Atmospheric pressure is sometimes expressed in terms of the number of inches in a column of Mercury that are supported by the downward push of the atmosphere. Meteorologists and weather people often do it this way. If the pressure is 1 bar, then the atmosphere will support a column of Mercury which is 29.5 inches tall. The inflation limit for automobile and bicycle tires is usually expressed in psi. The atmospheric pressure at the surface of the Earth is about 14.7 psi.
Atmospheric pressure is measured with a barometer .
Atmospheric pressure decreases with height above the surface of a planet because there is less total mass in the atmosphere above a reference point as the height of the reference point increases. Only the weight of the mass above the reference point contributes to the pressure at the reference height. On a clear and dark night, these moons are visible with binoculars. The nearest of these four Galilean moons to Jupiter is called Io. When the Voyager 2 spacecraft encountered Jupiter in July 1979, a photograph was taken of Io which captured the eruption of an active volcano.