Setting A Barometer - Under Pressure

You've just unpacked your new barometer or home weather station. Next question - how to set a barometer.

No matter what you've bought, unless you live at sea level, you'll probably have some trouble making sense of your first air pressure readings.

This is a very common problem during the barometer calibration phase. You follow all the instructions, then check your air pressure readings against a nearby weather station, only to find they appear to be way out.

What's going on?

Air Pressure 101

First some very basic facts. Air pressure measures the pressure caused by the weight of a column of air, extending to the top of the atmosphere. Effectively this is the top of the troposphere, the lower part of the atmosphere in which almost all our weather happens.

So if we assume the top of the troposphere is at the same height over a large area, then higher points on the land's surface will have less air above them. Less air = less weight of air = lower air pressure.

But the top of the troposphere is rather bumpy and wavy. Where the troposphere is thicker, the pressure is higher, and it is lower where the troposphere is thinner, and these highs and lows move across the planet. They control our weather, and the easiest way to understand their movement is to monitor the air pressure.

But if we climb a hill or mountain, we get closer to the top of the troposphere - less air above us equals lower air pressure. In fact, for every 8.2 meters, about 25 feet, that we climb, the air pressure drops by 1 millibar or hectopascal, about 0.03 inches of mercury. (For other ways of describing this drop in pressure with elevation visit the Air Pressure page).

So if we drove around the country with a portable or auto barometer, otherwise known as an altimeter, we would see the elevation change on the dial as we climbed a hill and drove down the other side, reflecting the changes in air pressure with height. Unless some way of standardizing air pressure with elevation was used, we would have the greatest difficulty in using smaller variations in overall air pressure to help us predict the weather. To achieve this, all local air pressures are recalculated back to the value they would have if the location was at sea level.

And the easiest way to do this is to calibrate all barometers back to sea level, so the value they give for air pressure is the sea level equivalent, not the actual value.

Setting a Barometer to Sea Level

OK, we know that air pressure decreases by 1 millibar for every 8.2 meters we gain in height, so setting a barometer should be a simple matter if we know the elevation.

Well, yes and no. In general terms, provided that the elevation is less than about 1000 meters or 3000 feet, that would work fine for most purposes. Any higher and you would need a different factory setting on your aneroid barometer, and as you go higher, some weather stations will stop registering a realistic pressure.

But if you want to have air pressure data which you can directly compare to official values you will have to do a little more.

There are three main reasons for this.

Firstly, at elevations over about 3000 feet - 1000 meters, the simple formula for compensating for altitude gets a little more complex.

Secondly, temperature also affects barometers, and temperature also decreases with altitude.

And thirdly, unless you live very close to the coast, air pressure varies from place to place as highs, lows, fronts and storms travel across the country. So if you live 100 miles or more (160 km) inland, your recalculated sea level pressure will probably be wrong - maybe not by much, but there are better ways of setting a barometer.

And fortunately there is a much easier way if you live reasonably close to an official weather station, or an airport.

On a calm clear day, preferably one where the weather map tells you that a high pressure cell is overhead, check the pressure data from your nearest airport (the local NWS website should have the info you need), and enter it into your system.

Compare the two values over the next few days, preferably in calm clear conditions during the middle part of the day. If the two values continue to correspond your problems are solved. If not, continue to compare and adjust until they do.

Avoid exceptionally hot or cold days, times when the weather is changing or stormy, and windy days, all of which can lead to noticeable local pressure variations.

Most home weather stations will allow you to set a reference sea level pressure equivalent, and all later readings will be based on that. All weather software programs also have this facility.

And if you have a standard mechanical aneroid barometer, you will find an adjustment screw which will let you do the same thing.

Depending on what you want from your weather station or your barometer, you can continue to refine your base pressure value. Well calibrated air pressure readings are necessary if your weather station is part of the official network, but from a personal point of view, it is the changes in air pressure that are important in understanding the weather, not the absolute values (as long as they are in the ball park). So there's no need to be worried if you can't manage a perfect match with another weather station. After all, who's to say that their equipment is perfect?

More About Barometers

Here's where to go to find out about the History of the First Barometers.

And while most scientific attention was concentrated on the mercury barometer, water based barometers were also in use, and in big numbers. They were known as the Weather Glass or Storm Glass. Follow the link to find out more about these elegant weather instruments.

But both of these barometers had their problems, not the least of which was keeping the mercury or water in its container. As technology improved, the better, more convenient Aneroid Barometer appeared, and that is the most common form we see today.

Want to make your own barometer? There are several pages of air pressure experiments and Instructions For Making Homemade Barometers, plus others with reviews of Modern Barometers. This link will take you back to the Top, or, when you're ready, here's how to return to the Home page.

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Last update 12/31/2006