Impact of mass human migration during Chinese New Year on Beijing urban heat island

The Chinese New Year (CNY, also called Spring Festival), which officially lasts for 7 days, is the most important festival in China. Chinese people in large cities usually return to their hometowns for family reunions before the CNY holiday and return afterward. Nearly half of Beijing's population has been reported to leave the city for family reunions before the CNY holidays in the past several years. Hourly automatic weather station data during CNY 2010-2015 were used to analyze the changes in temporal and spatial distribution of Beijing urban heat island intensity (UHII) and the impact of mass human migration on urban temperature. Soil moisture, 10-m wind speed, and cloud cover were considered and indicated nearly no change during the pre-CNY period (2-4 weeks before CNY) and CNY week, which means that UHII variation was mainly affected by the mass human migration. Daily UHII during CNY week was lower than during pre-CNY period. UHII for daily maximum temperature decreased by 55% during CNY week than the pre-CNY period (0.6 degrees C during pre-CNY period versus 0.27 degrees C during CNY week) due to mass human migration, which was much larger than the relative reduction in UHII for the daily minimum temperature (5%, 4.34 degrees C during the pre-CNY period versus 4.11 degrees C during the CNY week). The spatial distribution of the UHII difference between CNY week and the pre-CNY period is closely related to the locations of functional population zones. UHII for daily maximum temperature decreases most (80%, 0.40 degrees C during the pre-CNY period versus 0.08 degrees C during the CNY period) between the Third and Fourth Ring Roads (RRs), an area which experiences high frequency human activities and has high floating population percentage. This study can provide suggestions for optimizing the layout of urban space and land-use structures.