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 Barometric Pressure What is Barometric Pressure? The weight of the air that makes up our atmosphere exerts a pressure on the surface of the earth. This pressure is known as atmospheric pressure. Generally, the more air above an area, the higher the atmospheric pressure, this, in turn, means that atmospheric pressure changes with altitude. For example, atmospheric pressure is greater at sea-level than on a mountaintop. To compensate for this difference and facilitate comparison between locations with different altitudes, atmospheric pressure is generally adjusted to the equivalent sea-level pressure. This adjusted pressure is known as barometric pressure. In reality. the weather station measures atmospheric pressure. When entering the location's altitude in Setup Mode, the weather station calculates the necessary correction factor to consistently translate atmospheric pressure into barometric pressure. Barometric pressure also changes with local weather conditions, making barometric pressure an extremely important and useful weather forecasting tool. High pressure zones are generally associated with fair weather while low pressure zones are generally associated with poor weather. For forecasting purposes, however, the absolute barometric pressure value is generally less important than the change in barometric pressure. In general, rising pressure indicates improving weather conditions while falling pressure indicates deteriorating weather conditions. Formula: Simply, PSL = PS * R. where PSL is sea level pressure, PS is the unadjusted reading sensed by the Davis barometer, and R is the reduction ratio, which is determined as follows: First, TV (virtual temperature in the 'fictitious column of air extending down to sea-level) can be determined as follows. The result is in degrees Rankine, which is similar to Kelvin except It uses a Fahrenheit scale divisions rather than Celsius scale divisions, TV = T + 460 + L + C, where T is the average between the current outdoor temperature and the temperature 12 hours ago (in Fahrenheit) in whole degrees. L is the typical lapse rate, or decrease in temperature with height (of the "fictitious column of air"), as calculated by: L = 11 Z/8000, where L is a constant value with units in °F. Z is elevation, which must be entered in feet. The current dewpoint value and the station elevation are necessary to compute C. C is the correction for the humidity in the "fictitious column of air". It is complicated, and thus can only easily be calculated from a lookup table (provided in the attached table), The table consists of dewpoints in °F every 4°F and elevabons in feet every 1500 feet. Linear interpolation is highly recommended between points to obtain the correct reduced pressure value, For dewpoints below -76°F, C = 0; for dewpoints above 92°F, a dewpointof 92°F is assumed. The resultant humidity correction factor should be determined to 0-1°F resolution. Now, TV can be determined, From this, the following can be computed; Exponent = [ Z / ( 122.8943111 * TV ) ] Once this exponent is computed, R can be computed from the following-, R = 10[Exponent]. Thus, PSL= PS * (R) can be calculated. Pressure can be in any units (R is dimensionless) and still yield the correct value. This procedure is designed to produce the correct reduced sea-level pressure as displayed. This requires the user to know their elevatilon to at least ±10 feet to be accurate to every .01" Hg or ±3 feet to be accurate to every 0.1 mb/hPa. This is a simplified version of the official U.S. version in place now. The accepted method is to use lookup tables of ratio reduction values keyed to station temperature. These are based on station climatology, These values are unavailable for every possible location where a Davis user may have a station, thus this approach is not suitable. REFERENCE "Smithsonian Meteorological Tables". Smithsonian Institution Press, Washington, DC, 4" Ed. 1968. "Federal Standard Algorithms for Automated Weather Observing Systems used for Aviation Purposes", Office of the Federal Coordinator for Meteorological Services and Supporting Research, Washington, DC, 1988

 The Exploratorium would like to thank Davis Instruments for their generous support in setting up this weather station.