Weather Effects on Mobile Social Interactions: A Case Study of Mobile Phone Users in Lisbon, Portugal Santi Phithakkitnukoon1*, Tuck W. Leong 1 , Zbigniew Smoreda2, Patrick Olivier1 1 Culture Lab, School of Computing Science, Newcastle University, Newcastle, United Kingdom, 2 Sociology and Economics of Networks and Services Department, Orange Labs, France Abstract The effect of weather on social interactions has been explored through the analysis of a large mobile phone use dataset. Time spent on phone calls, numbers of connected social ties, and tie strength were used as proxies for social interactions; while weather conditions were characterized in terms of temperature, relative humidity, air pressure, and wind speed. Our results are based on the analysis of a full calendar year of data for 22,696 mobile phone users (53.2 million call logs) in Lisbon, Portugal. The results suggest that different weather parameters have correlations to the level and character of social interactions. We found that although weather did not show much influence upon people’s average call duration, the likelihood of longer calls was found to increase during periods of colder weather. During periods of weather that were generally considered to be uncomfortable (i.e., very cold/warm, very low/high air pressure, and windy), people were found to be more likely to communicate with fewer social ties. Despite this tendency, we found that people are more likely to maintain their connections with those they have strong ties with much more than those of weak ties. This study sheds new light on the influence of weather conditions on social relationships and how mobile phone data can be used to investigate the influence of environmental factors on social dynamics. Citation: Phithakkitnukoon S, Leong TW, Smoreda Z, Olivier P (2012) Weather Effects on Mobile Social Interactions: A Case Study of Mobile Phone Users in Lisbon, Portugal. PLoS ONE 7(10): e45745. doi:10.1371/journal.pone.0045745 Editor: Attila Szolnoki, Hungarian Academy of Sciences, Hungary Received March 9, 2012; Accepted August 24, 2012; Published October 10, 2012 Copyright: 2012 Phithakkitnukoon et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The study was supported by the School of Computing Science of Newcastle University. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: ZS is employed by Orange Labs. This does not alter the authors’ adherence to all the PLOS ONE policies on sharing data and materials. * E-mail: santi@newcastle.ac.uk Introduction Weather has been the subject of studies in many fields because of its relation to, and influence on, a number of phenomena related to human behavior. Many socioeconomic activities are strongly related to weather and climate [1]. People may choose to move and live in places because of particular climate and weather sequences in order to gain personal satisfaction and for a sense of well-being. Particular places may also be attractive to people because the climate and weather is conducive to economic activities. Tucker and Gillian [2] concluded from their compre- hensive literature review that physical activity vary with seasonality and identified poor or extreme weather condition as a barrier to physical activity. The levels of physical activity vary by geograph- ical region. For example in Scotland, The Netherlands, France, Canada, the levels of physical activity appear to be highest in summer and spring (April–August). However, the levels of physical activity are lowest in July in Texas (USA) as it is the hottest month. People’s thermal comfort thus plays an important role in their participation of physical activity. Stathopoulos et al. [3] found that air temperature is the dominant factor in influencing human comfort, but that wind speed, relative humidity, and solar radiation may also contribute to human perception of overall comfort. There are also significant gender differences in the perception of thermal comfort. Females have been found to be more sensitive to temperature levels, and less sensitive to humidity levels, than males [4–5]. Weather has also been shown to impact people’s mood. Humidity, temperature and hours of sunshine had the greatest effect on mood. High levels of humidity lowered concentration, increased sleepiness while rising temperatures lowered anxiety and skepticism [6]. The relationship between attempted or completed suicide and weather or climate is also observed. While researchers have yet to identify a specific weather condition associated with a generally higher risk for suicide, interactions between weather and seasonal effects, as well as environmental effects on brain functions and weather-related interactions show a contributory role [7]. Epidemiological data also found correlations between small positive air ionization due to changing weather conditions with increases in industrial and automobile accidents, suicide, crime, depression, irritability, and interference with central nervous functions [8]. Air ionization arising from electrical and electro- magnetic changes in the atmosphere are in part caused by the weather. Together with other factors such as temperature, humidity, noise, and so on, changes in ionization can affect our physical and psychological health [9]. Changes in weather have also been linked to public health. Patz et al. [10] discuss climatic variability as a determinant of disease and its importance in the construction of predictive models to support public health prevention strategies and programs. Through its impact on human behavior, weather has also been identified as a determinant of stock prices. Saunder Jr. [11] suggested including behavioral variables influenced by weather conditions in models of asset pricing. Akhtari [12] also found a PLOS ONE | www.plosone.org 1 October 2012 | Volume 7 | Issue 10 | e45745