A traditional flight simulator implements a flight model with normally 6 degrees of freedom and will use complex analysis methodologies like the Blade Element Model (X-Plane famously uses this method) which I barely remember from University. These methods produce reliable and interesting flight dynamics but are computationally quite intense! Here for example we can see the model for a single element of a blade, imagine doing this for 40 to 50 elements per aircraft!
Part of that building has been implementing semi-realistic thrust and lift modelling to our model, at the very core of this is a model of the atmosphere. Sea Power makes use of the International Standard Atmosphere (ISA) as defined in this resource kindly provided by NASA. The ISA defines how the atmosphere changes around us and helps explain why the outside air temperature at 30,000 ft is significantly below 0C.
