THE STANDARD ATMOSPHERE
For a standard reference, a concept called a standard day is used. In aviation, everything is related
to standard day conditions at sea level, which are 29.92 in-Hg (1013.2 mb) and 15°C (59°F). In
the lower atmosphere, and thus for most aviation applications, a 1000 foot increase in altitude will
result in a pressure decrease of approximately 1 in-Hg (34 mb) and a temperature decrease of 2°C
(3.5 °F). These values are the standard day pressure and temperature lapse rates.
STATION AND SEA LEVEL PRESSURE
Station pressure is the atmospheric pressure measured directly at an airfield or other weather
station. Sea level pressure (or Reported Altimeter Setting) is the pressure measured from the
existing weather if the station were at MSL. This can be measured directly at sea level, or
calculated if the station is not at sea level using the standard pressure lapse rate.
Surface analysis charts, such as the one in Figure 1-6, use MSL as the reference level for the
depicted isobars (to provide a common reference), even though the pressure was first measured at
a weather station. This is done so that daily pressure variations associated with weather systems
can be tracked as they move across the country, as mentioned above. If, instead, station pressures
were used, the pressure charts would depict the inverse of the land topography, reflecting the
contour lines of a map. Mountain tops would always have lows over them, and valleys would
have highs. In other words, high altitude stations such as Denver would always reflect lower
pressure than surrounding stations at lower altitudes regardless of the day-to-day pressure
variations that occur with passing weather systems. Thus, for pressure to be meaningful, all
stations (even those far from the ocean) will report sea level pressure.
To calculate sea level pressure use the formula: SLP = SP + Terrain correction (2 in-Hg/1000 feet).
Example: Barometer reads 28.95 and elevation at the station is 1050 feet.
SLP = 28.95 + 1.05
SLP = 30.00
Set 30.00 in your altimeter (Figure 1-7)
General Structure of the Atmosphere, and Atmospheric Temperature and Pressure 1-7