260,000 ft 1% other gases 78% 21% Nitrogen Oxygen A t m o s p h e r e Figure 9-2.  The composition of the atmosphere. In aviation weather reports, the units of pressure are inches  of mercury ("Hg) and millibars (mb) and 29.92 "Hg equals  1013.2 mb. This force or pressure is created by the moving  molecules act equally in all directions when measured at a  given point. if the temperature is much higher than standard, then the  density must be much lower. Air pressure and temperature  are usually measured, and using the gas law, density of the air  can be calculated and used to determine aircraft performance  under those conditions. Using a representative vertical distribution of these variables,  the standard atmosphere has been defined and is used for  pressure altimeter calibrations. Since changes in the static  pressure can affect pitot-static instrument operation, it is  necessary to understand basic principles of the atmosphere.  To provide a common reference for temperature and pressure,  a definition for standard atmosphere, also called International  Standard Atmosphere (ISA), has been established. In  addition to affecting certain flight instruments, these standard  conditions are the basis for most aircraft performance data.  In the METAR report, see  Figure 9-3 , the local altimeter  setting “A2955”, read as 29.55, is the pressure "Hg. In the  remarks (RMK) section of this report, sea level pressure  expressed as “SLP010”, the value expressed in millibars  (hPa), is used in weather forecasting. Dry air behaves almost like an ideal gas, meaning it obeys the  gas law given by P/DT = R, where P is pressure, D is density,  T is temperature, and R is a constant. This law states that the  ratio of pressure to the product of density and temperature  must always be the same. For instance, at a given pressure  Standard Atmosphere KAPA 1014522 11004KT 10SM SCT055 BKN120 00/M04 A2955 RMK A02 SLP010 T00001039 SLP010 = Sea level presure in hPa used in weather forecasting A2955 = Local altimeter setting 29.55 Figure 9-3.  Example of how pressure is used in an aviation weather report. 9-3