We’ve all become familiar with contrails — the cloud tracks that planes leave as they fly through certain altitudes:

But, did you know that ships apparently do the same thing? (I sure didn’t!)

UW professor of Atmospheric Sciences Cliff Mass (author, and weather blogger) has a nice post up discussing the phenomena. Both contrails and “ship trails” are produced when microparticulates released by combustion serve as seeds for condensation. Over the ocean, the typical droplet size is much smaller in ship exhaust than in the natural cloud cover, producing different reflectivity, leading to high contrast white ship trails. As Cliff discusses, you can imagine the interest that national security agencies might have in this effect….

(EDIT: Actually, it turns out that contrails are primarily due to vapor released during combustion, not nucleation, so the processes are somewhat different.)