The primary cause of corrosion of outdoor iron artifacts is rainfall and surface condensation resulting from rainfall and temperature-humidity fluctuations. To elucidate the specific effects of rainfall and dew on the surface corrosion of steel materials, this study conducted outdoor corrosion exposure experiments with control groups for both rainy and rain-free conditions, considering different orientations. Simultaneously, a thermal-humidity-corrosion model was established to predict the dynamic corrosion amount of steel in atmospheric environments. The outdoor corrosion exposure experiment in Nanjing's climate revealed varying annual corrosion rates for low carbon steel based on orientation, with horizontal corrosion averaging around 224 g/m2, about three times that of vertical corrosion. Corrosion rates peak in autumn and decrease in spring. Dew formation emerged as a key factor influencing outdoor steel sheet corrosion, surpassing rainfall impact. Simulation findings highlighted those thinner liquid films during dew formation led to higher corrosion current densities, resulting in more significant corrosion.