On February 2nd, 2016, we made an excursion to the Anna Tower together with my daughter´s form. The Anna Tower is located in the Deister hills, a small range of hills at about 20 kilometres southwest of Hanover. Its highest elevation ist Mt Bröhn near the Anna Tower with about 405 metres above sea level. We startet from the Nienstädter Pass at 277 metres above sea level. The car park there is covered with gravel and normally rather muddy, but that day it was frozen and hoar frost glittered everywhere. When we startet our excursion, the weather was bright and sunny. But as soon as we left the car park, I noticed a fibrous thing of brilliant white just beside the path which leads along the top of the range. At first sight, it looked like a sheared piece of wool from a sheep. But for me it was clear what it was: ice wool!
Up to then, I knew ice wool only from descriptions, and although I had been looking for it for years everytimes there was a light frost, I never found some. And now I found it right beside the path, without having searched for it! After having given others a hint on that phenomenon and explaining it, I took some photographs and then continued my way – slowly and even slower, because there were more and more tree branches which showed ice wool. After having found 10 of these ice wool formations, I roughly counted them, but when I reached 50, I stopped counting. It made no sense, because there was too much. I found a place where at least 20 branches and twigs with ice wool laid around. Not every ice wool formation was well defined, but there were also some very bizarre ones among them!
I arrived at the Anna Tower about one hour delayed. The weather was nice, but it was not really clear. There was a distinct inversion with a pronounced layer of mist, but without any mirages. I think, for this the hill is not high enough. (At really clear conditions you can see Mt Brocken in the east and the Porta Westfalica in the west from the Anna Tower).
Three weeks later, on February 27, I succeeded in finding ice wool at the Nienstädter Pass again. This time I was prepared better and brought a retro adapter to make macro photographs. Thus it was possible to take detailed pictures of the ice wool. Some parts of it had structures which reminded of chains of bacilli. Other parts just looked like shiny and transparent hair. The augmentation effect of the lens with retro adapter was not strong enough to unravel the structures here.
Ice wool is a physical and biological phenomenon which mainly appears on rotten and decayed wood in deciduous forest with mixed types of trees. It forms hairy ice curls of a brilliant white which remind of candyfloss. Sometimes it looks like paintbrushs with the uppermost parts cut away, others look like wool from a sheep, others remind of minerals or lichen. And sometimes it looks just like a thrown away paper tissue and is often mixed up with this from the distance. But it has always this hairy and cristalline structure which sometimes looks like chains or if it was covered with sugar.
Ice wool is caused by the activity of funguses which decompose rotten wood. During this process, water is set free which gets out of the dead wood through capillaries and freezes at temperatures around or slightly below 0 °C, forming these hairy ice structures. This works as long as the wood itself does not freeze. Contrary to hoar frost crystals, ice wool is formed by liquid water from the wood freezing outside while the atmosphere is not involved. It is not long ago that the process could really be clarified.
Ideal conditions for the formation of ice wool are given when after a period of mild weather with (light) rain the sky clears off at night allowing frost on the ground. So, when you have to scratch the ice off your car windows, there is also a chance of encountering ice wool in the forests. It can be found from October until the beginning of March, except during very cold periods. Best places to find ice wool are under oak and beech trees and maybe also under some larches. Other conifers are not suitable.
But you also need some good luck when looking for ice wool, just as it normally is not wide spread. Similar to the appearing of mushrooms and toadstools, there seem to be good and bad years. Even if the conditions may apparently be perfect, you will not automatically find ice wool. Locations also seem to play an important role as I could see for myself a short time ago. While ice wool was rather abundant up there along the path on the top of the range, I could not find any of it in the Deister forest near my home, although the tree population there is not very different from that at the Nienstädter Pass. Also here lots of branches and twigs in all stages of rotting are lying around, but there is not a single trace of ice wool to be found.
Author: Reinhard Nitze, Barsinghausen, Germany