Comparison of Barometric Pressure Readings with Weather Stations
Comparing the Measured Barometric Pressure with that reported by Weather Stations
All PASCO barometers directly report the actual barometric pressure, which is often referred to as the station pressure. The barometric pressure is the pressure exerted by the atmosphere at the measurement point as a result of weight per unit cross-sectional area of the "column" of all of the air above the point.
Measured barometric pressure cannot be directly compared with the barometric pressures reported by weather services, even if the barometric pressure is measured at the exact location of a weather service station because when weather stations report “barometric pressures”, they are actually reporting derived mean sea-level barometric pressures.
The mean sea-level barometric pressure attempts to reasonably estimate what the value of the barometric pressure would be if, at the station barometer’s horizontal geographical coordinates, a vertical hole were drilled through the Earth to sea level depth and a barometer placed at the bottom of that hole. This is done to have the reported barometric pressures be a useful predictor of weather patterns, rather than a static reflection of the Earth’s topography.
The pressure exerted by the hypothetical air column between the actual barometric location and sea level will be a function of the height (h or altitude), air temperature T(z), and the relative humidity RH(z) in the column.
The air temperature in the troposphere typically decreases linearly with height z, so one should not ignore that the temperature in the column is a function of height.
Below are certain models that attempt to determine what the sea-level barometric pressure would be from measurements of barometric pressure, temperature, and relative humidity.
Globe Protocol Sea Level Pressure Determination
The crudest estimate of the value of the barometric pressure at the virtual barometer assumes a standard atmosphere, with standard temperature and pressure. With these assumptions, one can determine mean sea level pressure P0 from the station barometric pressure Ph and the altitude h, which can be read off of a map. This is what is done in the Globe Protocol.
P0 [millibars](Ph, h) = Ph [millibars] + h / 9.2
h = station elevation [meters]The PS-2154A PASPORT Weather Sensor and PS-2174 Weather/Anemometer Sensor can both measure temperature and relative humidity at the station’s altitude. Using this information gives a better estimate of the pressure of the column of air. These values are used in the World Meteorological Organization Protocol (3.11.2) formula:
P0 = Ph (1+ h / 29.27 / TV)
Tv (p, T, RH)= T / (1-RH e(T)(1-ε)/p)
Creation Date: 08/30/2011