When the air is saturated you do get clouds. So on a day with a completely clear sky you can say there is no altitude where the air is saturated. If you have high cirrus clouds only you know the lower levels are not saturated but the higher levels are. And if you have a low cloud layer and clear above, then you know the lower level is saturated but the RH is less when you get higher.
It's hard to think of different parts of the sky being different from the part right next to it because we don't see the difference. But on a day with 20 SCT the air where the cloud is either has more water vapor or is colder than the air without clouds.
Boeing and the engine companies have thought that at -40 the amount of water vapor in the air will not be enough to cause a problem. The water should already be ice crystals so it will not stick to anything. However Boeing and GE have found that even these ice crystals can cause problems in their latest engines on the 747-8 and 787.
A kg of air (a bit less than a cubic meter at sea level) at 20c can hold 15 grams of water. At -40c at altitude it now takes a few cubic meters to equal a kg but the air can only hold .1 of a gram of water.