Where an altitude has to be related to pressure, temperature or density, CPACS uses a single atmospheric model: the ICAO Standard Atmosphere of 1993 (ICAO Doc 7488/3, Manual of the ICAO Standard Atmosphere, third edition), selected as ISA in <atmosphericModel>. It gives the temperature, pressure, density, speed of sound and the dynamic and kinematic viscosity over altitude, from -5000 m to 80000 m. An <altitude> in CPACS is the geopotential altitude H of the ISA manual (section 2.3), given in [m].
A temperature offset ΔT (<deltaTemperature> [K]) describes a day warmer or colder than the standard, which the ISA manual itself does not cover. At a given altitude, the pressure keeps its ISA value, so that the altitude remains a pressure altitude, and the temperature is shifted by the offset: T = TISA(H) + ΔT. The density ρ = p/(R T), the speed of sound and the viscosities follow from this temperature with the relations of the ISA manual.