A barometer was a weather instrument used in meteorology to measure barometric pressure, read in terms of units of millimeters of mercury or inches of mercury.
The atmospheric pressure of Taurus II was 70 mmHg. (TOS: "The Galileo Seven")
In 2376, Tom Paris joked that Lieutenant Commander Tuvok would have made a good barometer, as his Vulcan physiology was sensitive to neutronic gradients. (VOY: "Fair Haven")