Absolute humidity is the volume of water that is contained in a volume of air. The higher the temperature, the greater the volume of water it can contain. For example, at the average temperature at sea level and average pressure, of 15 C, a cubic meter of air can't contain more than 13 grams of water. When that happens, the air is saturated with moisture and the relative humidity is 100 percent. When the temperature goes down to 0 C (freezing point) the air can sustain only 5 gr/m3. The absolute humidity is then 5 gr/m3 but the relative one is still 100 percent because at saturation point, a.k.a. dew point temperature.
What happens when the temperature sinks below dew point? Well, the parcel of air has to get rid of some of its moisture by condensing it into tiny water droplets. But that can only happen by giving away energy in form of heat and the tiny molecules have virtually no mass to dissipate that heat energy. They have to touch anything like a dust, smoke particle, pollen or the bonnet of your car, to condense. Likewise, it must give heat energy to go from liquid to solid and that's why you can get tiny drops of water, in the clouds, that are under-cooled all the way down to -40 C!
Incidentally, when water goes from solid to liquid (melt) or liquid to gas (evaporates) it needs to take energy in form of heat and that's why we sweat; its evaporation cools down our skin.
So, when the temperature falls below freezing point, the water molecules in form of gas or vapour can't turn into liquid or solid before it touches something. And that's why you can observe ice rims on the branches of a tree, early in the morning, when the heat of the ground has radiated back into space, during the night, and the surface temperature has fallen under freezing point: the tiny gas molecules turn directly into ice crystal as they touch something with a mass.
