The barometer is the instrument that measures atmospheric pressure and is, historically, the most important weather forecasting tool. Invented by Evangelista Torricelli in 1643 using an inverted mercury tube, the barometer demonstrated that the atmosphere has weight — the column of mercury rose to about 760 mm (equivalent to 1013.25 hPa) as the air pressed down on the mercury reservoir.
Three main types exist: the mercury barometer (Torricelli tube — very precise but abandoned due to mercury toxicity), the aneroid barometer (a sealed metal capsule that deforms under pressure, moving a needle on a dial — the classic wall barometer found in homes and ships), and the digital barometer (using piezoresistive or capacitive sensors — the most common today, built into weather stations, smartphones, and even smartwatches with altimeter functions).
For weather prediction, the trend is far more informative than the absolute value: a rapid drop (>3 hPa in 3 hours) warns of an approaching depression bringing wind and rain, while a steady rise signals the return of an anticyclone and settled weather. The traditional phrase "the barometer is falling" is synonymous with deteriorating weather. Marine barometers have dial markings — "Very Dry", "Fair", "Change", "Rain", "Storm" — based on these trends. Today, barometric sensors embedded in smartphones make pressure data accessible to everyone, and many weather apps display the pressure tendency graph.