The infrared signature of an aircraft is generally calculated as the sum of multiple components. These components are, typically: the aerodynamic skin heating, reflected solar and upwelling and downwelling radiation, engine hot parts, and exhaust gas emissions. For most airframes, the latter two components overwhelmingly dominate the IR signature. However, for small targets--such as small fighters and cruise missiles, particularly targets with masked hot parts, emissivity control, and suppressed plumes- -aerodynamic heating is the dominant term. This term is determined by the speed of the target, the sea-level air temperature, and the adiabatic lapse rate of the atmosphere, as a function of altitude. Simulations which use AFGL atmospheric codes (LOWTRAN and MODTRAN)--such as SPIRITS--to predict skin heating, may have an intrinsic error in the predicted skin heating component, due to the fixed number of discrete sea-level air temperatures implicit in the atmospheric models. Whenever the assumed background temperature deviates from the implicit model atmosphere sea- level temperature, there will be a measurable error. This error becomes significant in magnitude when trying to model the signatures of small, dim targets dominated by skin heating. This study quantifies the predicted signature errors and suggests simulation implementations which can minimize these errors.