NOTE: With the introduction of the new Ecowitt SLP algorithm in 2025, this article can be considered to be deprecated. Please go to
www.barometer-wiki.ca/barometer for all the latest news, set-up advice, calibration techniques along with new guides, tutorials and how-to's.
This tutorial or “how-to” is 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, this guide is 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 - especially for the study of wind.
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 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 this forum – wxforum.net.
A helpful forum member got me going and soon after, my barometer was finally set up and operational.
Time to pay it forward!
Since we don’t know the experience level of a new personal weather station owner we can use three categories: beginner, intermediate and advanced. In this tutorial we’ll start at the beginner level.
For the beginner, we start with the basics of atmospheric pressure:
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 millibars or 1013.25 mb. The world-wide sea also has an
average temperature and it has been set to be 15C.
So those two numbers is our baseline. Intuitively, we know that the higher we go in the air, atmospheric pressure is 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 denser and warm air is lighter (less dense) – something that hot air balloons take advantage of.
There’s a formula that shows the relationship between temperature and pressure at different elevations/altitudes – we’ll deal with air density as a more advanced topic later.
No need to panic – this is not a math test. We are going to use an online calculator that does the calculations for you.
But where is our barometric sensor that records the atmospheric pressure? Is it outside on the array where the rain gauge is?
Depending on your model, it is inside either hidden in your display console, gateway or in that remote control looking device(WH32B) that has an alternating LCD display displaying inside temperature, inside humidity and the one that we want – barometric pressure. These measurements are in Imperial units. Unfortunately you can’t change the LCD display to metric – but you can change the console display to show metric pressure units.
For better precision and accuracy we need to change the pressure from inHg (inches mercury) to a metric measurement called hecto-pascals or hPa for short. Both hPa and mb are the same except your display console uses hPa.
Your weather station manual will tell you how to switch units to hPa.
We mentioned that pressure and temperature vary with elevation so we are going to calculate what your pressure and temperature should be for your location when the sea level temperature is 15C and the sea level pressure is 1013.25 hPa. You probably figured out by now that we need your elevation above sea level.
I like to use a satellite photo for this as you can clearly pin-point your location. The use of the Google Earth application (or web site) is ideal. All you need to do is cursor around to find your elevation above sea level. It is important to add additional meters or feet if your barometric sensor is on the second story for instance. By all means, take out a measuring tape to calculate the extra height above ground level.
There is a model of the atmosphere called the
International Standard Atmosphere or ISA. The best way to illustrate how it works is by way of an example. Let’s go to the digital dutch website and fire up the ISA calculator.
Example:
My barometric sensor elevation is 250 meters above sea level. According to the ISA model of the atmosphere, what is my ISA temperature and my ISA pressure @ 250 meters?
[ You are not allowed to view attachments ]
I’ve created a table by setting the minimum elevation to 0 meters (sea level) and setting the maximum to the desired elevation (250 m). I have unchecked the boxes for density, speed of sound and dynamic viscosity to highlight just temperature and temperature and switched the units to meters, celsius and millibars. The increment could be any number up to 250 but I chose 50 m to broadly illustrate how temperature and pressure declines from sea level elevation up to our location at 250 m.
The above table shows that yes, the higher in elevation we go, both temperature and pressure drop and that the temperature for our 250 m elevation is 13.375C and the atmospheric pressure is 983.575 mb/hPa. Note that the default “standards” we have previously discussed, are already present in the table: 1013.25 mb at sea level (0 meters) and 15C at sea level (0 meters).
OK. I have a confession. There is some math involved but it is pretty easy. We need to do a simple subtraction:
pressure @ 0 meters= 1013.250
pressure @ 250 meters = 983.575
elevation offset = 29.675 hPa (we will round this up to 29.7 hPa)
That 29.7 is important. Memorize it or write it down in your weather journal. It is sometimes called a barometer correction but in this forum we tend to call it a fixed offset or an
elevation offset. What this tells us that at any sea level pressure (not just at the standard 1013.25) you subtract 29.7 to get your pressure for your 250 m location.
OK. We are now ready to compare our readings with a close by airport. We need to find your airport's METAR ID code, e.g. ABCD and the airport's pressure reading(s), go to:
https://aviationweather.gov/gfa/#obs and click on the map and find your region. Click on the station that you want and you will see the station code for your airport.
Note: For the beginner, it would be best to compare to the airport's Altimeter or if you live outside North America use QNH, i.e Q1029. If you used the Configure button to decode the METAR report, Q1029 will be displayed as Altimeter: 1029 hPa.
Time to dig out the manual and re-read the barometer calibration section.
It will say something like setting your REL (relative pressure) to the airport.For the beginner, I will suggest that you set REL to the
airport’s Altimeter setting.
The manual is starting to make a bit more sense: we need to change the REL in the display console to 1029.7 (the airport’s Altineter reading) subtract the elevation offset of 29.7 and set the ABS (Absolute pressure) which is our location’s pressure to 1000.0. I will leave it to the reader to look up the instructions for their specific model in order to press the right buttons.
It is very important to match the airport REL reading
only when you and your airport are in the same weather system (the same high or low pressure zone). Why? Because the airport has an expensive, calibrated barometer and you are trying to closely match its readings. The airport should be reasonably close. If the airport is far away, chances are that the weather systems will be different most of the time which will make calibration difficult.
Essentially you are now done. You figured out the elevation offset and have matched the airport’s REL and have entered the REL and ABS into the display console. If you want, compare your REL reading with the airport’s Altimeter setting from time-to-time. If you are within 1.0 hPa of the airport reading you have successfully calibrated your barometer!
If you can’t seem to match the airport or your readings are drifting more than 1.0 hPa apart, continue on to Part II for more intermediate category calibration techniques:
New Method of calibrating your barometer Part II (a guide for the intermediate user)References:
ISA Standard Atmosphere online calculator:
https://www.digitaldutch.com/atmoscalc/table.htm
