Barometers work on air pressure. Inside an aneroid barometer there is a sealed capsule, often made if copper. Aneroid means that there isn’t any liquid involved. The main component of an aneroid barometer is the capsule or box, the capsule is evacuated (a vacuum) and the skin of the capsule is moved by the ambient air pressure which through a linkage moves the pointer on the barometer scale.
Usually there is a second pointer on a barometer scale that is moved by hand to the ambient setting of the day whereas the main pointer is driven by the aneroid capsule allowing a comparison between the two.
High pressure depresses the capsule, low pressure allows it to remain in shape, very low pressure may cause it to tighten, so long as the capsule is intact the barometer is an exceptionally accurate piece of equipment.
Because the traditional barometers have mechanical linkages they sometimes get ‘hung up’, a quick firm tap on the glass before taking the reading will usually cause the needle to settle in the correct place.
Why is barometric pressure so important?
Because it tells us what to expect from the weather, high pressure means high skies and fair weather whereas low pressure means rain, and inclement conditions.
By checking the way the ‘glass is moving’ i.e. the barometer, we are able to say whether the weather is likely to improve or deteriorate. Essential information for anyone setting out in a boat!
Falling pressure indicates that the mass of the atmosphere above a point is decreasing, which can happen as the result of various atmospheric processes. In general, weather systems with low pressure at their centers (cyclones) are associated with unsettled weather, clouds and precipitation – rain and strong winds.
Rising barometric pressure is often, though not always, a sign that the weather will soon clear and become or remain fair and sunny, i.e. good boating weather.
Air pressure moves from high to low, so, when warm air rises, cooler air will often replace it, so wind often moves from areas where it’s colder to areas where it’s warmer. The greater the difference between the high and low pressure or the shorter the distance between the high-and low-pressure areas, the stronger the wind will blow.
From any boating or commercial sea-going persons point of view knowing how to use a barometer is essential. Heading out into dangerous weather is only for the brave (or foolish) and those who have no choice. For leisure sailors and water sporting buffs a good working barometer can tell you if the conditions are going to stay good, improve, or deteriorate, if it is the latter, please stay on shore!