To convert inches of mercury to millibars, multiply the number of inches by 33.8637526. For instance, 31.01 inches of mercury equals slightly more than 1,050 millibars. The National Weather Service has an automatic pressure converter that takes measurements in inches, millibars and millimeters of mercury and calculates figures for each one. PC Weather Products features a chart that converts figures from 26.58 inches to 31.01 inches into millibars.
Most National Weather Service measurements are given in inches of mercury. At sea level, standard air pressure is 29.92 inches of mercury and 1,013 millibars. Inches of mercury is measured in how high a column of mercury rises in a barometer.
One millibar is equivalent to one-thousandth of a single bar, or 1,000 dynes. A dyne is the amount of force it takes to accelerate a one-gram object one centimeter per second squared. Most meteorological measurements take pressure readings from 100 millibars to 1,050 millibars.
The term bar comes from the Greek word "baros," meaning weight. Millibars are often used to measure pressure aloft as opposed to on the ground. In other parts of the world, millibars are known as hectopascals.
Air pressure changes gradually under normal conditions. Barometric readings change due to air density and temperature as the atmosphere cools and warms throughout the day and night. When the weather is calm, sunrise and sunset are the times of day when air pressure changes the most.