Weather Effects on Mobile Social Interactions Figure 3. Monthly weather conditions in Lisbon during the period of analysis (April 2006–March 2007). Note: the standard air pressure is 1,013.25 hPa. doi:10.1371/journal.pone.0045745.g003 proposed to explain the relationship between heat and violent criminal behavior includes alcohol consumption, vacations, leisure time, and the availability of social interaction. This is because alcohol consumption tends to increase when the temperature is high, while vacations also generally occur during periods of warm weather increasing the potential for social interactions, especially with family and friends. Furthermore, uncomfortable heat also tends to cause increased frustration, reducing one’s tolerance for annoyances. Weather also plays an important role in determining travel behavior and traffic demands. Cools et al. [14] show that travel patterns are strongly related to weather conditions, while changes in travel behavior caused by the weather are also dependent upon the purpose of the trip. Maze et al., [15] describe the influence of inclement weather on traffic safety, traffic demand, and traffic flow, while Agarwal et al., [12] discuss how changes in weather type (rain, snow, visibility) and intensities impact the speed, headway, and capacity of roadways. While the above review clearly affirms the importance of understanding the relationship of weather on human behavior, it is also clear that we need to find other ways that can help further shed light into this complex phenomenon. Inquiring into how weather shapes social interaction is crucial to understanding the weather’s relationship to human behavior. However, we have yet to find any significant studies that have looked at this particular relationship. To offer a way to investigate this twin concept of weather and human behavior and to establish an emergent understanding of this relationship, we describe an approach that leverages the recent widespread adoption of mobile phones, in particular its use as a platform for conducting large scale longitudinal ‘social sensing’. Other studies along this line have shown that mobile phones can be used effectively to sense aspects of human spatial behavior [16][17][18][19] and social interactions [20] [21][22]. Thus, in this paper, we demonstrate how longitudinal mobile phone data can allow us to empirically examine the relationship between weather conditions and social behavioral patterns. Methodology Ethics Statement This research was granted ethical approval from the IRB at Newcastle University, UK. The consent was not needed for this study because the data used in the study was anonymized by the operator before being transferred to the authors, in such a way that no connection could be made with any individual. The mobile phone users agreed to the terms and conditions for using the PLOS ONE | www.plosone.org 3 October 2012 | Volume 7 | Issue 10 | e45745