In order to present an accurate speed, altitude or vertical speed indication it is necessary to know the relationship between atmospheric values like temperature, pressure and air density. Since all the values are subjected to continuous changes, complicated measuring is needed in order to provide reliable indications. As you can imagine it takes a lot of effort to calculate such values and so a less complicated method is being used called the International Standard Atmosphere or ISA. The ISA assumes that the values of pressure, temperature and density at different altitudes are constant. In turn, these assumptions are based on established meteorological and physical observations, theories and measurements and so the ISA is accepted internationally. Later on we'll discuss the relationship between instrument settings and the ISA.
The ISA is based on the following assumptions:
Temperature decrease (1000ft)
Pressure decrease (1000ft)
Since both pressure and temperature decrease with altitude, the standard lapse rates can help calculate the temperatures and pressures you can anticipate at different altitudes. For temperature, the standard lapse rate is 1,98°C for each 1000ft of altitude.