Atmospheric pressure
On average, a column of air with a cross-sectional area of 1 square centimetre (cm2), measured from the mean (average) sea level to the top of Earth's atmosphere, has a mass of about 1.03 kilogram and exerts a force or "weight" of about 10.1 newtons, resulting in a pressure of 10.1 N/cm2 or 101 kN/m2 (101 kilopascals, kPa).
A column of air with a cross-sectional area of 1 in2 would have a weight of about 14.7 lbf, resulting in a pressure of 14.7 lbf/in2.
This is the atmospheric pressure normally given in weather reports on radio, television, and newspapers or on the Internet.
[citation needed] The altimeter setting in aviation is an atmospheric pressure adjustment.
In the United States, Canada, and Japan altimeter setting is reported in inches of mercury (to two decimal places).
The United States and Canada also report sea-level pressure SLP, which is adjusted to sea level by a different method, in the remarks section, not in the internationally transmitted part of the code, in hectopascals or millibars.
The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 1,050 hPa (15.2 psi; 31 inHg), with record highs close to 1,085 hPa (15.74 psi; 32.0 inHg).
The lowest measurable sea-level pressure is found at the centres of tropical cyclones and tornadoes, with a record low of 870 hPa (12.6 psi; 26 inHg).
Pressure (P), mass (m), and acceleration due to gravity (g) are related by P = F/A = (m*g)/A, where A is the surface area.
Pressure is proportional to temperature and inversely related to humidity, and both of these are necessary to compute an accurate figure.
At low altitudes above sea level, the pressure decreases by about 1.2 kPa (12 hPa) for every 100 metres.
The values in these equations are: Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate.
The highest adjusted-to-sea level barometric pressure ever recorded on Earth (above 750 meters) was 1,084.8 hPa (32.03 inHg) measured in Tosontsengel, Mongolia on 19 December 2001.
[10] The highest adjusted-to-sea level barometric pressure ever recorded (below 750 meters) was at Agata in Evenk Autonomous Okrug, Russia (66°53' N, 93°28' E, elevation: 261 m, 856 ft) on 31 December 1968 of 1,083.8 hPa (32.005 inHg).
[11] The discrimination is due to the problematic assumptions (assuming a standard lapse rate) associated with reduction of sea level from high elevations.
[13] The lowest non-tornadic atmospheric pressure ever measured was 870 hPa (0.858 atm; 25.69 inHg), set on 12 October 1979, during Typhoon Tip in the western Pacific Ocean.
[14] One atmosphere (101.325 kPa or 14.7 psi) is also the pressure caused by the weight of a column of freshwater of approximately 10.3 m (33.8 ft).
Conversely, 10.3 m is the maximum height to which water can be raised using suction under standard atmospheric conditions.
A typical gas-using residential appliance in the US is rated for a maximum of 1⁄2 psi (3.4 kPa; 34 mbar), which is approximately 14 w.g.
Similar metric units with a wide variety of names and notation based on millimetres, centimetres or metres are now less commonly used.
A rough approximation of elevation can be obtained by measuring the temperature at which water boils; in the mid-19th century, this method was used by explorers.
William Roy, using barometric pressure, was able to confirm Maskelyne's height determinations; the agreement was within one meter (3.28 feet).
