Pressure changes signal weather changes. A barometer measures that pressure. Early mercury barometers used a glass tube inverted in a dish of mercury; the height of the column indicated the weight of the air above.

Modern instruments include aneroid types with a sealed metal capsule that expands or contracts, and electronic sensors that convert pressure into digital readings. All require careful calibration and placement away from drafts or temperature extremes.

A rising barometer often points to improving weather; a falling one warns of approaching systems. Ships, airports, and home weather stations still rely on these instruments. Some digital models also log trends over hours or days.

• Measures atmospheric pressure
• Mercury, aneroid, and electronic types
• Rising values often mean clearer weather
• Essential for forecasting and altitude measurement