Many complex human and natural phenomena can usefully be represented as networks describing the relationships between individuals. While these relationships are typically intermittent, previous research has used network representations that aggregate the relationships at discrete intervals. However, such an aggregation discards important temporal information, thus inhibiting our understanding of the networks dynamic behaviour and evolution. We have recorded patterns of human urban encounter using Bluetooth technology thus retaining the temporal properties of this network. Here we show how this temporal information influences the structural properties of the network. We show that the temporal properties of human urban encounter are scale-free, leading to an overwhelming proportion of brief encounters between individuals. While previous research has shown preferential attachment to result in scale-free connectivity in aggregated network data, we found that scale-free connectivity results from the temporal properties of the network. In addition, we show that brief encounters act as weak social ties in the diffusion of non-expiring information, yet persistent encounters provide the means for sustaining time-expiring information through a network.