Pressure Right Now — Your Live Barograph
This virtual barograph brings that classic instrument online: it charts live pressure data for any location in your browser, tracing the trend just as a mechanical recording barometer would on paper. The barograph above is drawing a live record of atmospheric pressure for your location. As soon as the page finds you — or you search for a city — it pulls the last two days of pressure readings and traces them across the drum, just as a real clockwork barograph would ink them onto its chart. The pen arm swings to rest at the current pressure, and the line behind it shows exactly how the air has been behaving: climbing toward fair weather, sliding toward a storm, or holding steady. Beneath the drum you will find the current reading in hectopascals, the three-hour trend that forecasters watch most closely, the range over the last day, and a plain-language outlook.
This is the crucial difference between a barograph and an ordinary barometer. A barometer gives you a single number. A barograph gives you the story — and in weather, the direction and speed of change usually matter more than the value itself.
What a Barograph Is
A barograph is a barometer that writes. Inside the case sits the same pressure-sensing element as an aneroid barometer: a small sealed metal capsule (or a stack of them) that expands and contracts as the air pressure outside changes. But instead of that motion turning a single needle, it is amplified through a system of levers to a long arm carrying an ink pen. The pen rests against a sheet of graph paper wrapped around a drum, and the drum is turned slowly by a clockwork motor — one rotation per day on some instruments, one per week on others. As pressure lifts the pen up and down, and the drum carries the paper sideways beneath it, the pen inks a continuous curve of pressure against time.
The classic barograph, in its glass-and-brass case, was a fixture of ships' chart rooms, coastal weather stations and gentleman-scientists' studies from the nineteenth century onward. It needed no electricity, ran for a week on a wound spring, and produced a paper record that could be filed, compared and studied. Many are still working a century later.
Reading the Trace — Rising, Falling and Steady
The shape of the line is the whole point. A trace climbing from left to right means pressure is rising, and rising pressure is the signature of high pressure building overhead: sinking air, dispersing cloud, lighter winds and generally improving, settled weather. A line sloping downward means pressure is falling, the hallmark of an approaching low-pressure system or front bringing cloud, rain and strengthening winds. A roughly flat line means pressure is steady and the current weather is likely to persist.
Just as important as the direction is the steepness. A gentle slope means a slow, gradual change; a steep slope means rapid change and, usually, more vigorous weather. A sharply plunging trace is one of the oldest and most reliable storm warnings there is — sailors learned to respect a fast-falling glass long before anyone understood why it worked.
The Three-Hour Tendency
Professional forecasters pay special attention to the "pressure tendency" — how much the pressure has changed in the last three hours, and in which direction. It is reported in every synoptic weather observation for exactly the reason a barograph makes visible: the recent trend is a powerful short-term predictor. A fall of more than about 3 hectopascals in three hours signals a rapidly deepening system and often precedes strong winds; a steady rise of similar size marks a clearing, settling airmass. The live reading on this page calls out the three-hour change so you can apply the same rule the professionals use.
Barograph vs Barometer
An ordinary barometer answers "what is the pressure now?" A barograph answers "what has the pressure been doing, and how fast?" Both read the same atmosphere, but the barograph adds the dimension of time. That is why, for forecasting, the two were traditionally used together: a glance at the barometer for the current value, and a study of the barograph trace for the trend. If you only had one instrument aboard a ship heading into open water, the barograph was the one you wanted — because a storm announces itself in the slope of the line long before it arrives at your position.
Why the Drum Rotates
The rotating drum is what turns a pressure sensor into a recorder. Without it, the pen would simply move up and down over the same spot, overwriting itself into a meaningless vertical smear. By turning the paper steadily sideways — driven by the same kind of clockwork that runs a mantel clock — the drum spreads the pen's motion out along a time axis. On a daily drum, one full turn represents twenty-four hours, and the vertical gridlines on the chart paper mark the hours; on a weekly drum, each turn covers seven days. Change the paper, rewind the clockwork, and the record begins again.
Pressure Units — Hectopascals, Millibars and Inches
Atmospheric pressure is quoted in a few different units that all describe the same thing. The modern scientific and international standard is the hectopascal (hPa), and a typical sea-level pressure is around 1013 hPa. The millibar (mb) is numerically identical to the hectopascal — 1013 mb equals 1013 hPa — and is still widely used in marine and aviation weather. In the United States, pressure is often given in inches of mercury (inHg), where standard sea-level pressure is about 29.92 inHg. The barograph on this page reads in hectopascals, the unit used across most of the world's weather services.
Barograph Uses — Sea, Air and Station
The barograph earned its keep at sea, where an early warning of a falling glass could mean the difference between riding out a storm and being caught by it. It became standard equipment in ships' chart rooms and at coastal and airfield weather stations, where the paper traces were collected as a permanent climate record. Aviators used it to anticipate deteriorating conditions along a route. Even today, when electronic pressure sensors log data digitally, the barograph's way of showing the trend as a shape — instantly readable, no numbers to interpret — keeps the instrument popular with sailors, pilots, weather enthusiasts and anyone who wants to see the weather coming.
Multilingual Pressure Terms
Atmospheric pressure carries different names around the world: pression atmosphérique in French, Luftdruck in German, presión atmosférica in Spanish, pressione atmosferica in Italian, and pressão atmosférica in Portuguese. The instrument itself is a barographe, Barograph, barógrafo or barografo depending on the language — but everywhere, the rising and falling line means the same thing to the weather-wise eye.
A Short History of the Recording Barometer
The barometer itself dates to Evangelista Torricelli in 1643, but it was a static instrument — a column of mercury to be read by eye. The leap to recording pressure came with the aneroid capsule in the nineteenth century, which was compact and dry enough to drive a pen. By the 1860s, clockwork barographs with inked drums were being manufactured commercially, and they quickly spread through the world's growing network of weather observers. For nearly a hundred years, before electronic sensors and continuous digital logging, the paper barograph chart was one of the primary records of a station's weather. The instrument you see traced live on this page is the digital descendant of that long, elegant tradition.