Air saturation
 

Hello,

I am a little bit confused about air saturation. B737 FCOM says EAI must be on when icing condition exist or anticipated except during climb and cruise below -40 SAT. If the air is saturated at lower SAT as mentioned in this threat http://www.pprune.org/questions/4815...ngine-tai.html doesn't that mean it has water vapor already?

If your question is why EAI is not required below -40 deg, it has nothing to do with 'saturation'. Conventional icing is related to supercooled liquid water that impacts the cold aircraft surfaces and freezes - and the general belief is that supercooled liquid water can't exist below -40 deg (BTW, -40 C and -40 F are the same temp, so no need to differentiate).
This is not to be confused with "Ice Crystal Icing" - the problem that has been a recent issue for the GEnx engine on the 747-8 and 787. ICI is fundamentally different - when the ice crystals hit a cold surface, they just bounce off and are not a threat. But when the ice crystals hit a warm surface they can melt, additional crystal then cool the water enough that it re-freezes. EAI doesn't help with ICI.

My question is more likely related to Meteorology. I understand as the temp gets lower the less water vapor it can hold and hence the possibilities of icing formation is very low. Do you agree with this? I was only confused because in the other threat (which I posted in my original post) someone mentioned the air become 'saturated' which mean it's full of water vapor.

- I don't think anyone did!