Barometers are instruments used to measure atmospheric pressure. Divide the reading in inches of mercury by 0.0295 to convert it to a pressure in millibars. Some continental barometers are calibrated in centimetres of mercury. To convert this to a reading in millibars, first multiply the number of cm by 2.54 to get to inches of mercury, and then divide by 0.0295 as above. More information on units .
Atmospheric pressure varies with height above sea level as well as with atmospheric conditions.
Pressure maps shown on the television and in newspapers show pressures which are corrected to the values which would be measured at sea level. If a barometer's pressure reading is to be compared with pressure maps distributed by the Media, then it first needs to be set to read sea level pressure. The easiest way to set a barometer is to adjust it to give the same reading as that on a weather chart for your area at the same time. This is best done in atmospheric conditions when the pressure varying only very slowly. Don't adjust it when conditions are unsettled or windy.