When measuring air pressure, sometimes inches of mercury are used, and sometimes millibars. To convert inches of mercury to millibars, multiply the number of inches by 33.8637526 (or, 33.86 will give you an accurate enough measurement). To do the calculation in reverse, multiply millibars by 0.0295301 (or 0.03 for an estimate) to give you inches of mercury.

A barograph resembles the more familiar seismograph, but instead of tracking earthquakes it draws lines on paper indicating changes in air pressure.