Temporal Evolution and Extremes of Urban Thermal and Humidity Environments in a Tibetan Plateau City

To elucidate features of the recent urban thermal–humidity climate, this study interrogates high-density observational data (2018–2023) from Xining, a key urban area on the Tibetan Plateau (TP), focusing on recent changes and extremes. Results show that the summer urban heat island intensity (UHII) has intensified in recent years, marked by a surging frequency of extreme heat island days and increased variability, as exemplified by the maximum hourly UHII increasing from 3.95 °C to 6.60 °C during 2018–2023. Conversely, the summer urban dry island intensity (UDII) exhibited a clear weakening, yet this is accompanied by a dramatic increase in transient extreme events, characterized by a sharp rise in weak dry island occurrences and the emergence of urban moist islands. Furthermore, the hourly UHII is dominantly modulated by atmospheric humidity and temperature conditions, and these influences displayed a pronounced diurnal asymmetry, being strongest at night while weak or even reversed during the pre-noon hours. These findings underscore the escalating thermal risks and complex humidity dynamics in this highland city, providing critical insights for urban planning and climate adaptation strategies in similar environments.