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start [2026/02/01 19:44] – [Altimetry, METAR and Q-codes] adminstart [2026/05/14 20:28] (current) – [Barometer troubleshooting] admin
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 Barometers and their pressure readings and related calculations can be difficult to understand, so much so that until now, only a very small percentage of station owners have successfully managed to properly set-up or even use their barometers. However, this all changed with the introduction of Ecowitt's new automatic barometer firmware update in 2025. Barometers and their pressure readings and related calculations can be difficult to understand, so much so that until now, only a very small percentage of station owners have successfully managed to properly set-up or even use their barometers. However, this all changed with the introduction of Ecowitt's new automatic barometer firmware update in 2025.
  
-Ecowitt's new sea level pressure (SLP) firmware update is a radical departure from the previous algorithm, where sea level pressures were somewhat crudely approximated using a simple fixed offset. From now on, SLP (mean sea level pressure) will be automatically and continuously calculated based on changes in station pressure, outside temperatures and humidity. This new update will greatly increase the accuracy of sea level pressure calculations. It will also make the initial set-up of the barometer much easier, requiring the user to only input their barometer altitude above sea level. No more manual calculations required!+Ecowitt's new sea level pressure (SLP) firmware update is a radical departure from the previous algorithm, where sea level pressures were somewhat crudely approximated using a simple fixed offset. SLP (mean sea level pressure) is now automatically and continuously calculated based on changes in station pressure, outside temperatures and humidity. This new update greatly increases the accuracy of sea level pressure calculations. It will also make the initial set-up of the barometer much easier, requiring the user to only input their barometer altitude above sea level. No more manual calculations required!
  
  
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 </alert> </alert>
  
-One of its first important uses was for sailing ships more than three hundred years ago. Sailors 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 analogue  +One of its first important uses was for sailing ships more than three hundred years ago. Sailors 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, ships still have analogue (dial type) barometers on board should all the electronics fail.
-(dial type) barometers on board should all the electronics fail.+
  
-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.+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 other 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.+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 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. But the barometer is different. Unlike most weather sensors, it has predictive qualities, which is a key element of weather forecasting.
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 //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.// //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.//
  
-====Sea Level Pressure - visualizing a concept====+====Sea Level Pressure - visualizing a concept (Example #1)====
 updated 07 Jan 2026 updated 07 Jan 2026
  
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 The weather station on the 40th floor is 400 feet above sea level (10 ft/floor) and is displaying an ABS of 998.7 hPa. You call your friend on the ground floor (at sea level) and ask him what his pressure is. He says it's 1013.2. You glance at your Ecowitt display console and the REL is also displaying 1013.2, the same as your friend's weather station on the ground floor at sea level! Fantastic! The algorithm is working perfectly! The weather station on the 40th floor is 400 feet above sea level (10 ft/floor) and is displaying an ABS of 998.7 hPa. You call your friend on the ground floor (at sea level) and ask him what his pressure is. He says it's 1013.2. You glance at your Ecowitt display console and the REL is also displaying 1013.2, the same as your friend's weather station on the ground floor at sea level! Fantastic! The algorithm is working perfectly!
  
-However, what type of magic is going on here? How does my display console "know" what the sea level pressure is at sea level when I am 400 feet above sea level??+How does my display console "know" what the sea level pressure is at sea level when I am not at sea level??
  
 Actually, you've already told your barometer how high it was above sea level when you first set up your barometer. You had entered 400 feet as your altitude in the console, so the algorithm "knows" to add a 400 foot pressure adjustment (14.5 hPa) to whatever the current ABS pressure is showing on the display. The console's algorithm calculates and adds back an extra 14.5 hPa of pressure to the current ABS pressure to compensate for the thinner air 400 ft above sea level. Actually, you've already told your barometer how high it was above sea level when you first set up your barometer. You had entered 400 feet as your altitude in the console, so the algorithm "knows" to add a 400 foot pressure adjustment (14.5 hPa) to whatever the current ABS pressure is showing on the display. The console's algorithm calculates and adds back an extra 14.5 hPa of pressure to the current ABS pressure to compensate for the thinner air 400 ft above sea level.
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 Sea level pressure can be measured if you are located at sea level or estimated/calculated if you are not. Sea level pressure can be measured if you are located at sea level or estimated/calculated if you are not.
  
