Fog in an A320 cabin

[Spitoon]

A while ago I was down the back of an A320. It was a bright but slightly humid early morning departure. During the take-off roll a fog appeared at vents at the top of the window panel (i.e. just below the overhead bins). The fog lasted for about 6 inches before dissipating quickly. It stopped being produced after the aircraft was passing 2000ft or so without any noticable change of engine power at the time.

I presumed this was a result of the VAC system combined with the humidity inside and outside the cabin. Is this right, or maybe there was some fault? I've not seen it again since - and I've been on far too many early morning A320 flights for my liking lately - is it common?

It was very pretty.

[Tacolegend]

Very very common when it is very humid out. Ive always noticed it when the humidity was higher than 80%.

[nosewheelfirst]

Yip we managed to get the bus looking like one of the clubs in Ibiza last night

[Wirelock]

try an unstable frequency IDG.... then u get ibiza in the cabin.... disco style lights

[john_tullamarine]

All to do with relative humidity. Air can hold a given proportion of water vapour according to the ambient conditions, including temperature (absolute humidity). As the temperature varies, the absolute humidity capability varies, with the result that the relative humidity goes up and down inversely with temperature (higher temperature for the same water content results in lower relative humidity and vice versa).

If the temperature drops sufficiently for the RH to get to 100% (dewpoint) then we start to see the now excess water vapour condensing out as visible water droplets (cloud).

There are various mechanisms to achieve this situation ..

(a) add some colder air (as in your aircon example)

(b) chill the air mass (eg adiabatic cooling associated with expansion .. such as in the vortex flow associated with propellers, flap tips, nacelle chines .. etc. .. the reduced pressure inside the vortex results in a temperature drop ... increased RH ... condensation

Similar thing is at work with cloud formation.

Worse in the tropics ... I paxed a couple of legs on a Tu154 .... got a shower rather than a cloud ...