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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 ]
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.
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 ...
(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.)
SLP176 indicates the current barometric pressure extrapolated to sea level is 1,017.6 hPa (30.05 inHg). P0002 indicates that 0.02 inches (0.5 mm) of liquid-equivalent precipitation accumulated during the last hour. T10171017 is a breakdown of the temperature and dew point in eight digits separated into two groups of four. The first four digits ...
To the left of the cloud shape in the center of the station model is the symbol depicting present weather. The present weather symbol depicts the current weather which normally is obstructing the visibility at the time of observation.
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).
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.