On many occasions the ground temperature and pressure will not exactly be equal to ISA standard conditions. In these cases it is possible to adjust the atmosphere model to suit local conditions. As the depth of the atmosphere is very small (125Km-150Km) local variations in temperature and pressure will substantially effect the full depth of the atmosphere.
Where local sea level temperature is above 15oC an ISA+ model is used. In this model the complete atmosphere is incremented by the temperature difference between the current sea level temperature and the standard value of 15oC. For example, on a 20oC day, an ISA + 5 model is used. Temperature at all levels of the atmosphere model are incremented by 5oC. With this adjustment, the previous formula for pressure, density, viscosity and speed of sound variation can still be used.
No adjustments are made for ground surface altitude, all calculations are done based on model starting at sea level. If the local ground level locally is well above sea level then the atmospheric model will still be based on sea level and will start at the altitude of the ground compared to sea level.