Vibration-based monitoring (VBM) is widely applied for assessing the structural health of bridges, where tracking changes of modal characteristics is used for damage assessment. Natural frequencies are the most commonly employed modal characteristic for this purpose, but have the drawback that they are sensitive to temperature changes and not very sensitive to local stiffness variations. Recent case studies, primarily on steel railway bridges, have demonstrated that strain mode shapes offer a much higher sensitivity to local damage and are significantly less influenced by temperature. This study investigates the impact of temperature and solar irradiation on the natural frequencies and strain mode shapes of a post-tensioned concrete bridge through a long-term monitoring campaign. In agreement with previous research findings, a significant impact of temperature and solar irradiation on the natural frequencies is observed. However, also the strain mode shapes are found to be substantially temperature-dependent, mostly attributed to the composite nature of the structure and the non-uniform temperature distribution caused by solar irradiation. For vibration-based damage detection, this implies that damage has to be sufficiently large to be detected through changes in strain mode shapes, if environmental variability is not accounted for.