Inches of Mercury
Inches of Mercury (inHg) are used to measure pressure in the imperial system. Traditionally this unit is used in the United States for barometric pressure in weather reports. It is the pressure from a column of mercury exactly one inch high at standard gravity.
mm of mercury
The millimeter of mercury (mmHg) is pressure unit traditionally used in meteorology. It is the pressure from a 1 mm column of mercury at 0°C under normal gravity.