FlyingKiwi_73, RIO-3, the one on Mi-8, contains strontium-90, which has a half-life of 28 years. However, it's not the half-life that is primarily important but rather the quantity of the isotope.
There are also other physical properties of water that can be measured to assess air humidity, e.g. intensity of its infrared or NMR spectral lines, etc. Measuring them properly is a tricky business, but it's perfectly doable. It can even be done remotely - you have probably heard of cloud base measurements by a ground-based lidar.