This chapter investigates how seasonal variations shape human thermal comfort in outdoor environments. A field study in Guangzhou, a city with hot summers and mild winters, was conducted to assess the sensitivity of thermal perception to key microclimatic parameters. Using standardized regression coefficients, the influence of air temperature, mean radiant temperature, wind speed, and relative humidity was examined across different seasons, alongside the performance of three thermal indices: PET, UTCI, and SET. Results demonstrate that human thermal sensitivity is season-dependent, with air temperature exerting the strongest effect and relative humidity playing a minimal role. Among the indices, SET provided the most consistent alignment with observed seasonal responses. These findings highlight the importance of considering seasonal dynamics in outdoor thermal comfort research and provide evidence for climate-responsive urban planning and design.