-As most of us live above sea level, the new Ecowitt SLP algorithm will calculate the sea level pressure based on your barometer's elevation (altitude) and station pressure, Additionally, it will also factor in current outside temperature and humidity to give as a truer representation of the atmosphere.+As most of us live above sea level, the new Ecowitt SLP algorithm will calculate the sea level pressure based on your barometer's elevation (altitude) and station pressure, Additionally, it will also factor in current outside temperature and humidity to give us a truer representation of the atmosphere.
  
 <alert> <alert>
 If two or more weather stations are at different altitudes, you can "equalize" or "normalize" each of them to a common elevation/altitude (sea level) by adjusting their station pressures to an estimated sea level pressure. If two or more weather stations are at different altitudes, you can "equalize" or "normalize" each of them to a common elevation/altitude (sea level) by adjusting their station pressures to an estimated sea level pressure.
 </alert> </alert>
- + 
 +====Sea Level Pressure - visualizing a concept (Example #2)==== 
 +**updated 07 March 2026** 
 + 
 +If you do a general search for a visual illustration of sea level pressure, you will undoubtedly find quite a few illustrations/drawings of mountains and weather stations at different elevations and calculated (but different) sea level pressures.  
 +For a new weather station owner, these illustrations may not be all that helpful. 
 + 
 +To better illustrate the concept of //sea level pressure,//we will do something a bit different. 
 + 
 +Rather than starting with station pressure and calculating sea level pressure, we will do the reverse. We will start with the sea - specifically the standard sea level pressure of 1013.25 hPa and work our way up into the atmosphere and calculate the appropriate temperature and station pressures at our weather stations using the standard model of the atmosphere.  
 + 
 +Let's say we have three Ecowitt weather stations on a small ocean island located at three different altitudes. One of the stations, STN "A" is located exactly at sea level - not far from the beach. The other two stations, STN "B" and STN "C" are located on two small mountains. All three stations are under the same, stable atmospheric pressure system. If we were to drive around this small island in a boat, we can confirm that the pressure at sea level all around the island is indeed, 1013.25. 
 + 
 +The current temperature is 15C at sea level and we know that the atmosphere will get cooler as we go up higher into the mountains where STN "B" & STN "C" are located. 
 + 
 +STN "A" is located at sea level. It is directly measuring sea level pressure of 1013.25 hPa at sea level. Because all pressures (measured or calculated) are the same at sea level, the REL value will be the same as the ABS value. Therefore, our Ecowitt console located at sea level near the beach (STN "A"), is showing ABS = 1013.25 and REL = 1013.25. 
 + 
 +Our second Ecowitt console is STN "B" located at 200 meters above sea level. The station pressure (ABS) @ 200 m is now lower by about 23.75 hPa from what it was at sea level because we are higher into the atmosphere and the air is thinner and cooler. Our Ecowitt console is now displaying ABS = 989.45 and REL = 1013.25. The outside temperature at STN "B" has dropped from 15C to 13.7C. 
 + 
 +STN "C" is the highest station located at 500 meters above sea level and because we are much higher, the station pressure has dropped considerably and is now 58.64 hPa less than the pressure at sea level. Therefore, our station pressure @ 500 meters is 954.61. The console display indicates ABS = 954.61 and REL = 1013.25. The outside temperature has dropped a bit more and is down to 11.75C. 
 + 
 +As we know, both pressure and temperature drop the higher we go up in the atmosphere and we can see that the ABS values and outside temperatures are dropping as we move farther up into the mountains. 
 + 
 +We have three weather stations located at three different elevations. All three have different station pressures and different temperatures: The Ecowitt algorithm is calculating sea level pressure - displayed on our consoles as REL (relative pressure): 
 + 
 +1. STN "A"  ABS =1013.25, REL = 1013.25 @ 15C @ 0 m (sea level) 
 + 
 +2. STN "B"  ABS = 989.45, REL = 1013.25 @ 13.7C @ 200 m 
 + 
 +3. STN "C"  ABS = 954.61, REL = 1013.25 @ 11.75C @ 500 m   
 + 
 +ABS = measurement of absolute pressure/ station pressure 
 +REL = algorithmic calculation of relative pressure/sea level pressure 
 + 
 +We can observe the following: 
 + 
 +The REL of 1013.25 (sea level pressure) is the same at all three stations even though all three stations are at different elevations, station pressures and temperatures.  
 + 
 +Since the sea level pressures are the same we could plot all three stations on a weather map and draw an isobar (lines of equal pressure) just like the meteorologists do.  
 + 
 +**SUMMARY** 
 + 
 +We can now see what they mean by converting (normalizing) a station pressure to an equivalent sea level pressure. All three stations (A, B & C) have different station pressures, different altitudes and have different temperatures. Yet all of them yield the same sea level pressure of 1013.25. Because we have normalized or standardized all station pressures to a common altitude (sea level), we can now make valid comparisons between all three stations. 
 + 
 +**What is the point of calculating sea level pressure if your weather station is not at sea level?** 
 + 
 +Meteorologists use barometer readings from thousands of weather stations in order to draw isobars. Isobars are the curving lines of equal mean sea level pressure. These lines (isobars) represent the familiar HIGH and LOW pressure systems we see every day on weather maps and forecasts. Drawing isobars is just a matter of connect-the-dots. See link below. 
 + 
 +Resource: https://www.noaa.gov/jetstream/analyze-appair-pressure-map 
  
