A barometer measures the atmospheric pressure or the pressure in the air. The atmospheric pressure is the weight of a column of air from where the barometer is and up to the top of the atmosphere. The higher up you get, the lower the pressure. The pressure in the air decreases with altitude. The weather conditions affect the measuring, but as a principle we can say that the barometric pressure diminishes with 1 hecto Pascal (hPa) per 8 meters. That is to say: If you live 80 meters above sea level, the pressure will be 10 hPa lower than at sea level.
There are different types of barometers: mercury barometers, water barometers and aneroid barometers (no fluid), big ones and pocket versions etc.
A barograph is an instrument that transfers the measuring of the atmospheric pressure from a barometer to a roll of paper. On this paper you could then read the air pressure over time.
The Fram Museum’s barograph is made by Richard Frères in Paris and was made in the 1890s.
When Roald Amundsen and the other participants on the South Pole expedition in 1911 needed to find their height above sea level (altitude), they had to know the atmospheric pressure. That is why they brought both small and big barometers in addition to the tables and almanacs necessary for their calculations. Amundsen brought furthermore a hypsometer to find the atmospheric pressure. This is an instrument that measures the boiling point of a fluid, for instance water. Water and other fluids boil at lower temperatures when the air pressure decreases, and therefore the hypsometer will indicate what the pressure is at a given place and with that indicate the actual height above sea level. “The boiling this evening gave 9 200 feet,” says Amundsen in his diary on the 27th of November 1911. Two weeks after this he is wondering if Shackleton on his Nimrod Expedition 1907-09 could have carried any hypsometer at all on his way to the Pole where he reached 88⁰S: “And now the hypsometer this evening shows t