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Atmospheric pressure, also known as air pressure or 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 1,013.25 millibars , [ 1 ] 760 mm Hg , 29.9212 inches Hg , or 14.696 psi . [ 2 ]
On the top right corner of the model for a surface weather map is the pressure, showing the last two integer digits of the pressure in millibars, or hectopascals, along with the first decimal. For instance, if the pressure at a certain location is 999.7 hPa, the pressure portion of the station model will read 997.
Measurements of barometric pressure and the pressure tendency (the change of pressure over time) have been used in forecasting since the late 19th century. [66] The larger the change in pressure, especially if more than 3.5 hPa (2.6 mmHg ), the larger the change in weather can be expected.
With a temperature lapse rate of −6.5 °C (-11.7 °F) per km (roughly −2 °C (-3.6 °F) per 1,000 ft), the table interpolates to the standard mean sea level values of 15 °C (59 °F) temperature, 101,325 pascals (14.6959 psi) (1 atm) pressure, and a density of 1.2250 kilograms per cubic meter (0.07647 lb/cu ft).
Barograph – an aneroid barometer that records the barometric pressure over time and produces a paper or foil chart called a barogram; Barometer – an instrument used to measure atmospheric pressure using either water, air, or mercury; useful for forecasting short term changes in the weather; NOAA continental US weather forecast map for ...
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An automated prognostic chart of the 96-hour forecast of 850 mbar geopotential height and temperature from the Global Forecast System. Atmospheric models are computer programs that produce meteorological information, including prognostic charts, for future times at given locations and altitudes. [10]
Forecast times are spelled out as digits on the 24-hour clock, for example "two-three-double-O", and barometric pressures are pronounced as whole numbers, for example "a thousand and five". [16] With regard to the timing of weather events, the words "Imminent", "Soon" and "Later" are used and are tightly defined.