Why is the air in airplanes so dry?
If you ever traveled a long distance by plane, you may have noticed that the air is extremely dry: The eyes start burning, the skin is dry and you get thirsty. But why is the air not moistened so that the passengers are more comfortable?
There are actually two reasons. First, the air at the altitudes where the plane is flying is extremely cold (say, -60 °C and below) and consequently dry. It contains practically no water. This air is compressed by the turbines and used as cabin air. Any water in the air would come from people breathing or would have to be added. Such a "moisturizing" system costs money, takes up space and has weight, so an airline would have to think if the added cost is worth it.
Second, airplanes are usually made of high-strength aluminum alloys that can corrode in contact with water. They also have a lot of electrical cables running all over. And they fly at high altitude where it is very, very cold. So at least some parts of the hull are cold, too. This means that moisture from the air inside the plane could condense to liquid water on cold parts and cause trouble. Dry cabin air can reliably prevent that: If the dew point (resp. frost point in that case) of the cabin air is below the freezing point of water, there can be no liquid water condensing. Only frost can form, i.e. water vapor turns directly into ice, which does not cause corrosion problems. And a frost point of roughly 0 °C corresponds to a relative humidity of 25 % at 20 °C. If the humidity in the cabin is desired to be above 25 %, one would have to take special measures: Corrosion-resistant materials or a completely sealed vapor barrier on top of the cabin's thermal insulation. Such a barrier would prevent the moisture from reaching colder parts. But it would make the insulation more complex and add weight...