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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 ]
There, barometer readings fell to 956.0 mb (28.23 inHg). [2] In Detroit, air pressure fell to 28.34 inches of mercury (960 mbar). At around the same time, the absolute low pressure was measured at Sarnia, in Southwestern Ontario, Canada, where barometer readings decreased to 955.5 mb (28.22 inHg). [2]
For example, if a barometer located at sea level and under fair weather conditions is moved to an altitude of 1,000 feet (305 m), about 1 inch of mercury (~35 hPa) must be added on to the reading. The barometer readings at the two locations should be the same if there are negligible changes in time, horizontal distance, and temperature.
Weather stations typically have these following instruments: . Thermometer for measuring air and sea surface temperature; Barometer for measuring atmospheric pressure; Hygrometer for measuring humidity
(A reading of 922.5 mbar (corrected to MSL) was reported from Omagh in Co. Tyrone (Harding 1887) the graduation and calibration of the barometer were not considered satisfactory enough “to quote the reading as trustworthy”, the storm minimum over the Northern Ireland was thought to be close to 924 mbar.)
The bar is a metric unit of pressure defined as 100,000 Pa (100 kPa), though not part of the International System of Units (SI). A pressure of 1 bar is slightly less than the current average atmospheric pressure on Earth at sea level (approximately 1.013 bar).
It is used for barometric pressure in weather reports, refrigeration and aviation in the United States. It is the pressure exerted by a column of mercury 1 inch (25.4 mm) in height at the standard acceleration of gravity. Conversion to metric units depends on the density of mercury, and hence its temperature; typical conversion factors are: [1]
The changing height of the mercury in the barometer was recorded on a continuously moving photosensitive surface. [5] By 1847, a sophisticated temperature-compensation mechanism was also employed. Ronalds’ barograph was utilised by the UK Meteorological Office for many years to assist in weather forecasting and the machines were supplied to ...