A barometer is the instrument that measures atmospheric pressure and inspired the unit called the bar.
The first successful barometer was developed by Evangelista Torricelli in 1643. He used a sealed glass tube filled with mercury and inverted it into a mercury reservoir. Air pressure pushed on the reservoir, supporting a column of mercury whose height changed with atmospheric pressure.
Modern instruments may use mercury, aneroid capsules, or electronic sensors. Meteorologists use pressure readings to help identify weather systems: falling pressure often accompanies unsettled conditions, while rising pressure commonly occurs with improving weather.
The bar is a metric unit of pressure equal to 100,000 pascals. Standard atmospheric pressure at sea level is approximately 1.013 bar, or 101,325 pascals. A barometer is not the same as a thermometer, which measures temperature.