The Italian physicist Evangelista Torricelli was the first to discover air pressure, in the 17th century. Torricelli also developed the mercury barometer, an instrument used to measure air pressure. The measurement of air pressure is used for many things, including blowing up tires and predicting the weather.
Air pressure is the weight and force of air pressing down on the earth. Air pressure constantly changes due to weather systems and altitude. Low air pressure usually signals stormy weather, while high air pressure typically means clear skies and calm weather. Using a barometer to measure air pressure helps meteorologists accurately predict the weather.