Pressure is a physical force exerted over a surface. Specifically, it is defined as a force (F) divided by unit area (A). When expressed relative to a pure vacuum, this quotient is referred to as absolute pressure. When referred to relative to atmospheric pressure it is called gauge pressure. Pressure is an important concept in many chemical and physical processes. For example, many types of reactor vessels and boilers must be monitored to ensure they do not exceed specific pressure limits, lest they rupture. In atmospheric science, an understanding of air pressure, also known as barometric pressure, is critical for making weather predictions.

Pressure is expressed in various units depending on the application. For example, air pressure is usually given in terms of atmospheres (atm), torr, bars, or millimeters of mercury (mm Hg at 0°C). (Although air pressure varies with altitude and weather conditions, on average the...