# why is atmospheric pressure corrected to sea level

This requires the use of enabled and capable browsers. This script determines the error in an observed barometer reading, based on elevation above (or below) sea level of the barometer and the median standard outside air temperature. The barometer is 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 elevation of the barometer and the known and observed barometer reading, in any of the possible designations, then click on Calculate. Actual calculations are done on inches of mercury, feet and median standard degrees F. Results are yielded in both inches of mercury and millibars (mb).

The metric unit hPa (hectoPascal) is for all practical purposes, identical to the pressure unit designated as millibars; in reality, there is a very small difference, but the scientific community accepts them as the same. There is also an elevation correction chart, for a given temperature, below the calculator. 1 atm = 29. 92 in Hg (inches of mercury)
1 atm = 1013. 20 millibars 1 atm = 14. 7 psi (pounds force per square inch) 1 atm = 1013. 20 hPa (hectopascals) To further complicate matters, there are actually 2 values for an atmosphere. The first is generally called "standard" and the other is termed as "scientific". These are based on standard atmosphere calculations. Other pressure correction calculators are, and. subtracted from barometer readings when below the datum plane. Updated 8. 12. 11 Barometric pressure is also known as sea level corrected pressure, and is what the weather station and airports report because itБs useful for pilots and making weather assessments. б б Barometric pressure is not the actual air pressure where you are, rather itБs a number thatБs corrected to sea level. б б In order to determine the actual air pressure where you are (which is what the ballistics program cares about), you have to account for the effects of altitude. б б However if you have a handheld weather meter like a Kestrel, you can measure Station Pressureб directly which is the actual air pressure where you are. б б This is the preferred method of inputting pressure data because itБs one less input and relies on only one measurement instead of two.

A common error is to mistake station pressure for barometric or vice versa. б б The consequence of this error is that the wrong air density gets applied which degrades the accuracy of trajectory predictions. б б This error is increasingly more severe the higher up you are above sea level.

Refer to the image on the right for proper set-up of the atmospheric pressure inputs. б Note the reference altitude is set to 0 ft in the Kestrel which indicates itБs displaying uncorrected station pressure, and the Pressure is Absolute box is checked in the program indicating itБs using station pressure. To further clarify the output from the Kestrel, here is an excerpt from the Kestrel userБs manual: БSome final notes Б If you wish to know the actual or station pressure for your location (such as for engine tuning), simply set the reference altitude on the BARO screen to Б0Б. б In this case, the Kestrel Meter will not make any adjustment and will display the measured value. (Engine tuning and sometimes refer to atmospheric or station pressure as Бabsolute pressure. Бб These applications are concerned with the actual air density, as opposed to pressure gradients relating to weather, so barometric pressure is less useful. Б

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