Weather Effects on Mobile Social Interactions Figure 8. Probability mass of the number of connected strong ties/hour under different weather conditions. doi:10.1371/journal.pone.0045745.g008 minutes (Fig. 4(c)). Wind speed, on the other hand, does not appear to have any much influence upon talk time (see Fig. 4(d)). In summary, the individual weather parameters do not appear to impact on the average duration of people’s phone calls (which is around three minutes), but under certain weather conditions, we found an increase in the likelihood for people to make longer calls, i.e., calls longer than six minutes. Colder weather or extreme humidity appeared to increase the likelihood of longer call durations. Days of very high air pressure conditions also appeared to increase the likelihood of above average call durations while the wind speed appeared to show no influence. The time (in hours) for each band of the weather parameters experienced by the people in this study is shown in Fig. 5. Weather effect on degree (number of social ties) Typically we connect to a number of different people using our mobile phone and for a range of purposes. The number of people in an individual’s mobile phone network can be used to understand the structure of that individual’s social network [28], and this can also facilitate many applications such as prediction of mobile phone viruses [29] and the assessment of socioeconomic opportunity [30]. Here we attempted to see the correlation between the change in weather conditions and the number of social ties people connect to. Using the same approach to understand the influence of weather conditions upon the duration of phone conversations, we examined the correlation between degree and different weather parameters. For example, the number of social ties people connect to for different bands of temperature. Because the observation time lengths of each weather parameter O˜s bands were different (as shown in Fig. 5), we decided to normalize the number of ties in relation to time into ‘ties per hour’. This ensures that the results would not be influenced by the differences in observation time lengths. We observed that at the extreme bands of temperature (5uC– 10uC) and (30uC–35uC), people tend to connect to fewer number of social ties when compared to other temperature bands (as shown in Fig. 6(a)). In other words, people tend to be less connected with their social ties during either very cold or very warm temperatures (the annual temperature range of Lisbon from our dataset range from 5uC–35uC). We observed a similar correlation with the humidity level. At humidity bands of 20%– PLOS ONE | www.plosone.org 8 October 2012 | Volume 7 | Issue 10 | e45745