A rough-hewn hand rail in the Park was covered with what I thought at first was frost. However, there was something odd about it: the ice was in the form of tiny towers. They looked different than all the hoar frost I have seen and more like ice extrusions.
Tiny towers of ice grow upward from the wooden hand rail. How were they formed?
The ice of hoar frost, below, is formed by the condensation of water vapour from the air. The arriving molecules move across an ice crystal to fit into the crystal lattice. These frost crystals look very different than the ice seen, above, on the wooden hand rail.
The ice on the handrail looked more like ice extrusions (geologists call them ice needles, but that is a term that means something different to meteorologists). When seen on the ground, the precursor of ice extrusions is water seeping into cavities in the soil. Then with low overnight temperatures, the water in these cavities freezes. When water freezes, it expands, and the ice now forces its way upward as extrusions rising out of the soil, and even lifts dirt atop it.
Not only do the towers on the wood look like tiny versions of the ice extrusions rising from soil, but they have formed on the cavity-rich sapwood portion of the wood grain rather than on the denser heartwood portion. It seems that our recent rains had filled the cavities in the sapwood with water which froze causing ice extrusions to grow out of the wood.