Mean sea level pressure calculator (converter) – Advanced formula

References

1. Barometers serve several very important purposes:

   Knowing the atmospheric pressure at different levels in the atmosphere allows us to track weather systems, such as high and low pressure, and therefore make good weather forecasts.

   A form of barometer, called an altimeter, allows airplane pilots to know what height they are in the air; this is crucial to their ability to maintain the altitude that they need to fulfill their flight plan, and to land in poor visibility (by knowing their altitude in comparison to the altitude of the airport runway).

   David R. Cook (2001) Climate Research Section Environmental Science Division Argonne National Laboratory, online at: http://www.stab-iitb.org/newton-mirror/askasci/wea00/wea00031.htm

2. Pressure Definitions

   Station Pressure is the pressure observed at a specific elevation and is the true barometric pressure of a location. It is the pressure exerted by the atmosphere at a point as a result of gravity acting upon the "column" of air that lies directly above the point. Higher elevations above sea level experience lower pressure since there is less atmosphere on which gravity can act. In another words, the weight of the atmosphere decreases as one increases in elevation.

   Mean Sea Level Pressure is the pressure reading most commonly used by meteorologists to track weather systems at the surface. It is a "reduced" pressure, which uses observed conditions rather than "standard" conditions to remove the effects of elevation from pressure readings. This reduction estimates the pressure that would exist at sea level at a point directly below the station using a temperature profile based on temperatures that actually exist at the station. In practice the temperature used in the reduction is a mean temperature for the preceding twelve hours. Mean sea level pressure should be used with caution at high elevations as temperatures can have a very profound effect on the reduced pressures, sometimes giving rise to fictitious pressure patterns and anomalous mean sea level pressure values.

   Altimeter Setting is the pressure reading most commonly heard in radio and television broadcasts. It is not the true barometric pressure at a station. Instead it is the pressure "reduced" to mean sea level using the temperature profile of the "standard" atmosphere, which is representative of average conditions over selected area. The altimeter setting is the pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude (above mean sea level) of the aircraft on the ground at the location for which the pressure value was determined. The altimeter setting is an attempt to remove elevation effects from pressure readings using "standard" conditions.

   Pressure definitions from: http://www.weather.gov/bou/pressure_definitions

3. Low pressure versus high pressure: A low pressure system, or "low," is an area, where the atmospheric pressure is lower than that of the area surrounding it. Conversely, a high pressure system, or "high," is an area, where the atmospheric pressure is greater than that of the surrounding area.

   Saying that the pressure is high or low is relative to the characteristics of the air surrounding the air mass. If you say that the pressure is high when it's above average and low when it's below average is an inadequate definition.

   High and low pressure could be defined by saying that, on average, the air in the Troposphere is generally rising in a low pressure system and sinking in a high pressure system. The rising or sinking of air results in a weather system having a low or high pressure, respectively.

   David R. Cook (2000) Climate Research Section Environmental Science Division Argonne National Laboratory, online at: http://www.newton.dep.anl.gov/askasci/wea00/wea00386.htm

4. Air pressure and air temperature: In a very stable atmospheric situation, as is often found in equatorial desert areas, the heating of the atmosphere "in front of" the rising Sun in the morning will actually cause the atmospheric pressure at the ground to start to increase before sunrise.

   David R. Cook (2015) Climate Research Section Environmental Science Division Argonne National Laboratory, online at: http://www.newton.dep.anl.gov/askasci/wea00/wea00386.htm

5. Mean Sea-level pressure formulas: http://integritext.net/DrKFS/correctiontosealevel.htm, LAPLACE GENERAL EQUATION THE REDUCTION OF BAROMETRIC PRESSURE DrKFS_net.pdf

   h= height of the upper station (m)

   hz= height of the lower station (m)

   Z= h-hz (m)

   p= atmospheric pressure at the upper station (hPa)

   p0= atmospheric pressure at the lower station (hPa)

   R= mean radius of the earth (6367324 m)

   0m= mean temperature of the air column (C)

   e= mean pressure of aqueous vapour in the air column (hPa) - formula used for saturation pressure

   b= mean barometric pressure of the air column (hPa)

   upsilon= lattitude of the stations (degrees)

   K= Barometric constant (hypsometric constant) K=delta.bldelta1.M (18400 m)

   alfa= coefficient of thermal expansion of the air (0.0037)

   k= constant depending on the figure of the earth (0.0026)

   delta= density of mercury at 0C (13.6)

   delta1= density of dry air at 0C (0.001293)

   M= Modulus of common logarithms (0.43)

6. Pressure effects on weather:

   Motions caused by circulations are building blocks in the atmosphere and are creating our weather.

   Low pressure systems circulate anticlockwise north of the equator and clockwise south of the equator because of Earth’s spin and the Coriolis Effect (cyclonic flow) and this forces air upward (and results in clouds and precipitation). (SEE DIAGRAM)

   High pressure systems circulate clockwise north of the equator and counterclockwise south of the equator (anticyclonic flow) and cause a sinking motion in the atmosphere (causes clear and sunny sky). (SEE DIAGRAM)