (Sea Level Pressure)

Other than the larger international US airports, the majority of METARs (meteorological aerodrome reports) located in the U.S. often don’t publish SLP values at all, They usually report only Altimeter. In Canada, METARs always publish both Altimeter and SLP on every report. In Europe, you would undoubtedly see only QNH on a METAR report.

Here are a few examples of METAR reports from the U.S., Canada and Europe. I have highlighted the pressure values in bold. Note the use of “Q -codes” in Europe.

Note: customary units for Altimeter(setting) is in English/Imperial units. For example, a METAR report would indicate 30.02 inHg (inches of mercury) as A3002. Customary units for SLP are in metric, usually in millibars (mb). In a METAR report, 1013.2 mb would be shortened to SLP132. The decimals are dropped for both Altimeter and SLP and the preceding 10 for SLP is dropped as well. In Europe and other countries, a reading of 1017 mb would be reported as Q1017. In a METAR report using a “Q-code”, the pressure reported is in whole integer units; Q1016, Q1017, Q1018 etc. There are no decimals.

Here are some actual METAR reports and their reporting of pressure (in bold):

Anchorage International Airport, Anchorage, Alaska, US: PANC 081017Z 16005KT 10SM -RA SCT020 BKN028 OVC060 07/06 A2905 RMK AO2 P0001 T00670061

Vancouver International Airport, Canada: CYVR 081000Z 09007KT 15SM FEW080 FEW220 13/11 A2986 RMK AC1CI1 AC TR CI TR SLP113

Heathrow Airport, England: EGLL 080950Z AUTO 29003KT 220V340 9999 NCD 18/13 Q1025 NOSIG

Note: You may notice that Station pressure is absent from METAR reports. METAR does not publish station pressure values. To obtain all three pressures; station pressure, altimeter and SLP, you will have to use a weather data service like mesowest or in some cases you might be able to obtain all pressure values directly from the data stream from a AWOS or ASOS station.

Here is a excerpt from the data output of a Canadian AWOS station (XML format):

1. <element name=“stn_pres” uom=“hPa” value=“1001.0”>
2. <element name=“mslp” uom=“hPa” value=“1025.0”>
3. <element name=“altmetr_setng” uom=“inHg” value=“30.23”>

Note that the station pressure that is absent in METAR reports is present in the above data stream from the AWOS. Also note that Canada prefers the use of hPa for station pressure and SLP (mslp) over mb (millibars). Altimeter(setting) is the exception - it is always in inHg (inches of mercury)

This is where meteorology and Altimetry diverges a bit. Pilots, naturally, need to know how high they are flying, so barometers (altimeters) are very important. If you are studying to become a pilot, your flight instructor will certainly cover the subject of indicated altitude versus true altitude. The instructor might say; “Your altimeter is lying to you!” and go over the saying; “High to low — watch out below!”.

The instructor is really referring to the change in air density if you are flying from a high pressure system into a low pressure system or flying into much colder air. Either way, the indicated altitude as shown on the cockpit altimeter could indicate that you are flying higher than you really are. And that is not a good thing if visibility is poor, and you can't eyeball the ground below. Pilots can compensate for these “errors” by applying manual corrections to their altimeters.

Ok. By now, you must be wondering how all of this aviation stuff is of any use to land-based weather stations. After all, we don't fly our barometers!

Actually, this discussion is very relevant to us amateur weather observers, especially for pressure corrections due to temperature changes.

If your data source (METAR, MesoWest,AWOS/ASOS) displays both Altimeter and SLP info, you might notice what appears to be odd behaviours in comparing Altimeter with SLP pressure values. At times, Altimeter (setting) can be higher than SLP, yet at other times lower. Occasionally, we might see that both values are the same. The magnitude of these “spreads” may widen or narrow with the seasons as well. In a warm/hot summer, the spread will narrow and in a cold winter environment, the spread greatly widens.

The “Q” rules:

For elevations/altitude above MSL (Mean Sea Level):

• If the outside temp < ISA temperature then QNH < QFF
• If the outside temp = ISA temperature, then QNH = QFF
• If the outside temp > ISA temperature then QNH > QFF

What is the ISA temperature? The ISA (International Standard Atmosphere) temperature for your location depends on your elevation. According to the ISA model of the atmosphere, every elevation (or altitude), has a temperature assigned to it. For example, the ISA temp at sea level is 15 °C, the ISA temp for a 300m elevation is 13.1 °C, the ISA temp for 500m = 11.75 °C, etc.