 ==== Barometers - why do you need to calibrate them? ==== ==== Barometers - why do you need to calibrate them? ====
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 Why devote an entire wiki to just one weather sensor? Why devote an entire wiki to just one weather sensor?
  
-Barometers, can be one of the most difficult weather sensors to set up properly. My informal annual survey of surrounding weather stations reveals that only a few percent of weather station owners have managed to successfully set up and calibrate their barometers. +Barometers, can be one of the most difficult and frustrating weather sensors to set up properly. My informal annual survey of surrounding weather stations reveals that only a few percent of weather station owners have managed to successfully set up and calibrate their barometers. 
  
 Hopefully, these guides, tutorials and how-to's will shed some light on the subject of barometers - what they are measuring, how to set them up and how to keep them calibrated and accurate, Hopefully, these guides, tutorials and how-to's will shed some light on the subject of barometers - what they are measuring, how to set them up and how to keep them calibrated and accurate,
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 ===== Barometer troubleshooting ===== ===== Barometer troubleshooting =====
-updated 23 Dec 2025+updated 14 May 2026
  
 Ecowitt rolled out a new firmware update in 2025 that dynamically calculates SLP (sea level pressure) factoring in elevation, station pressure,temperature and humidity. This new update is a radical departure from the previous algorithm, where sea level pressures were somewhat crudely approximated using a simple fixed offset. Ecowitt rolled out a new firmware update in 2025 that dynamically calculates SLP (sea level pressure) factoring in elevation, station pressure,temperature and humidity. This new update is a radical departure from the previous algorithm, where sea level pressures were somewhat crudely approximated using a simple fixed offset.
  
-Most barometer issues are usually caused by barometer calibration problems. All barometers drift over time which means they require regular recalibration to restore their full accuracy. New barometers can drift the most during their initial 12 month wear-in period . Therefore, it is important to check and verify barometer accuracy at least once or twice a year.+Most barometer issues are usually caused by barometer calibration problems. All barometers drift over time which means they require regular recalibration to restore their full accuracy. New barometers can drift the most during their initial 12 month wear-in period. Therefore, it is important to check and verify barometer accuracy at least once or twice a year.
  
