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How does a barometer work?

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Quick answer

A barometer works by balancing the push of the air against something that moves. In a mercury barometer, air pressure holds up a mercury column about 760 mm tall at sea level. In an aneroid barometer, a sealed metal capsule flexes as pressure changes, and levers turn that motion into a needle reading.

A barometer measures air pressure by letting the weight of the atmosphere push against something that responds measurably, such as a column of mercury or a sealed metal capsule. Rising or falling readings show whether air pressure is changing, which helps forecasters predict weather and lets instruments estimate altitude.

Step by step

  1. Air molecules above the instrument press down with a force set by the weight of the atmosphere overhead.
  2. That pressure pushes on the open mercury dish or on the flexible walls of a sealed metal capsule.
  3. Higher pressure forces the mercury column taller or squeezes the capsule; lower pressure lets the column drop or the capsule expand.
  4. A scale beside the tube, or levers linked to a needle, converts that change into a reading in hectopascals or millimetres.
  5. Forecasters track how fast the reading changes, since a quick fall often warns of an approaching storm.

Key things to understand

  1. The atmosphere has weight, pressing down at sea level with about 1,013 hectopascals of pressure on average.
  2. In a mercury barometer, that air pressure holds up a column of mercury roughly 760 mm tall inside a sealed glass tube.
  3. An aneroid barometer instead uses a sealed, partly emptied metal capsule that squeezes or swells as outside pressure changes.
  4. Levers and a spring magnify the capsule's tiny movements so a needle sweeps across a printed dial.
  5. Falling pressure often signals approaching cloud and rain, while rising pressure usually means settled, clearer weather.

Labelled diagram

  1. Air pressure

    The weight of the atmosphere pushes down on the instrument from all sides.

  2. Mercury column

    Air pressure on the dish holds mercury about 760 mm up the sealed tube.

  3. Aneroid capsule

    A sealed, partly evacuated metal box flexes inward or outward with pressure.

  4. Levers and needle

    Magnify the capsule's tiny movement into a sweep across the dial.

Inside a mercury and an aneroid barometer

Real-world examples

  • India Meteorological Department weather stations record pressure readings that feed cyclone and monsoon forecasts.
  • A hiker's watch uses a built-in pressure sensor to estimate altitude while climbing a mountain trail.
  • A ship's crew watching an aneroid barometer drop sharply prepares for strong winds and rough seas.
  • Aircraft altimeters are aneroid barometers calibrated to show height instead of pressure.

Common mistake

✗ Myth: A barometer's 'Rain' or 'Fair' label directly predicts the weather.

✓ Fact: Those dial words are rough guides. The direction and speed of pressure change, combined with local conditions, say far more about coming weather than any single reading.

People also ask

What does a falling barometer mean?
A falling barometer means air pressure is dropping, usually because a low-pressure system is approaching. Rising air in a low cools and forms clouds, so falling pressure often comes before cloud, wind and rain. A very rapid fall can signal a strong storm or cyclone nearby.
How does an aneroid barometer work?
An aneroid barometer contains a sealed metal capsule with most of the air removed, held open by a spring. When outside air pressure rises, it squeezes the capsule slightly; when pressure falls, the capsule expands. Levers magnify these tiny movements and turn a needle across a calibrated dial.
What is normal barometric pressure?
Average sea-level air pressure is about 1,013 hectopascals, equal to 760 mm of mercury or 29.92 inches. Everyday readings at sea level usually fall between roughly 980 and 1,040 hectopascals. Readings are adjusted to sea level so that places at different heights can be compared on weather maps.
Who invented the barometer?
Italian physicist Evangelista Torricelli built the first mercury barometer in 1643. He filled a glass tube with mercury, upended it in a dish, and saw the column settle at a height the air pressure could support.
Why use mercury instead of water?
Mercury is about 13.6 times denser than water, so air pressure supports a column only around 76 cm tall. A water barometer would need a tube about 10 metres high, far too tall for practical use.
How does a phone measure air pressure?
Many smartphones contain a tiny electronic pressure sensor with a thin silicon membrane over a sealed cavity. Pressure bends the membrane, changing its electrical resistance, and the phone converts that signal into a pressure or altitude reading.
Why does air pressure fall as you climb?
Air pressure comes from the weight of air above you, so climbing leaves less air overhead. Near sea level pressure drops by roughly 12 hectopascals for every 100 metres of height, which is why barometers double as altimeters.

Part of the guide: Water & Weather Phenomena →