Dummies Guide to Barometric Pressure and How to Choose From the Barometers On Sale

Accurate use of a barometer to monitor changes in barometric pressure is the lynchpin of weather forecasting.

This short article is intended to be of use to anyone who is thinking about buying a barometer for their own home or as a present. It will give broad overview of what is available and hopefully point you in the direction of the type that will suit you best.

Let’s start with a short introduction:

What Is A Barometer?

A barometer is a device that measures barometric pressure, what kind of fool do you take me for?

What then is barometric pressure?

It is the pressure exerted by the weight of the air above. It’s easiest to understand if you picture a column of air stretching up into the atmosphere.

The downward pressure of that column is what is being measured by a barometer. This incidentally explains why barometric pressure (otherwise referred to as atmospheric pressure) changes with height.

If you are standing at the top of a mountain the column of air is shorter than if you are standing on the valley floor and therefore the pressure exerted is less.

So, since pressure varies with height, if you move you barometer from one place to another you will almost certainly be at a different elevation and therefore won’t you change the reading? You are absolutely right (how astute you are).

To avoid every meteorological organisation in the world having to mount their barometric pressure sensors at the same elevation all readings are corrected to sea level. Whenever you hear a pressure figure given in a forecast it has been corrected to sea level.

A formula is used to adjust the reading given by the barometer to derive the pressure it would have read had it been located at sea level.

Remembering that barometric pressure is the weight exerted by a column of air will help understand that almost always the adjusted figure will be higher than the physically measured one.

What Does A Barometer Measure?

The unit used for atmospheric pressure measurements is the millibar (mB). In scientific publications you will often see the hectopascals (hPa) unit used instead (but it couldn’t be easier to convert, 1mB = 1hPa). The only exception is the in the USA where they still use millimetres of mercury (mmHg) for their barometric pressure measurements.

If you are wondering how to adjust your local barometric pressure reading to sea level then there is a fairly fearsome formula to do so, but broadly speaking 10 meters or 33 feet of height corresponds to 1mB.

Therefore if your measurement location was 50m above sea level then you would add 5mB to your measured reading. In reality the formula also factors in air temperature (which will change the density in the column of air above and therefore also change the pressure-per-metre of air).

However for all practical purposes 10m/33ft per mB will prove accurate.

Ok, that’s probably a lot more than you wanted to know. But at least now you can hang around in front of your barometer waiting for some fool to come past who you can impress with some technicalities. Let’s look at what types of barometer are available.

Mercury Barometer:

It may still be possible to buy a mercury barometer in the shops but if so I haven’t seen any. They look a lot like a committee-designed thermometer:

They are extremely accurate but probably a non-starter for the home, people are (rather sensibly) reluctant to have large quantities of toxic substances in fragile glass containers on their walls.

The Household Favourite, the Wall-Mounted Barometer Dial:

A rotating needle points to the current pressure and usually the words “Rain”, “Change” and “Fair” appear with ill-deserved confidence in bold text. More accurate might be “low pressure” “middling pressure” and “high pressure”.

The device itself holds no memory of the recent pressure history and so it’s a mystery known only to the manufacturers exactly how they know whether or not it is going to rain.

In terms of accuracy, they should be pretty good, they all work using an aneroid sensor  (aneroid simply means without liquid, it is essentially a sealed box that expands and contracts with changes in external air pressure, this movement is transferred to the dial on the front of the display).

They will require periodic adjustment but this is a straight forward process, much like setting an alarm clock.

Of course they come in all kinds of styles from the beautiful antique marine style brass dial to the plywood-mounted with the warning “Frae Bonny Scotland” printed above. You pays your money and you takes your choice. Prices range accordingly of course.

 

Digital Barometer Pressure Sensors and Displays

These days there are a large range of inexpensive LCD style barometer displays, often incorporating indoor temperature and even little pictograms of storm clouds (the digital equivalent of the barometers’ outrageously optimistic rain, change or fair and probably not much more reliable but definitely more fun).

I have one that combines its function with that of being a wireless doorbell, it cost £15 and is every bit as accurate as these facts imply, but I still enjoy using it.

Digital sensors do come in all styles and price ranges. If you buy a good one you can typically also get graphs of the pressure trends and automatic adjustment for sea level pressure.

The Rotating Drum Barograph:

The king of barometric pressure measurement and my personal favourite is the rotating drum barograph. It looks a bit like the seismographs that you see in earthquake disaster films. Depending on the model chosen you will typically have a chart on a drum that rotates over a period of a week or a month and is linked to an aneroid barometer which in turn moves an ink pen up and down the rotating paper chart in response to pressure changes.

They are usually clockwork (part of the charm of ownership is the routine of winding up the rotating drum mechanism). The pressure is set with a small manually adjusted knob which shifts the physical position of the pen on the paper charts. Once set it very rarely needs resetting.

Some models use a disposable felt-tip pen for the trace on the charts, others the more traditional nib and ink reservoir. The latter is lovely and feels more in keeping with nature of these instruments but I have never yet seen one that wasn’t caked in ink smears. Replacement pens and chart paper are generally available (for some reason ship’s chandlers often seem to sell this type of equipment).

They are extremely sensitive, I once worked in an air conditioned office which had one and it was possible to tell if the door to the office had been opened because there would be a small kick in the pen trace on the chart where the pen had reacted to the pressure change in the room caused by the door opening.

 

 

Rotating drum barographs are undoubtedly objects of beauty (and usually extremely accurate too) unfortunately this is typically reflected in the price, expect to pay upwards of £600 for a good one, but then these are heirlooms.

Ultimately, you choose the barometer that you can live with and matches your budget.