Handy barometric pressure definition Air Pressure? If you had a good barometric pressure definition you'd determine what that means and how significant it really is. And you would know how hard does the air press against us. Under Pressure Pressure drops as you to up in altitude. How quickly? A good rough estimate is that for every 50,000 feet, about 15 km, you go up, the pressure drops by around 90% from what it was before. The most familiar ways to define air pressure are either in inches of mercury supported, or mm of mercury (used for blood pressure), or force per unit area. Common examples of units in a barometric pressure definition are PSI, pounds per square inch, and Pascals, which are Newtons per square metre. A Newton is roughly a quarter of a pound. And scientists have named Pascals and Newtons after dead guys, like the one at the top of this page. Meteorologists, ever so odd, like to use millibars. So here's another barometric pressure definition: A millibar is 100 Newtons per square metre and is also known as a hectaPascal, hPa. Standard pressure Finally, we also use a unit of normal barometric pressure known as an atmosphere. As you might guess, it has a value equal to a typical sea-level atmospheric pressure. One atmosphere is 101,325 Pa, but the International Union of Pure and Applied Chemistry, preferring round figures, declares it as 100,000 Pa, or 100 kPa. Real atmospheric pressures vary from these amounts because of