 If you are having difficulties with the setup of your barometer or your barometer readings just don't seem to be quite right, there are some things to check or double-check: If you are having difficulties with the setup of your barometer or your barometer readings just don't seem to be quite right, there are some things to check or double-check:
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 Procedure: Procedure:
  
-1. **Verify that your station altitude is correct.** Station altitude is the total height of the barometer above mean sea level. For Ecowitt equipment, the barometer (its sensor) can be located inside the display console, Wi-Fi gateway or in a separate 3-in-1 device that has the inside temperature, humidity, and pressure sensor, e.g. WH32B. Other models or brands may have the barometer on the outside array.+1. **Verify that your station altitude is correct.** Station altitude is the total height of the barometer above mean sea level. For Ecowitt equipment, the barometer (its sensor) can be located inside the display console, Wi-Fi gateway or in a separate 3-in-1 device that has the inside temperature, humidity, and pressure sensors, e.g. WH32B. Other models or brands may have the barometer on the outside array.
  
 2. Starpath.com (barometer calibration firm) recommends Google Earth in order to **determine your ground elevation above mean sea level.** Your barometer will likely be located inside a house or apartment so you will have to manually **measure and add the additional height above ground level** in order to obtain the total height of your barometer. 2. Starpath.com (barometer calibration firm) recommends Google Earth in order to **determine your ground elevation above mean sea level.** Your barometer will likely be located inside a house or apartment so you will have to manually **measure and add the additional height above ground level** in order to obtain the total height of your barometer.
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 **Why choose Altimeter setting or QNH instead of SLP to calculate our station pressure?** **Why choose Altimeter setting or QNH instead of SLP to calculate our station pressure?**
  
-We will choose to use altimeter setting values instead of using SLP because altimeter ignores local temperatures and humidity making the math far simpler and easier.+We will choose to use altimeter setting values or QNH instead of using SLP because altimeter setting and QNH ignores local temperatures and humidity making the math far simpler and easier.
  
-Is the above procedure better than using a calibrated reference barometer?. No, it is not. It is a compromise Consider it as an initial setting.+Is the above procedure better than using a calibrated reference barometer?. No, it is not. It is a compromise. Consider it as an initial setting.
  
 **SUMMARY:** **SUMMARY:**
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 **Resources:**  **Resources:** 
  
-QFE calculator: This calculator uses the term QFE instead of station pressure. QFE is the aviation equivalent to station pressure. +QFE calculator: This calculator uses the term QFE instead of station pressure. QFE is the aviation equivalent to station pressure. Click on the QFE button to start and enter in your elevation and sea level pressure/QNH. 
-See example:https://www.sensorsone.com/elevation-station-qfe-sea-level-qnh-pressure-calculator/?ab=2&v0=300&f0=m&v1=975&f1=hPa&f2=hPa + 
 +See:https://www.sensorsone.com/elevation-station-qfe-sea-level-qnh-pressure-calculator 
  
 Note: For North America and jurisdictions that use altimeter setting instead of QNH, put the altimeter setting value of your airport into the QNH input box of the the calculator. Note: For North America and jurisdictions that use altimeter setting instead of QNH, put the altimeter setting value of your airport into the QNH input box of the the calculator.
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 **Question**: //What is the SLP algorithm actually calculating?// **Question**: //What is the SLP algorithm actually calculating?//
  
-**Answer:** The new algorithm is constantly adding or subtracting pressure in response to cooler or warmer air at your location. Cold air is heavier (more pressure) as the algorithm is compensating by adding pressure. In summer under warm/hot conditions, the algorithm will actually reduce your REL pressure because warm/hot air is lighter (less pressure).+**Answer:** The new algorithm is constantly adding or subtracting pressure in response to cooler or warmer air at your location. Cold air is heavier (more pressure) so the algorithm is compensating by adding pressure. In summer under warm/hot conditions, the algorithm will actually reduce your REL pressure because warm/hot air is lighter (less pressure).
  
