Atmospheric pressure (cont.)

• measured with mercury barometer or aneroid barometer
• Units are lbs/square in., inches of mercury or millibars (We'll use millibars = mb)
• station pressure - Actual measured pressure.
• sea level pressure - The atmospheric pressure at mean sea level, either directly measured or most commonly mathematically converted from the measured station pressure.  This allows one to compare pressures at different locations even though they are at different elevations.
• Simple formula to convert from station pressure to sea level pressure is

P(sea level) = P(station) + 100 mb/km x Altitude
where the Altitude is in km.
For example, the typical station pressure for Tucson is 925 mb and the altitude is 0.779 km. What would the sea level pressure be for Tucson using the simple conversion formula?  P(sea level for Tucson) = 925 mb + 100 mb/km x 0.779 km
                                     = 925 mb + 77.9 mb
                                     = 1002.9 mb
• More accurate rule of thumb would be Pressure = 1013mb x exp(-Altitude/8.5), where Altitude is the altitude above sea level, but you don't need to remember this formula.  Just hope that the altimeter in the airplane does remember it.
• altimeter pressure - The value of atmospheric pressure to which the scale of a pressure altimeter is set.  In the U.S. the setting represents the pressure required to make the altimeter indicate the elevation of the airfield.