If you've ever observed the skies above your head before, then you might have watched as high-flying jet airplanes left white trails behind them. Some of these trails stay around for longer periods of time, while others disappear more quickly, but what are they?
Conspiracy theorists are going to yell, "chemical trails!" around every corner, but that's not actually what causes these contrails.
Instead, they're attributed to a reaction from the jet's exhaust, which expels hot water vapor into the cold upper atmosphere. There, the hot water vapor quickly crystallizes and forms an icy cloud like what you see when you exhale your breath on a cold day in the winter.
But why do some disappear more quickly than others? Experts say it depends on the humidity levels in the sky, and higher humidity causes the trails to stay around longer. Sometimes, this method gets used for weather prediction since it can signal when high humidity levels or rain are nearby.