The lowest recorded non-tornadic atmospheric pressure, 87.0 kPa (870 mbar or 25.69 inHg), occurred in the Western Pacific during Typhoon Tip.
Typhoon Tip, known in the Philippines as Typhoon Warling, was the largest and most intense tropical cyclone ever recorded. The forty-third tropical depression, nineteenth tropical storm, twelfth typhoon, and third super typhoon of the 1979 Pacific typhoon season, Tip developed out of a disturbance within the monsoon trough on October 4 near Pohnpei in Micronesia. Initially, Tropical Storm Roger to the northwest hindered the development and motion of Tip, though after the storm tracked farther north, Tip was able to intensify. After passing Guam, Tip rapidly intensified and reached peak sustained winds of 305 km/h (190 mph) and a worldwide record-low sea-level pressure of 870 hPa (25.69 inHg) on October 12. At its peak intensity, Tip was the largest tropical cyclone on record, with a wind diameter of 2,220 km (1,380 mi). Tip slowly weakened as it continued west-northwestward and later turned to the northeast, in response to an approaching trough. The typhoon made landfall in southern Japan on October 19, and became an extratropical cyclone shortly thereafter. Tip's extratropical remnants continued moving east-northeastward, until they dissipated near the Aleutian Islands on October 24.
U.S. Air Force aircraft flew 60 weather reconnaissance missions into the typhoon, making Tip one of the most closely observed tropical cyclones. Rainfall from Tip indirectly led to a fire that killed 13 Marines and injured 68 at Combined Arms Training Center, Camp Fuji in the Shizuoka Prefecture of Japan. Elsewhere in the country, the typhoon caused widespread flooding and 42 deaths; offshore shipwrecks left 44 people killed or missing.
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.