QFF changes values depending on outside temperatures that are above or below the ISA temp. If the temperature of the outside air is greater than the ISA temp (warm air is less dense than cold air), then QFF pressure must be corrected downwards. Conversely, when the temps fall below the ISA temp (for your specific elevation) QFF must be adjusted upwards (cold air is denser).

Just like in aviation, our barometers are lying to us as air density continually changes the QFF value. In practical terms, the REL value showing on your display might be too low in the morning when it is cooler and too high in the afternoon when it is much warmer. Basically, the “Q” rules tells us that there is a relationship between QNH (Altimeter) and QFF (SLP). The spread between QNH and QFF changes dynamically depending on temperature.

Where are we going with this and why are aviation Q-rules relevant for our terrestrial weather stations?

The main takeaway is that the relationship between QFF/SLP and QNH/Altimeter values constantly change due to changes in temperature. Temperature changes air density. Density changes pressure. If you are graphing SLP vs Altimeter, you might see the lines cross and reverse position as temperatures exceed the ISA temperature during the day. The next time you look at a METAR, take a closer look at the “spread” between Altimeter (setting) and SLP.

updated 03 Sept 2024

PROBLEM: “Help! I've tried to find a calibration procedure in the manual, except there is only a couple of sentences about calibration, and it did not make much sense to me. The manual said something about matching an airport's reading. My readings don't match — not even close. I think my barometer must be defective.”

SOLUTION:

Barometers are pretty robust sensors. Although it is possible that any sensor could be defective out-of-the-box, it is far more likely that the barometer just needs calibrating.

It is best if you start over. If you have a GW series gateway, this is easy to do so. Go to the calibration page and change your Absolute and Relative offsets back to zero. If they are both are zero, then you already have the factory default settings.

If you have a display console, you will see a Absolute value and a Relative value. Even if you changed these values or messed them up, there is no need to do a factory reset back to the original default pressure readings and lose all your other settings.

Go to Calibration - display console or Calibration - Wi-Fi gateway. to do a thorough re-calibration.

PROBLEM: “The manual say's to use the airport to calibrate, but my weather station is at a completely different altitude than my airport. Shouldn't both be at the same altitude?”

SOLUTION:

In setting up their barometers, owners of weather stations are often concerned about their airport’s elevation being different from their weather station’s elevation. We assume that we can't compare our weather station with an airport at a different elevation than ours. The manual says to compare and calibrate your barometer with a close-by airport. How can you do that if the airport is a lot higher (or lower) than your weather station?

You would be correct — you can't directly compare pressures at different elevations. If you did, high elevations would always show low pressure and low elevations would always show high pressure. It is important to note that station pressure values are reduced down to mean sea level elevation so that one can make valid pressure comparisons between weather stations that are at different elevations. Mean sea level elevation is the common denominator that is used. All the fancy algorithms and equations do is to convert your station pressure to what it would be at sea level elevation. Therefore, the isobars you see on weather maps are all the station pressures from all the weather stations that have been converted (reduced) to sea level pressure at sea level elevation, so everyone is on the same level playing field.

PROBLEM: “I thought I had calibrated my barometer properly, but colder weather has arrived, and my readings seem to be drifting badly. Sometimes the readings appear to be OK, but most of the time I am way out.”

SOLUTION:

You might be trying to match your weather station's REL value with the SLP value at the airport. SLP values go up and down not only in response to changes in pressure, but to changes in temperature. Temperatures will cause SLP values to fluctuate as SLP continually compensates for changing air density. Colder air is heavier/denser and warm air is lighter/less dense. Ambient/Ecowitt equipment do not have the necessary algorithms to compensate and correct for temperature. To obtain the best results in all weather conditions, make sure you calibrate to the airport's Altimeter setting instead of SLP.

PROBLEM:

I followed the instructions to calibrate my barometer to the Altimeter setting at my airport. However, on some days, my readings don't seem to be as accurate. After a day or two, the problem goes away. How can I fix this?

SOLUTION:

The Ecowitt weather stations uses a fixed offset in order to estimate Altimeter. In reality, the atmosphere isn't completely linear. The fixed offset is calculated assuming the Altimeter reading at the airport is equal to 1013.25 hPa. If the current reading at your airport is much higher or lower than 1013.25 hPa, then your readings will drift a bit. This behaviour is normal. Readings will become accurate once again when Altimeter pressure returns closer to the average of 1013.25 hPa. If not, you will have to re-adjust your ABS (Absolute value) until your REL value matches the airport Altimeter value of 1013.25 hPa.

