Weather Effects on Mobile Social Interactions Figure 4. Probability mass of talk time under different weather conditions. doi:10.1371/journal.pone.0045745.g004 services with the network operator that their communications may be recorded and analyzed for improving services. Datasets In this study, we used anonymized Call Detail Records (CDR) of mobile phone users in Lisbon, Portugal over the course of one year: from April 2006 to March 2007. The data was collected for billing purposes by an European telecom operator. To safeguard personal privacy, individual phone numbers were anonymized by the operator before leaving their storage facilities, and were identified with a security ID (hash code). The CDR comprised the voice call information: timestamp, caller’s ID, callee’s ID, call duration, caller’s connected cellular tower ID, and callee’s connected cellular tower ID. The dataset did not contain information relating to text messages (SMS) or data usage (Internet). The location of the mobile phone user was recorded as the nearest connected cellular tower location when the user made or received a call. The dataset provided us with social and spatial characteristics of the mobile phone users over an extensive temporal window of observation. From the original dataset of over 1.3 million mobile phone users in Portugal, only the subjects, whose home and work location were estimated to be in the Lisbon study area, were selected for this study. The home and work locations were estimated based on the spatial distribution of the call activity intensity during nighttime and daytime [16]. The home and work locations estimated using this method have been shown to be reasonable proxies of human geography based on a comparison study using the Portuguese census data [16]. This led to the consideration of 22,696 subjects from the dataset, which accounted for 53,210,299 CDR for this study. Following [23] we only considered reciprocal communica- tions in inferring the social network for each subject. On average, each subject was connected to about 24 different social ties (degree) and spent about three minutes on a phone call. The probability mass function of the subjects’ talk time (call duration) and degree are shown in Fig. 1. The other set of data used in this study was a historic record of weather conditions, which was obtained from the Weather Underground Agency of Michigan (www.weatherunderground. com). The Weather Underground had been used as the weather provider for the Associated Press and weather reports for some newspapers (including the San Francisco Chronicle) as well as the PLOS ONE | www.plosone.org 4 October 2012 | Volume 7 | Issue 10 | e45745