A barograph is a barometer that is outfitted with a writing arm that records rises and falls in barometric pressure on a rotating scroll of paper. Barographs were once the preeminent tool of weather forecasters, mariners and others who closely followed pressure changes over time. Collecting and studying the scrolls provided great insight into fronts, weather systems and general atmospheric changes, and it allowed a record of these patterns to be kept for posterity. In the modern era, computerized records and digital barographs have largely replaced the self-recording versions, although antique models still are prized for their craftsmanship.
Barometers have been around since at least the mid-1600s, but early models were quite rudimentary. Most were mercury-filled capsules that fluctuated with atmospheric shifts. To record weather patterns, barometer owners had to carefully monitor the devices and engage in diligent note-taking. The introduction of the barograph in 1843 provided a more self-contained way of keeping track of barometric changes.
Barograph technology is based on the functionality of aneroid barometers. Aneroid barometers were some of the first so-called “advanced” barometers, because they depended not on rising liquid but rather on an aneroid capsule cell, which works in many ways like an alloy battery. These barometers indicate pressure changes with a spinning dial that is synchronized to numbers on a flat face that resembles a clock.
In a barograph, the aneroid’s central dial is oriented outward, toward a roll of paper. Most of the time, the barometer in not large enough to reach the paper. For this reason, most barographs feature stacks of barometers, usually four to seven of them, depending on size. The top barometer holds the recording arm.
A roll of barograph paper stands immediately adjacent to the barometer stack in most models. The roll turns slowly, usually powered by quartz clock movement, and the barometers are fixed. As the pressure changes and the arm moves, it makes a mark on the paper.
The earliest barograph was outfitted with a sharp metal arm that recorded pressure by making scratches on a roll of foil. More modern examples use felt-tip pens on graph paper that is optimized to indicate the day, the time and the relative measurement of pressure. The majority of barograph charts record a full week at a time, although the specifics vary from device to device.
Barographs have special importance at sea, where predicting weather patterns is often essential for safe navigation. Transporting an ordinary barograph on deck often proves to be troublesome, however, because the constant movement of the ship skews the device’s recording mechanism. For this reason, a specific marine barograph has been developed, in which the recording device and charting paper both are specially anchored and weighted to withstand a great deal of surface motion.
Most of recording barographs still on the market are antique models. New barographs are almost exclusively digital, which allows scientists and forecasters to digitize, amalgamate and quickly compare stored charts. The workmanship that is demonstrated in many of the original models was so precise that many of them still operate in perfect working order, and they often command very high prices.