User:Milton Beychok/Sandbox

Atmospheric pressure at a given point in Earth's atmosphere is the downward force per unit area exerted upon a horizontal surface at that point by the weight of air above that surface. Atmospheric pressure at sea level will vary with geographic location, the temperature and humidity of the air and with the weather conditions. In fact, a change in the sea level atmospheric pressure usually indicates an upcoming change in the weather. Since air temperature and humidity as well as the weather change with the annual seasons (i.e., winter, spring, summer and fall), the sea level atmospheric pressure changes with the seasons.

Standard values of atmospheric pressure at sea level

In 1954, the 10th Conférence Générale des Poids et Mesures (CGPM) adopted a standard atmosphere for general use and defined it as being precisely 1,013,250 dynes per square centimeter (101,325 Pa).BIPM Definition of the standard atmosphere</ref> This value was intended to represent the average atmospheric pressure at the average sea level at the latitude of Paris, France, and as a practical matter, truly reflects the average sea level pressure for many of the industrialized nations (those with latitudes similar to Paris).

The International standard atmosphere (ISA) as used by International Civil Aviation Organization (ICAO) is also defined as being 101,325 Pa.

In chemistry, the original definition of "Standard Temperature and Pressure" by the International Union of Pure and Applied Chemistry (IUPAC) was a reference temperature of 0 °C (273.15 K) and pressure of 101.325 kPa. However, in 1982, the IUPAC recommended that for the purposes of specifying the physical properties of substances, the "standard pressure" should be defined as 100,000 Pa (1 bar).

However, the sea level atmospheric pressure of 101,325 Pa (as defined by the CGPM and by the IUPAC prior to 1982) continues to be very commonly used and its unit of measurement unit is the atmosphere (symbol: atm) defined as being 101,325 Pa. The difference between an atm and a bar is about 1%, which is not significant for many applications, and is within the error range of many common pressure gauges.

 The following units are equivalent, but only to the number of decimal places displayed: 760 mmHg (torr), 29.92 inHg, 14.696 PSI, 1013.25 millibars.  One standard atmosphere is standard pressure used for pneumatic fluid power (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries. 

In 1999, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of specifying the properties of substances, “the standard pressure” should be defined as precisely 100 kPa (≈750.01 torr) or 29.53 inHg rather than the 101.325 kPa value of “one standard atmosphere”. This value is used as the standard pressure for the compressor and the pneumatic tool industries (ISO 2787). (See also Standard temperature and pressure.) In the United States, compressed air flow is often measured in "standard cubic feet" per unit of time, where the "standard" means the equivalent quantity of moisture at standard temperature and pressure. For every 1,000 feet you ascend the atmospheric pressure decreases 4%. However, this standard atmosphere is defined slightly differently: temperature = 20 C ( {{{3}}}), air density = 1.225 kg/m³ (0.0765 lb/cu ft), altitude = sea level, and relative humidity = 20%. In the air conditioning industry, the standard is often temperature = 0 C ( {{{3}}}) instead. For natural gas, the petroleum industry uses a standard temperature of 15.6 C ( {{{3}}}), pressure 101.56 kPa ( psi). (air pressure)