When employing vibration-based damage detection methods for monitoring the structural health of bridges, it is often possible to increase damage feature sensitivity by focusing on local as opposed to global vibrations. This heightened sensitivity, however, comes at a cost: by moving towards the higher frequency ranges and more local behavior, the effects of environmental variability become increasingly pronounced. In an attempt to characterize and quantify the effect of specifically temperature variations on the local vibrations, an extensive long-term monitoring campaign was performed on the Haringvlietbrug, a steel box-girder bridge in the Netherlands. Temperatures were measured on various components of the bridge, including the top and bottom of the asphalt layers. It was found that complex temperature gradients are formed especially in situations where the radiation from the sun strikes the bridge at oblique angles. More importantly, the temperature variations in the asphalt layers were found to strongly impact the natural vibration properties of the bridge in the targeted high-frequency ranges. The obtained results provide valuable indications as to the environmental parameters to monitor when designing vibration-based structural health monitoring systems for local damage detection in bridges.@en