Fig. 2: Change trend and significance of SUHI characteristic in urban expansion areas across 31 Chinese cities during 2000–2018.

a SUHI_Ra during the daytime. b SUHII_Mean during the daytime. c SUHII_Max during the daytime. d SUHI_Ra during the nighttime. e SUHII_Mean during the nighttime. f SUHII_Max during the nighttime. The color of the points represents change rate, and the cross markers on the points indicate significance at the 0.05 level.