Degree Under extreme temperature conditions (very cold, very warm), people tend to talk to a smaller number of social ties. Under humidity bands of 20%– 100%, people tend to talk to a smaller number of social ties. Under extreme air pressure conditions (very high, very low), people tend to talk to a smaller number of social ties. Under windy condition (when wind speed becomes stronger than 5 km/h), people tend to talk to a smaller number of social ties. Weather Effects on Mobile Social Interactions Tie Strength Under extreme temperature conditions (very cold, very warm), people tend to connect to a smaller number of social ties. However, they maintain their connections with strong social ties more than with weak ties. Under humidity bands of 20%–100%, people tend to connect to a smaller number of social ties. However, they maintain their connections with strong social ties more than with weak ties. Under extreme air pressure conditions (very high, very low), people tend to connect to a smaller number of social ties. However, they maintain their connections with strong social ties more than with weak ties. Under windy condition (when wind speed becomes stronger than 5 km/h), people tend to connect to a smaller number of social ties. However, they maintain their connections with strong social ties more than with weak ties. Table 1. Summary of results. Temperature Humidity Air Pressure Talk Time No effect on average talk time. Colder weather increases the likelihood of call duration to be longer than 6 min. No effect on average talk time. Under extreme humidity (very high, very low) there is an increased likelihood for people’s call time to be longer than 6 min. No effect on average talk time. Very high air pressure increases the likelihood of calls to be longer than 6 min. Wind Speed No effect on talk time. doi:10.1371/journal.pone.0045745.t001 ties is shown in Fig. 7(c). Note that at this stage, this figure does not yet reflect the influence of different weather parameters. We then computed the ratio of this relationship with each different weather parameter. The results in Fig. 10 show us that although people were connected to fewer social ties when the weather conditions were uncomfortable, people lost a higher proportion of connectedness with weak ties when compared to their connectedness with strong ties. In summary, under presumably uncomfortable weather condi- tions, i.e., very cold, very warm, very high air pressure, very low air pressure, or windy conditions, people are more likely to interact with fewer social ties within their mobile phone network but the connection loss of the weak ties was higher than that of the strong ties. This result is also in line with the findings of [22], which established that the social interaction with strong ties is less dependent upon extrinsic factors than weak ties. Conclusions The ubiquity of personal communication technologies, such as mobile phones, means that these technologies can themselves be used to study the relationship between external factors, such as weather conditions and social behavior. We used a number of proxies for social interactions, including time spent on phone calls, number of connected social ties, and the strength of those connected ties. We considered the relationship between these proxies and a number of weather factors including temperature, relative humidity, air pressure, and wind speed. By analyzing our longitudinal data, which includes a full calendar year of communication records for 22,696 mobile phone users (53.2 million call logs), we have discovered a number of interesting results (result summaries are given in Tables 1); our main findings are as following: N weather conditions were found to have a correlation with mobile phone social interactions; N weather conditions do not greatly influence people’s average call time; N as the weather becomes increasingly cold, or when the air pressure is high, the likelihood of longer calls (more than six minutes) increases; N under (presumably) uncomfortable weather conditions (i.e., very cold/warm, very low/high air pressure, and/or windy) people tend to communicate with fewer social ties; N although people interact with a smaller number of social ties during uncomfortable weather conditions, they tend to maintain their connections with strong ties more than with weak ties. Nonetheless, there are some limitations to the observations we present in this study. With differences in weather conditions in different parts of the city, the weather information gathered from the three weather stations used in the study may not in reality represent the actual weather conditions experienced by the people in this study. In addition, these people might be indoors and thus, shielded from certain types of weather conditions such as wind and extreme temperatures. Although different weather parameters work in complex ways to influence actual weather, we did not consider this overall complexity in the current study. Future studies can take this into consideration in order to obtain a more detailed understanding of the overall effects of weather. The effect of special events such as holidays, national and social events were not considered in the study. These events may have some influence on the results through the number and duration of calls made. This too remains to be explored in our future studies. The weather conditions of the observed individual and the person they are connected to will also be taken into account. Finally, the mobile data offered insights into only a small part of the overall individual’s sociability. The results obtained in this study are within the constraint of our tie strength calculation. Our future research direction will need to consider the interplay of weather parameters and other extrinsic factors that may also have an impact on social interaction such as the day of the week or weekly activity patterns, time of the day, holidays, vacations, and special social events. This study does offer another way to understand how and under what conditions weather have an influence upon our use of the PLOS ONE | www.plosone.org 11 October 2012 | Volume 7 | Issue 10 | e45745