Figure 6. Probability mass of degree/hour under different weather conditions. doi:10.1371/journal.pone.0045745.g006 interests. By contrast, weak ties represent mere acquaintances. Granovetter defined the strength of a tie as ‘‘a combination of the amount of time, the emotional intensity, the intimacy (mutual confiding), and the reciprocal services’’ ([26, p. 1361]). We adopted a similar approach to [23] using the amount of time spent in communication and reciprocity as proxies. By computing tie strength based on call duration normalized by the average as given by Eq. (1) we can categorize ties accordingly. s(i)~ c(i) N 1 X N i~1 c(i) , 1 ð Þ where s(i) is the tie strength with tie i, c(i) is the total call duration with tie i, and the denominator is the average call duration of all associated ties where N is the number of associated ties. We then classify ties for which s(i)v1 as weak ties while strong ties are ties for which s(i)§1. In this study we investigated the relation between weather condition on social interactions by using call duration (time spent on Weather Effects on Mobile Social Interactions phone calls), degree (number of connected social ties), and kind of social tie (the strength of connected ties) as indicators of social interactions, and using the climatic condition data collected from three different locations of the weather stations in Lisbon (temperature, relative humidity, air pressure, and wind speed) as our weather parameters. Results Although certain weather parameters depend upon others, in this study we considered the effect of each weather parameter on individual indicators of social interactions. The following results show how the range of each weather parameter correlates to individual indicators of social interactions – i.e., we retrieved CDR and examined the distribution of each indicator of social interaction under specific weather conditions. Weather effect on call duration Talking with people on a mobile phone is a way of maintaining and developing social relationships [20] and is therefore a good indicator of social relation strength [26]. To find out how the PLOS ONE | www.plosone.org 6 October 2012 | Volume 7 | Issue 10 | e45745