Weather Effects on Mobile Social Interactions Figure 7. Probability mass of the number of strong ties, weak ties, and the ratio of the number of strong ties to the number of weak ties. doi:10.1371/journal.pone.0045745.g007 weather impacts conversation time, we examined the statistical distribution of call durations similar to Fig. 1 for different ranges of each weather parameter. To make the analysis easier and to allow us to more easily discern any emergent patterns we created a set of ranges, or bands, for each weather-related parameter. Based on the climate history (Fig. 3), temperature was considered between 5uC and 35uC and grouped into six bands, each with a 5-degree span; humidity was evenly divided into five bands in the range from 0% to 100%; air pressure in six even bands between 990– 1050hPa; and wind speed in four bands between 0–20 km/h. The statistical distribution of the talk time for each level of each weather parameter was computed as a probability mass function (pmf). This pmf is a normalized histogram on the logarithmic scale, where normalization allows comparisons across different bands of each weather parameter, and a logarithmic scale was used because of the nature of the statistical distribution of the call duration [27]. The pmf is a discrete probability distribution and its value represents the likelihood of a random variable (in our case call duration). The results show that the average call duration does not vary greatly when compared to different environmental temperatures (Fig. 4(a)). However, the lower the temperature (i.e., as temper- ature drops), we notice an increase in the likelihood for phone conversations to be longer than six minutes. In other words, there is a correlation between the drop in temperature and the likelihood that a call will be longer than six minutes. This result suggests that the colder the day, the more likely that people would make longer phone calls. The humidity level exhibits a similar correlation on conversation duration. Again, while different humidity levels did not influence the average duration of phone conversation, there is an increase in likelihood for conversations to be longer than six minutes when the humidity is either very high (80%–100%) of very low (0%–20%) (Fig. 4(b)). Air pressure also does not appear to influence peoples’ average call duration. However, at very high air pressures, i.e., above 1,040hPa (where 1,013.25 hPa is the normal air pressure), there is an increase in likelihood for phone calls to last longer than the average three PLOS ONE | www.plosone.org 7 October 2012 | Volume 7 | Issue 10 | e45745