D.C. It was first used in weather reporting in 1914. Here is some background.
Pressure is defined as force per unit area.
The force exerted by a column of mercury is the product of the mass in the column and the acceleration due to gravity. Gravity varies with distance from the center of the earth and, because the earth is spinning, with latitude. At identical pressures, therefore, the height of a column of mercury varies slightly with latitude and elevation. The observed height of a column of mercury must be corrected for these effects so that the corrected height is equivalent to the true air pressure. Bjerknes realized that this made an inch of mercury in a barometer a flawed, varying unit of air pressure.