I was on a commercial flight in a 737 stationary at idle on a taxiway. It had recently stopped raining so the relative humidity was likely near 100% and the air temperature was about "light jacket" level. I was in a window seat forward of the engine intake so I could see the whole intake.
Every 30 seconds or so a cloud would burst into existence across the entire jet intake and be sucked away in perhaps one quarter of a second. Was it really a tiny cloud that I was seeing and what bit of thermodynamics was causing it? Why was it periodic?
(Yes, I know "Boeing 737" under-specifies the engine type but it should give an approximate size and configuration. Intuitively I expect that the engine design was not crucial for the effect, but I've not had any university-level physics, so my intuition is worth what I'd paid for it.)