Weather Effects on Mobile Social Interactions Figure 5. Time (in hours) for each band of weather parameters considered in this study. doi:10.1371/journal.pone.0045745.g005 Google search engine. Some researchers have also used Weather Underground’s dataset in their investigations, e.g., [24] and [25]. The dataset included logs of the climatic conditions: temperature (uC), relative humidity (%), air pressure (aPh), and wind speed (km/h), every 30 minutes from three weather stations situated in different locations in the city: Lisbon International Airport, Portela, and Cais do Sodre (as shown in Fig. 2). The average weather conditions (monthly) during the period of the analysis in this study (April 2006–March 2007) are shown in Fig. 3 with standard deviation bars. For our analysis, weather condition values were taken as the average of values recorded from all three weather stations. For example, the value of the temperature at a given time from the three stations is reported as an average temperature for that given time in this study. The average distance between cell tower and weather station is 747.5 meters. Analysis Social interactions refer to the interactions between two or more individuals. In mobile social networks, the sociability of an individual can be estimated based on the overall amount of time spent (talking) on the mobile phone, and the number of people in his/her personal network. In addition, the strength of social relationship can also offer an insight into the type of social ties (e.g., strong ties, weak ties) being connected under different situations (e.g., weather conditions). To quantify a social link’s strength, we adopted the theory developed by Mark Granovetter in his milestone paper of 1973 [26] in which he categorized ties as one of two types: strong and weak. Strong ties are people who are socially close to us and whose social circles tightly overlap with our own. Typically they are people we trust and with whom we share several common PLOS ONE | www.plosone.org 5 October 2012 | Volume 7 | Issue 10 | e45745