Urban heat island effects, which are the phenomenon wherein higher ambient air temperatures are measured in cities than in rural areas, have worsened urban thermal environments over the past decades. This study aims to analyze the effects of urban physical environments on the duration of high air temperature, using climate data collected from 217 Automatic Weather Stations in Seoul, Korea. In order to specify radiation trapping effects, interaction effects between sky view factors (SVF) and albedo values were analyzed using multiple regression analysis. The results indicate that increases in commercial and traffic areas lead to longer durations of high air temperature and that high urban porosity shortens the duration of high air temperature by improving urban ventilation. This study also indicates that the duration of high air temperature has a negative association with SVF; however, an analysis of interaction effects indicates that high-albedo materials diminish the positive effects of high SVF largely because of radiation trapping effects. These findings suggest that urban ventilation paths, high SVF, and materials with an appropriate albedo value play important roles in improving thermal comfort conditions, such as the duration of high air temperature.