Ever bump into an acquentaince who is having a bad day? Maybe he's upset about work; you pause, nodding sympathetically as he goes on about his terrible boss and unhelpful colleagues. Up until the chance encounter, you were in a great mood. But chances are the exchange will dampen your day, at least slightly.

That's because, as numerous studies have suggested, other people's moods are as easy to catch as their germs, a phenomenon called emotional contagion. We tend to mimic the behavior of those around us (your acquaintance frowns, so you frown), which triggers genuine feelings of sadness. It's not all bad, of course. It's just as possible for us to "catch" a positive mood as it is for us to pick up a negative one.

But does this same process work over social media? Are emotions expressed online also contagious?

Yes, suggests a new study by researchers at the University of California, San Diego, Yale University and Facebook, who analyzed data from millions of Facebook posts over the last three years.

Related: Want to Spread Your Message? An Angry Tweet May Be the Answer

The researchers started with a neutral variable: the weather (rain, after all, doesn't discriminate between happy and sad people). Turns out, a rainy day affects the overall mood of a user's activity on Facebook, raising the number of negative posts by 1.2 percent and decreasing the number of positive posts by the same amount.

The study also found that each sad status posted on a rainy day resulted in an additional negative post by a Facebook friend living in a city where it wasn't raining. Positive posts had an larger effect than negative ones; Each positive post resulted in a further 1.8 positive posts by a user's friends (i.e. each positive post created almost two additional positive posts), whereas each negative post resulted in 1.3 additional negative posts by friends.

While researchers behind the study speculated on the weighty significance of their findings, suggesting that emotions could "ripple through social networks to generate large-scale synchrony that gives rise to clusters of happy and unhappy individuals," the study only looked at how rain affected the general slant of Facebook posts, and the effect was quite small.

What’s more, the validity of previous research conducted by the study's lead author, James Fowler, has been criticized by some experts. His widely covered 2007 paper, which suggested that obesity is socially contagious, came under fire for an apparent lack of statistical significance to support its findings.

Related: Winter Weather = Bad Mood? It's More Complicated Than That.

Still, the study raises interesting questions about emotion and social media. As we continue to engage with one another online, it seems natural that our mood would be affected by what we see on Facebook, Twitter or any other site where we go to interact. But does an online encounter carry the same weight as a face-to-face one?

Much of the reason that we 'catch' each other's moods is because we see, first hand, the markers of an emotion (the grimace, the tears, the goofy grin). And, as social creatures, we replicate them. As compelling or detailed or graphic as a Facebook post may be, can it even come close to rivaling the immediacy or contagiousness of actual human emotion? Can it be as powerful?

The authors of the study seem to believe it can, citing the way new technologies allow individuals to express themselves to a wider audience. "As a result," they wrote, "we may see greater spikes in global emotion that could generate increased volatility in everything from political systems to financial markets."

So, Facebook users, think about that the next time you turn to the social network to mope or vent.

Related: Connecting With Customers: How to Market to Their Emotions