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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. 

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 

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. 

## Standard Atmosphere 

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. 

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