A study by researchers at the University of California, Yale, and Facebook has found that moods can spread virally through social media sites such as Facebook.
Using data from millions of Facebook users, the researchers examined the impact of rainy days.
They found that for every one person directly affected by rain, one to two others would also feel the impact.
The study was published in online scientific journal Plos One.
"What people feel and say in one place may spread to many parts of the globe on the very same day," wrote the report's authors.
They added the data suggests that "online social networks may magnify the intensity of global emotional synchrony".
Positive spreads faster
Researchers have long known that emotions can be spread through people via face-to-face interaction, but the new frontier is to examine whether the effect translates to social media interactions.
The researchers - some of whom were Facebook employees at the time the research was carried out - analysed the emotional content of billions of updates posted to Facebook between January 2009 and March 2012.
To test whether emotions spread, they looked at how updates changed when it rained.
They found that negative Facebook posts increased by 1.16% and positive posts decreased by 1.19% in response to gloomy weather.
They then looked at the posts of people who were friends with those impacted by rain, but who lived in cities where the weather was not necessarily as bad.
The result? Every sad post generated an extra 1.29 more negative posts than normal among people's friends.
Surprisingly, every happy post had an even stronger impact: if a user posted an upbeat statement, an extra 1.75 positive posts were generated.
"These results imply that emotions themselves might ripple through social networks to generate large-scale synchrony that gives rise to clusters of happy and unhappy individuals," wrote the authors of the report.
"New technologies online may be increasing this synchrony by giving people more avenues to express themselves to a wider range of social contacts," they said.
"As a result, we may see greater spikes in global emotion that could generate increased volatility in everything from political systems to financial markets."