Outdoor spaces provide the growing urban population with social, health, environmental, and economic benefits. A thermal comfort model is needed to aid in the design of attractive urban outdoor spaces. In an outdoor environment, a person's thermal comfort changes with the dynamic weather conditions. However, most of the outdoor thermal comfort models in the literature are for steady-state conditions. To develop a dynamic thermal comfort model, this study observed the responses of 26 human subjects from West Lafayette, Indiana, USA, and Tianjin, China, to a wide range of outdoor thermal environments. The study monitored the subjects' skin temperatures, recorded their thermal sensations, and measured several outdoor environmental parameters. Analysis of the test data showed that the thermal load, the mean skin temperature, and the change rate of the mean skin temperature of the subjects tested were the most important parameters affecting their thermal comfort in the outdoor spaces. These three parameters were integrated as predictor variables into a comfort model for predicting the outdoor thermal sensation. The model uses the thermal load to evaluate the thermal environment, and the mean skin temperature, and its change rate to consider dynamic changes in the thermal state of the human body. The validity of the model developed in one region was tested with the use of data obtained from the other region. (C) 2017 Elsevier B.V. All rights reserved.