A barometer is a scientific instrument that is used to measure air pressure. Pressure tendency, which is derived from barometric readings, can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis to help find surface troughs, pressure systems and frontal boundaries.
Barometers and pressure altimeters (the most basic and common type of altimeter) are essentially the same instrument, but used for different purposes. A pressure altimeter is used to estimate altitude by measuring the pressure of the atmosphere surrounding the altimeter and comparing the result to the expected atmospheric pressure at various altitudes, while a barometer is kept at a constant altitude and measures subtle pressure changes caused by weather and elements of weather. The average atmospheric pressure on the Earth's surface varies between 940 and 1040 hPa (mbar). The average atmospheric pressure at sea level is 1013 hPa (mbar).
