Hygrometers are slow and not so reliable. There are, however, various icing sensors, e.g. radioisotope-based ones, which contain a small dose of beta-active isotope and a particle counter. They are installed on Mi-8 helicopters and a few other Russian types. Even a very thin layer of ice on the sensing rod absorbs the radiation, reducing the count rate. Beta particles don't travel more than ~1 m in the air, so it's safe to the aircraft occupants, though on the ground you are supposed to put a lead sheath on it.
...Actually, on second thoughts, I think I know a way or two to design a responsive and reliable hygrometer, but it will certainly cost a lot more than the ones used in meteorology.