PROBLEM:

“As recommended, I purchased a reference, calibrated barometer, so I could properly calibrate all my barometers. I noticed that my REL values on my consoles do not match the QNH value on the reference barometer, even though the ABS values match perfectly. Why doesn't QNH match my REL values exactly?”

SOLUTION:

The manufacturer of Ambient and Ecowitt weather stations have designed their barometer firmware so that a single fixed offset (REL offset) can be used to approximate Altimeter (setting) or QNH. By choosing this simple fixed offset method, the manufacturer makes the assumption that the atmosphere is perfectly linear and that ABS (absolute pressure) moves lock-in-step with REL values (relative pressure). In reality, the atmosphere is not linear (it's on a curve) which requires complex algorithms to calculate properly. Ambient and Ecowitt lack the necessary algorithms that are required, so REL values are unable to perfectly calculate and track Altimeter (setting) or QNH values.

PROBLEM:

“I live too far from my airport and/or my airport has a different climate. How can I calibrate my weather station barometer?”

SOLUTION:

Calibrating using an airport is not ideal. They use different methods to measure/calculate pressure than our personal weather stations. The best way to calibrate is to use a reference barometer side-by-side with your barometer and adjust your ABS value for accuracy. See the article, “Calibrating-the best way” for more information. Please also note that Altimeter or QNH values are unaffected by local climate conditions like temperature and humidity. Altimeter and QNH use the standard atmosphere model which uses fixed parameters determined by the model.

PROBLEM:

“I still don't get it. Just give me the setting to make it work.”

SOLUTION:

If all else fails and you are finding it impossible to calibrate your barometer, you can always use the “walk-away” method of calibrating as a last resort.

Use a close-by airport and assume it has the “true” pressure value. All you have to do is change the REL value to match the Altimeter setting or QNH value at the airport.

That's it - you are done! You just “walk-away”.

updated 22 April 2024

Absolute pressure(ABS) is the pressure reading from your barometric sensor. When calibrated, it is also known as station pressure.

Altimeter (setting) is station pressure that has been reduced to sea level based on elevation.

Altitude is the height of an object compared to sea level. It is used synonymously with the term “elevation” although technically, elevation is the height above sea level to a point on land. An airplane has altitude but a hiker has elevation.

ABS offset is used to adjust the absolute pressure reading (ABS) up or down for calibration purposes.

Barometric pressure has various meanings. NOAA/NWS defines it as a pressure reported by a barometer, or it could be atmospheric pressure. However, the World Meteorological Organization has no definition for “barometric pressure”. Some weather station manufacturers refer to sea level pressure as “barometric pressure.” Because of multiple and sometimes contradictory definitions, In this wiki, I will define barometric pressure as a pressure from a barometric sensor.

Plateau effect Any station at 305 meters (or above) are considered to be plateau stations and requires an additional pressure correction called a “plateau correction”. The “plateau correction” reduces SLP when the current temperature of the station is greater than the annual mean temperature. Similarly, the “plateau correction” increases SLP when the current temperature is less than the annual mean temperature.

Relative pressure (REL) represents what the pressure would be at sea level elevation if our weather station’s barometer was located down there. REL can refer to Altimeter or SLP.

REL offset is the fixed difference in pressure between your location's pressure and the pressure at sea level using standard atmosphere conditions.For meteorological purposes one might say; “station pressure has been corrected to sea level pressure.”

Sea level pressure in a generic sense, can refer to METAR SLP or Altimeter (setting)/QNH. Both are sea level pressures because both are station pressures that have been reduced (corrected) or “normalized” to sea level. Each is calculated differently.

SLP is station pressure that has been reduced to sea level based on elevation, temperature, humidity (optionally) and other factors (plateau effect and other empirical adjustments).

Station elevation has two different definitions. In aviation, station elevation is usually the runway elevation or aerodrome elevation. For personal weather stations, station elevation refers to the elevation/altitude of the barometric sensor.

Station pressure For personal weather stations, it is the sensor pressure at station elevation. In aviation, station pressure is called QFE, which is a calculated amount. The barometric sensor height above (or below) a reference point (runway or aerodrome elevation) is reduced to the reference point using a special “removal” correction.