The increased interest during the last decade in the infrared signature of (new) ships results in a clear need of validated infrared signature prediction codes. This paper presents the results of comparing an in-house developed signature prediction code with measurements made in the 3 5 μm band in both clear-sky and overcast conditions. During the measurements, sensors measured the short-wave and long-wave irradiation from sun and sky, which forms a significant part of the heat flux exchange between ship and environment, but is linked weakly to the standard meteorological data measured routinely (e.g., air temperature, relative humidity, wind speed, pressure, cloud cover). The aim of the signature model validation is checking the heat flux balance algorithm in the model and the representation of the target. Any uncertainties in the prediction of the radiative properties of the environment (which are usually computed with a code like MODTRAN) must be minimised. It is shown that for the validation of signature prediction models the standard meteorological data are insufficient for the computation of sky radiance and solar irradiation with atmospheric radiation models (MODTRAN). Comparisons between model predictions and data are shown for predictions computed with and without global irradiation data. The results underline the necessity of measuring the irradiation (from sun, sky, sea or land environment) on the target during a signature measurement trial. Only then does the trial produce the data needed as a reference for the computation of the infrared signature of the ship in conditions other than those during the trial.