Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1013.25 millibars (unit now deprecated), 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.
In most circumstances, atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. As elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation. Because the atmosphere is thin relative to the Earth's radius—especially the dense atmospheric layer at low altitudes—the Earth's gravitational acceleration as a function of altitude can be approximated as constant and contributes little to this fall off. Pressure measures force per unit area, with SI units of pascals (1 pascal = 1 newton per square metre, 1 N/m2). On average, a column of air with a cross-sectional area of 1 square centimetre (cm2), measured from 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.
2001Dec, 19
A record high barometric pressure of 1085.6 hPa (32.06 inHg) is recorded at Tosontsengel, Khövsgöl, Mongolia.
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Events on 2001
- 6May
Pope John Paul II
During a trip to Syria, Pope John Paul II becomes the first pope to enter a mosque. - 11Sep
September 11 attacks
Two hijacked aircraft crash into the World Trade Center in New York City, while a third smashes into The Pentagon in Arlington County, Virginia, and a fourth into a field near Shanksville, Pennsylvania, in a series of coordinated suicide attacks by 19 members of al-Qaeda. A total of 2,996 people are killed. - 14Sep
September 11, 2001 attacks
Historic National Prayer Service held at Washington National Cathedral for victims of the September 11 attacks. A similar service is held in Canada on Parliament Hill, the largest vigil ever held in the nation's capital. - 17Sep
September 11 attacks
The New York Stock Exchange reopens for trading after the September 11 attacks, the longest closure since the Great Depression. - 7Oct
War in Afghanistan (2001-2014)
The Global War on Terrorism begins as a result of the September 11 attacks. The U.S. invasion of Afghanistan initiates with an air assault and covert operations on the ground.