An aneroid barometer is a device that measures atmospheric pressure without using any liquid. Instead of the column of mercury found in traditional barometers, it relies on a small, sealed metal chamber that physically expands and contracts as air pressure changes. The word “aneroid” comes from the Greek for “without liquid,” which is exactly what sets it apart from older mercury-based designs.
How It Works
At the heart of every aneroid barometer is a hollow metal capsule with corrugated (ridged) sides. Most or all of the air has been pumped out of this capsule, creating a partial vacuum inside. Because the capsule is nearly empty, it responds sensitively to the weight of the atmosphere pressing on it from the outside. When air pressure rises, the atmosphere squeezes the capsule slightly flatter. When pressure drops, the capsule springs back and expands.
A system of small levers and linkages amplifies these tiny movements and transfers them to a pointer on a circular dial, much like the face of a clock. The dial is marked with pressure values so you can take a reading at a glance. Some models also include a second “reference” needle you can set manually to track how pressure has changed since the last time you checked.
One inherent challenge is temperature. Metal expands when it warms up and contracts when it cools, which could produce false readings if the capsule changed size for reasons that had nothing to do with air pressure. Better-quality aneroid barometers include a compensating element, often a small strip made of two bonded metals that flex in the opposite direction of temperature-related errors, canceling them out.
A Brief History
Mercury barometers had been around since the 1640s, but they were fragile, bulky, and filled with toxic liquid. In 1843, French inventor Lucien Vidie built the first practical metallic barometer. He received his initial patents in 1844. Vidie’s design used a corrugated metal diaphragm supported by 33 tiny helical springs, all sealed inside a strong evacuated metal box, according to the Smithsonian’s instrument collection. That core concept, a springy vacuum capsule driving a pointer, remains essentially unchanged in the aneroid barometers sold today.
Units on the Dial
Depending on where you live and what the barometer is designed for, the dial may show one or more pressure scales:
- Inches of mercury (inHg): The standard in the United States. Standard sea-level pressure is about 29.92 inHg.
- Millibars (mb) or hectopascals (hPa): Used internationally and in meteorology. Standard sea-level pressure is 1013.25 mb. One millibar equals one hectopascal.
- Millimeters of mercury (mmHg): Common on older European instruments. Standard pressure is about 760 mmHg.
Many home barometers display inches of mercury on the outer ring and millibars on an inner ring. To convert between them: 1 inHg equals roughly 33.86 mb.
Using Pressure Trends to Read the Weather
A single pressure reading tells you relatively little. What matters is the direction and speed of change over several hours. NOAA’s traditional guidelines for interpreting a barometer break it down clearly.
Falling pressure generally signals worsening weather. In warm conditions, a drop can indicate approaching thunderstorms. A sudden, steep fall points to high winds. If it’s already raining and the barometer keeps falling, expect more prolonged wet weather. The lowest readings tend to come when wind and rain arrive together.
Rising pressure points toward clearing skies, but the pace matters. A steady rise followed by consistently high readings suggests fair weather is settling in for a day or two. A sudden spike, on the other hand, often means the good weather won’t last long. In winter, rising pressure can signal frost or snow rather than sunshine.
If the needle keeps bouncing around with no clear trend, the weather itself is likely to be unsettled. As a general rule, upward motion means conditions are improving, and downward motion means they’re deteriorating. Checking your barometer at the same time each morning and evening gives you a reliable sense of the trend.
Where Aneroid Barometers Are Used
The most familiar version hangs on a wall at home, often built into a decorative wooden case alongside a thermometer and hygrometer. But the same aneroid principle shows up in far more critical places. Aircraft altimeters are essentially aneroid barometers calibrated to read altitude instead of pressure. Because atmospheric pressure drops at a predictable rate as you climb, the instrument translates that pressure change into feet or meters above sea level.
Hikers and mountaineers carry pocket-sized aneroid barometers (or wristwatch versions with built-in sensors) for the same reason: tracking elevation and watching for sudden pressure drops that warn of incoming storms. Weather stations use precision aneroid instruments called barographs, which attach the capsule mechanism to a pen that draws a continuous pressure trace on a rotating paper drum, creating a visual record of pressure changes over days or weeks.
Aneroid vs. Mercury Barometers
Mercury barometers are extremely accurate and were the gold standard for centuries. A glass tube filled with mercury sits inverted in a dish of mercury, and the height of the column directly reflects atmospheric pressure. The trade-off is obvious: they’re tall (about 30 inches), heavy, breakable, and contain a hazardous substance. Most countries have phased them out of routine use.
Aneroid barometers are compact, portable, and safe. They need no liquid and can be mounted in any orientation. The downside is that they can drift over time. The metal capsule gradually loses some of its springiness, so readings may shift. Occasional calibration against a known reference (like the pressure reported by your local weather station) keeps them accurate. For most home users, checking once or twice a year is enough.
Tips for Getting Accurate Readings
Place your barometer indoors, away from direct sunlight, heating vents, and exterior doors where gusts of wind can cause momentary pressure fluctuations. Mount it at eye level so you can read the dial straight on rather than at an angle. If your model has a set-hand (reference needle), adjust it to match the current reading each time you check. The gap between the reference needle and the active needle the next time you look tells you exactly how much pressure has changed.
If your barometer’s reading doesn’t match the official pressure for your area, keep in mind that weather stations report pressure corrected to sea level. If you live at higher elevation, the raw pressure at your location will always be lower than the reported value. Many aneroid barometers have a small adjustment screw on the back that lets you calibrate the reading to match the sea-level-corrected figure, which makes comparisons with forecasts much simpler.