### VARIATION DUE TO LOCAL GROUND CONDITIONS

Where local sea level temperature is above 15^{o}C 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 15^{o}C. For example, on a 20^{o}C day, an ISA + 5 model is used. Temperature at all levels of the atmosphere model are incremented by 5^{o}C. 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.