Urban Expansion Reshapes Surface and Atmospheric Heat Islands and Moisture Regimes in NCR-Delhi, India: Evidence from In-Situ and Satellite Observations

High-frequency climatic extremes in rapidly urbanizing areas are becoming prominent and often reflected through enhanced thermal stress, changed moisture conditions, and heavy diurnal asymmetries, but the quantification of their spatio-temporal changes is still underestimated. This study focuses on that aspect in the National Capital Region of India with long-term satellite-derived land surface temperature (MODIS 2003-2021) with high-resolution in-situ measurements of air temperature, humidity, and wind (AWS-IMD). A spatio-temporal analytics framework, based on physical diagnostics, time-series mining, and interpretable pattern learning, is used to describe surface and atmospheric urban heat islands (UHI), urban dry islands (UDI), and the question of emergent thermal hotspots at urban-peri-urban-rural gradients.

Results indicate an increase in surface thermal extremes, where daytime SUHI warming rates are approximately 0.19°C/ yr in urban cores and as high as 0.23 °C /yr in inner-urban regions. Increase in the night-time surface temperature was more prominent, especially in inner-city areas (~0.15 °C /yr), a phenomenon suggesting the rise of nocturnal heat stress. The atmospheric UHI peaks were as high as 2.0-2.3 °C, particularly during winter mornings and pre-monsoon nights. The space-time cube hotspots analysis reveals that the persistent hotspots experienced between 2003 and 2011 have evolved to become more intense and expanse beyond 2011 with evident outward movements to the peri-urban areas. At the same time, dry seasons in urban dry islands were highly coupled between thermal and moisture extremes with −13 to −15 g /m³ (urban dry islands). In general, the results show that there is a systematic increase and spatial expansion of coupled heat and dry island extremes, which implies that urban areas with rapid urbanization are changing to more volatile and persistence urban thermal stress regime.