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What is an aneroid barometer?

And how exactly does it work?

By Iain Barbour

Barometers were designed to measure atmospheric, or barometric, pressure predominantly, but also have the capacity to measure altitude. Available in two varieties- mercury or aneroid- barometers measure atmospheric pressure to give an indication of short-term weather.

As opposed to a mercury barometer, aneroid barometers are non-liquid, smaller, and less inclined to break and spill, making them perfect for use aboard boats. The aneroid barometer has a sealed metal chamber called an aneroid cell (or capsule) which is made from an alloy of beryllium and copper.

They are similar to a clock face as they are usually circular, with multiple hands, and labelled sections indicate the amount of pressure (given in a numeral) around the edge. Some aneroid barometers can also have weather labels, as you can see in the picture.

How does an aneroid barometer work?

Barometers work by measuring atmospheric pressure. But how does that work in practice?  The atmosphere is made up of layers of air wrapped around the Earth, that have weight. This air presses against everything it touches as gravity pulls it towards the planet.

This is the pressure that barometers measure, which is then expressed numerically in hectopascals - denoted as hPa. The aneroid barometer’s sealed chamber expands and contracts, depending on the amount of atmospheric pressure around it.
Mechanical tools within the barometer then measure how much the chamber expands or contracts and display this using a hand on the face of the instrument. This can allow skippers to interpret the weather and their altitude above sea level.

Which type of pressure indicates good weather?

The numerical hectopascal rating denotes the current number of atmospheres, pointing to either high or low pressure.

High pressure
If you are lucky enough to record a high pressure on your aneroid barometer, you can expect clear, dry, and stable weather. 

Low pressure
Aneroid barometer definitions of low-pressure point to days that require more than a jacket. Low pressure predicts unsettled, wet weather that can be changeable.

Do we use barometers on the RYA Day Skipper and Yachtmaster courses?
Part of the syllabus for both courses course includes meteorology, which is the science of the atmosphere and its phenomena, including the weather and the climate.

It is essential for skippers to have an understanding of the weather as it impacts hugely on the boat’s passage, and therefore using an aneroid barometer, as well as other meteorological tools, is crucial.

As such, the RYA Day Skipper and Yachtmaster courses ensures that skippers have all of the theoretical skills they need to understand atmospheric pressure and predict the weather- as well as having the confidence to change their passage plans accordingly.

As well as being able to repeat the theory lessons until you are fully comfortable with the material, you can also practice using an aneroid barometer on the water and in your home. You can install an aneroid barometer in your house and check it daily to predict the weather! Just be sure to install the barometer somewhere it will not be knocked, and away from all possible heat sources, including radiators, fireplaces, and log burners, as heat can affect its function.

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