‌Pressure‌ is a word with many connotations, some of them more foreboding than others: ‌Your tire pressure looks low. The pressure is on for the final exam. The barometric pressure drop signals a strong chance of ...‌

Wait. Was that part good news, or was it bad news? You are probably familiar with the term "barometric pressure" (often "atmospheric pressure") if you're a regular watcher or listener of weather forecasts. But have you ever stopped to wonder what its contribution to weather actually is?

To set the stage, we look at how pressure is measured. Like a lot of physical quantities, pressure comes in many units. ‌Pascals‌‌‌, ‌‌measured as kilograms per meter per second squared in base units or Newtons per square meter, are an SI unit of pressure – a metric unit of pressure. There are many other units that can describe pressure from millibars to psi (pounds per square inch), but all of them describe some amount of force distributed over a certain area.

Pressure is often described as a force, and it probably feels like one to you when you ponder the physical concept. That's not quite accurate, but it's close. ‌Force is mass times acceleration‌ (including the acceleration owing to gravity); ‌pressure is force per unit area‌. If you apply force to your skin with the corner of your cell phone and then the same amount of force using a pin, you'll appreciate the difference.

When measuring pressure that results from the weight of air (weight having units of force), the molecules in air are distributed evenly above the surface of the ground, choosing any area allows for the measurement of pressure as long as you know its exact magnitude. This works because the net force is assumed to act vertically downward with respect to this chosen system.

As hinted at already, ‌atmospheric pressure‌ is a measure of the weight of air molecules at ground level over a particular surface area. You are not aware of this pressure because, like all organisms on land, the species to which you belong is adapted to perceive this pressure as "neutral." But it is actually considerable, nearly half that of a well-inflated car tire.

The atmosphere consists mainly of the diatomic gas molecules nitrogen (N₂) and oxygen (O₂). Other players include CO₂ (carbon dioxide) methane (CH₄) and locally varying mounts of water vapor (H₂O); while constituting only a small percentage of the air overall, methane and carbon dioxide exert significant effects as greenhouse gases. which contribute to warming of the planet.

A device called a ‌barometer‌ is used to measure air pressure. (A panoply of pressure gauges exist; the old-style one used to measure blood pressure goes by the easy-to-recall name of ‌sphygmomanometer‌.) The principle is simple; because air has mass, it will tend to push other fluids (liquids or gasses) out of its way with its own weight.

A barometer contains a base reservoir of mercury into which a tube sealed at the top has been inserted. The top of the mercury column naturally rests at the level of normal standard atmospheric pressure, which is about 101,325 pascal (Pa) or 101.325 kilopascals (kPa). If the pressure drops, this is reflected by a lower mercury level, as the liquid is being pushed upward less forcefully; when pressure rises, so does the mercury level.

Blaise Pascal was a French mathematician and scientist who pioneered many fields of measurement and physics. The pascal (Pa) unit of measurement is named him, and it is used to describe the pressure, or force over area, on an object.

Pascal units are not actually convenient for measuring pressure on the scale of most everyday things. As we just saw, once atmospheric pressure (a common measurement on Earth) is over 100,000 Pa. However, pascal units can still be very useful as a unit of measurement because they are a part of the international system of units (SI units). As such, they can be defined in base SI units, and they can be easily scaled to greater values. Defining pascals in terms of other common units can be very useful for conversions:

where N is newtons, m is meters, and J is joules.

The metric system also makes it easy to convert within its own units. This can be useful to scale pascals and SI units to these greater more common measurements while still retaining standard units:

Other units typically used to express atmospheric pressure and the value of normal atmospheric pressure in these units include ‌millimeters of mercury‌, also called ‌torr‌ (760 mmHg or torr); ‌inches of mercury‌ (29.9 in Hg); ‌pounds per square inch‌ (14.4 psi); ‌bars,‌ where 1 bar = 100 kPa (1.013 bars) – sometimes also expressed as millibars (1 mbar = 100 Pa); and the aforementioned ‌standard atmospheric pressure‌, where 1 atm = 101,325 Pa (1 atm is the approximate pressure at sea level).

Pressure is an incredibly useful tool when describing the real world impacts of force on systems and materials. Geophysicists study pressure in the Earth’s mantle and core to better understand tectonic stresses and geological movement. Mechanical engineers use pressure in other measurements like Young’s Modulus, which quantifies the tensile strength of certain materials, and in meteorology, the study of weather, meteorologists use pressure to describe and predict weather patterns.

See the Resources for a calculator that allows you to convert between the aforementioned pressure units, plus a great many more.