Inches of water
Inches of water (inH₂O) is a pressure unit from the Imperial system. An inch of water is defined as the pressure from a one inch column of water at 4°C (39.2°F) (which is the temperature at which water is densest).
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.