What caused a fluctuating cloud to form in a jet engine intake on a humid day? 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.)
 A: Is it possible you were seeing an intake vortex. There is a YouTube video here. Since you say it had recently stopped raining there would presumably have been standing water on the ground for the engine to suck up.
Your description suggests a more diffuse cloud than the rather well defined cloud caused by the vortex, but it's possible the vortex was forming and dissipating and you got a more diffuse cloud every time the vortex dissipated.
A: My take is that while the engine is working sucking in air, the pressure in the vicinity of the input falls. The humidity of the surrounding is high and due to the low pressure, that volume  is supersaturated in H2O ( water vapor), and  it forms a local cloud. 
PV=nRT  If the sucked in volume is constant, pressure drop means temperature drop too, conducive to cloud formation given the dust available close to the tarmac.
The pulsing would be the result of the vortex John mentions. The pressure will not be uniformly low but will follow the vortex pattern, a wavy pattern which would tend to time the formation of the cloud ( more humidity/less humidity).
