The first condition that is conducive to hair-ice formation is rainfall on the few days prior to the formation. If this is followed by a clear and cold night with temperatures of -1 to -5 degrees, finding hair ice is a possibility.
Temperatures should not get too cold, as the water in the dead wood must not freeze completely. It is also ideal if there is just a slight breeze and the air is not too dry. Otherwise the hair ice will sublimate, i.e. change directly from a solid to a gaseous state.
If all the conditions are present, it is worth making the effort to be in the forest at sunrise. The first rays of sunlight show off the hair ice beautifully, but the filigree hairs melt away quickly in direct sun.
Why does hair ice form?
Now to the difficult question of why hair ice forms in the first place.
Alfred Wegner was the first to describe the formation of hair ice back in 1918. It is now known that the phenomenon only develops on dead wood in which a certain winter fungus (Exidiopsis effusa) is living. Why this happens is not yet know for sure.
What we do know, however, is that liquid water in the wood pores has a lower surface tension and a lower melting point than free water. If the supercooled water from the wood freezes on the surface, more water is sucked out of the wood and the filigree hair grows in length.
A more detailed explanation by Christian Mätzler, hair ice researcher (Interview in german from page 5).