Gravity, Elevation And Temperature Barometer
Reading Correction Calculator
This calculator requires the use of Javascript enabled and capable browsers. This script determines the error in an observed barometer reading, based on gravity differences in different latitudes, barometer elevation differences from sea level, and on temperature expansion and contraction differences. Because the Earth is not a perfect sphere, the acceleration due to gravity depends on latitude. The radius of the earth is greater at the equator than at the poles. This means that gravity is least at the equator and greatest at the poles. Thus at smaller latitudes the mercury in the barometer tube weighs less, and will require a greater length to balance a given pressure. The barometer is calibrated for a latitude of 45 degrees, at sea level. For latitudes less than 45 degrees, the correction should be subtracted from the observed height; for latitudes greater than 45 degrees the correction should be added. The metal housing of the barometer and the mercury both expand as the temperature increases. However, they expand at different rates. The barometer is calibrated for 0 degrees C, at sea level. For different temperatures a correction must be applied to compensate for the different expansions of the housing and the mercury. For temperatures greater than 0 degrees C, the correction should be added to the observed height; for temperatures less than 0 degrees C, the very minor correction should be subtracted. Contraction differences are extremely small and almost negligible. For elevation, the barometer is also calibrated at sea level; the higher the elevation, the lower the pressure. With the barometer at elevations above sea level, the correction should be added since it is a negative value; below sea level, the value is also added but the value is positive. The default values are typical for the general lower desert area of Southern California. If you wish to show below sea level changes, just use a negative number for the appropriate elevation. To use the calculator, enter the temperature, the elevations, the latitude and the known and observed barometer reading, in any of the designations, then click on Calculate. Results are returned in both inches of mercury and millibars. If you don't know the latitude, you can use our
USA longitude and latitude map to get an idea of the general area. There are also temperature, elevation and latitude correction charts below the calculator. Other pressure correction calculators are for temperature correction only, for altitude correction only, for gravity (latitude) correction only, for temperature and altitude correction.
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