Instead of using pounds per square inch, barometers in the U.S. measure the pressure in inches of mercury. This is how high the pressure would push mercury into a tube that has the top sealed off from the air. A reading of 29.92 inches of mercury is the same as 14.7 pounds per square inch. Rising air pressure signals that clear weather is likely. Our storms and fronts index has links to a lot more information on the relation between air pressure and weather.
Millibars are a direct measure of pressure, like pounds per square inch, but in the metric system.
A bar is a force of 100,000 Newtons acting on a square meter, which is too large a unit to be a convenient measure of Earth's air pressure. Inches of mercury, the number used for surface air pressure in the U.S., is not a direct measure of pressure. Instead, inches of mercury tell you how high the pressure pushes the mercury in a barometer . The use of direct pressure measurements goes back to the late 19th century when the great Norwegian meteorologist Vilhelm Bjerknes, the leader in making meteorology a mathematical science, urged weather services to use direct pressure measurements because they can be used in the formulas that describe the weather. The standard atmospheric pressure at the Earth's surface of 1013.25 millibars is equal to 101,325 Pascals.