  People nowadays express their opinions in online spaces, using different
forms of interactions such as posting, sharing and discussing with one another.
How do these digital traces change in response to events happening in the real
world? We leverage Reddit conversation data, exploiting its community-based
structure, to elucidate how offline events influence online user interactions
and behavior. Online conversations, as posts and comments, are analysed along
their temporal and semantic dimensions. Conversations tend to become repetitive
with a more limited vocabulary, develop at a faster pace and feature heightened
emotions. As the event approaches, the shifts occurring in conversations are
reflected in the users' dynamics. Users become more active and they exchange
information with a growing audience, despite using a less rich vocabulary and
repetitive messages. The recurring patterns we discovered are persistent across
a wide range of events and several contexts, representing a fingerprint of how
online dynamics change in response to real-world occurrences.
