Hoar Frost - another neat nature trick

I came across an interesting specatacle at the weekend - a hoar frost. I think what I saw was a particularly good example. Unfortunately I had just my camera phone to snap it with, so the images are to say the least a little poor (but better than no phone at all!). The linked-to snaps though at least give you an impression of how amazing this is.


Apparently, a hoar frost is a result of particularly unusual circumstances - when the object of the frost is colder than the air temperature, but both below freezing (usually near freezing though). Essentially, as I understand it, what makes it unique is that the water goes straight from vapour to solid form rather than via the dew stage. It therefore has a much more crystalline appearance (needle-like in this case and all in one direction off each branch as if windswept).

As you can also possibly pick out, the crystals would form along the whole length of each branch - not just in spikes. This is shown particularly well on the shrub photo.


It was also quite a spectacle to see them falling from the trees later on - like snow falling off but more painful if hit - typically one large part of a tree at a time. We had perhaps a 2" layer of crystals below the trees which shows just how much ice had formed in such a comparatively short time. Interestingly you could go half a mile in any direction and the frost was not a hoar frost, so this was very, very localised. So we played Sunday tennis as usual when I would have expected that location to have an even more severe frost.


This photo shows the crystals after they had fallen from the trees onto a table. Some of the crystals were considerably longer than 1 inch (25mm), yet they came from a branch perhaps just 5mm in diameter!