The influence of population movements on the urban relative humidity of Beijing during the Chinese Spring Festival holiday

In China, hundreds of millions of people move out of the large cities before the Chinese Spring Festival (CSF) or Chinese New Year holiday and return after every year, spurring the greatest short-term migration of human beings on earth. Surface relative humidity is a key meteorological variable that has profound impacts on the lives of city dwellers. However, it remains unknown to what extent or how mass human migration during the CSF holiday can affect urban relative humidity. Here, we focus on the period of 2005-2014 with the Olympic year of 2008 excluded, and present observational evidence for the impacts of human migration on urban relative humidity with Beijing used as an example. Note that urban relative humidity variations induced by population movements are relative to nearby non-urban values in this study. Averaged over the study period, Beijing's urban relative humidity increased by 4.00% during the CSF week relative to the non-holiday background period, statistically significant at the 99% confidence level. In relative terms, this change amounts to 76% of relative humidity difference between urban and nearby non-urban areas during the background period. This significant increase of urban relative humidity during the CSF week is also apparent in central China's Zhengzhou, supporting the robustness of the signal. These findings offer vital yet previously unknown evidence for population movement impacts on urban relative humidity from the observations, and can add value to our understanding of the human-cities nexus. (C) 2017 Elsevier Ltd. All rights reserved.