 **Question:** //My REL readings don't seem to exactly match the local airport's SLP reading. Why not?// **Question:** //My REL readings don't seem to exactly match the local airport's SLP reading. Why not?//
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 **Answer**: In the old firmware (fixed offset system) you could set your REL to the airport's Altimeter (setting) or, possibly, SLP (for very low elevations). However, there were major accuracy issues with under-reporting SLP pressures in colder temperatures. Much better results were obtained by setting our weather station's REL to the airport's Altimeter (setting). This enabled us to calibrate our barometer with better precision, but there were still issues with Altimeter accuracy "drifting" at high (or low) pressures. **Answer**: In the old firmware (fixed offset system) you could set your REL to the airport's Altimeter (setting) or, possibly, SLP (for very low elevations). However, there were major accuracy issues with under-reporting SLP pressures in colder temperatures. Much better results were obtained by setting our weather station's REL to the airport's Altimeter (setting). This enabled us to calibrate our barometer with better precision, but there were still issues with Altimeter accuracy "drifting" at high (or low) pressures.
  
-The new firmware barometer algorithm (introduced in 2025)calculates SLP only. If necessary, you can still use the official [[https://www.weather.gov/epz/wxcalc_altimetersetting|NWS Altimeter (setting) online calculator]] to get one-off Altimeter readings.+The new firmware barometer algorithm (introduced in 2025) calculates SLP only. If necessary, you can still use the official [[https://www.weather.gov/epz/wxcalc_altimetersetting|NWS Altimeter (setting) online calculator]] to get one-off Altimeter readings.
  
 **Question:** //Why do I need to calibrate the barometer? All my other sensors don't require calibrating. Why should I bother?// **Question:** //Why do I need to calibrate the barometer? All my other sensors don't require calibrating. Why should I bother?//
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   - Pressure directly measured at sea level is a sea level pressure.   - Pressure directly measured at sea level is a sea level pressure.
-  - For meteorological purposes, pressure measured at any elevation higher than sea level is mathematically reduced to sea level elevation. It is the elevation that is reduced - not pressure. "Pressure reduction to sea level" is not a good description for what it really is. It would better described if we said; "station pressure is converted to an equivalent sea level pressure." A mathematical equation calculates the additional pressure required (based on the weather station's elevation above sea level) to convert the station pressure to what the pressure at sea level would be. This estimate is known as a sea level pressure. In aviation, this estimated sea level pressure is referred to as //Altimeter (setting) or QNH//.+  - For meteorological purposes, pressure measured at any elevation higher than sea level is mathematically reduced to sea level elevation. It is the elevation that is reduced - not pressure. "Pressure reduction to sea level" is not a good description for what it really is. It would better described if we said; "station pressure is converted to an equivalent sea level pressure." A mathematical equation calculates the additional pressure required (based on the weather station's elevation above sea level) to convert the station pressure to what the pressure at sea level would be. This estimate is known as a sea level pressure. In aviation, this estimated sea level pressure is referred to as //Altimeter setting or QNH//.
   - As above, the third sea level pressure is also reduced to sea level elevation, although the equation uses additional local climate factors (temperature and humidity) as well as the elevation of the station to estimate the pressure at sea level. This estimate is also known as a sea level pressure. In an airport METAR report, this sea level pressure is abbreviated as SLP. In aviation, SLP is referred to as a "Q-code" called QFF.   - As above, the third sea level pressure is also reduced to sea level elevation, although the equation uses additional local climate factors (temperature and humidity) as well as the elevation of the station to estimate the pressure at sea level. This estimate is also known as a sea level pressure. In an airport METAR report, this sea level pressure is abbreviated as SLP. In aviation, SLP is referred to as a "Q-code" called QFF.
  
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 We need to do the following to see if our ABS reading of 1000 hPa is accurate. 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 we should have the same REL reading as the airport. The pressure reading at the airport is called the Altimeter (setting) or Altimeter.+If our barometric sensor is perfectly accurate, then we should have the same REL reading as the airport. The pressure 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. Suppose the current Altimeter reading at the airport is 1036.5 mb. However, our barometer REL shows 1035.5. They do not match.
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