Weather Effects on Mobile Social Interactions Figure 9. Probability mass of the number of weak ties/hour under different weather conditions. doi:10.1371/journal.pone.0045745.g009 100%, people also tend to be connected with fewer people within their mobile social network (as shown in Fig. 6(b)). As for the air pressure, we found three distinct patterns. Firstly, for the air pressure band of 1010hPa–1030hPa (the band where normal air pressure falls into), people were found to be likely to be connected to 0.004 social ties/hour. Secondly, at air pressure bands of 1000hPa–1010hPa and 1030hPa–1040hPa (which are respectively lower and higher than the normal), we found that people were connected to less number of their social ties, around 0.002 ties/hour. Thirdly, at extreme air pressure bands (990hPa– 1000hPa and 1040hPa–1050hPa), we found that people connected to the least number of their social ties, around 0.0004 ties/hour. This can be seen in Fig. 6(c)). In terms of wind speed conditions, the result in Fig. 6(d) shows that on days where there is little or no wind (0–5 km/h), people were found to be connected to 0.006 ties/hour. However, when wind speed was stronger than 5 km/h, connection to social ties were found to drop slightly, down to 0.002 ties/hour. To summarize, under extreme weather conditions that might lead to feelings of discomfort, people were found to interact with fewer social ties via their mobile phones. Weather effect on the kinds of connected social ties The results so far suggest that the weather condition has an influence on the time people spent talking on their mobile phone and the number of people they communicate with. We have learned that colder temperature, extreme humidity, and high air pressure can lead to an increase in the likelihood of long phone calls (i.e., more than six minutes), and at the same time extreme weather conditions were found to reduce the number of contacted social ties. With this in mind, we wanted to find out what kind of social ties people maintained connections to during those presumably uncomfortable weather conditions. We did this by applying the concept of tie strength (as developed by Granovetter [26] and Onnela et al., [23]) to work out the number of strong and weak ties in people’s mobile phone network described previously (in the analysis section). The overall distributions of strong and weak ties in our dataset are shown in Fig. 7(a) and (b) respectively. It turns out that peoples’ connectedness to the number of strong ties within their network is not greatly influenced by the temperature, humidity, or wind speed. However we found a more noticeable correlation with extreme air pressure conditions (990hPa–1000hPa and 1040hPa– PLOS ONE | www.plosone.org 9 October 2012 | Volume 7 | Issue 10 | e45745