What causes low barometric pressure?

Other vacuum units occasionally encountered in the literature include micrometers of mercury, the barometric scale, or as a percentage of atmospheric pressure in bars or atmospheres. Low vacuum is measured in the United States also in inches of mercury (inHg) below atmospheric pressure. "Below atmospheric" means that the absolute pressure is equal to the atmospheric pressure (29.92 inHg) minus the vacuum pressure in inches of mercury. (This is effectively a gauge pressure.)Thus a vacuum of 26 inHg is equivalent to an absolute pressure of 29.92 inHg − 26 inHg = 4 inHg.