Most studies of thermal actions on bridges have been devoted to concrete structures. In this paper, we focus our analysis on steel box girder bridges. The statistical analysis is based on data collected over a three-year period by temperature gauges placed in appropriate places on the Normandy Bridge, France. Temperature–time series are generally known to contain a deterministic and a random part. The deterministic component is due to the natural seasonal fluctuation over a whole year. The same phenomenon has been observed in the case of the Normandy Bridge. At design stage, engineers need to know the extreme fluctuations of thermal actions (average temperature and thermal gradient) to ensure the durability and safety of a structure. Design codes prescribe extreme values called characteristic values defined on a return period of 50 years. The statistical analysis allows the calculation of these characteristic values as well as frequent and quasi-permanent values.