Sue Cox (via email) asked:
When it's warm during the day and freezes at night, in the morning you sometimes see in the mud these tiny icicles that look as if the water in the mud was expelled really fast into the air and were frozen mid leap. They can be several inches high and are very fragile. What causes this?
This is likely to be caused by water condensing out of the atmosphere onto a point. Once there is a nucleation centre, such as an ice crystal in the mud on which more ice can form. This small crystal of ice would not only offer a nucleation point, but would also separate the condensed water vapour from the relatively warm Earth, encouraging freezing on this, slightly higher, point. This would lead to a ‘stalagmite‘ of ice forming from atmospheric water vapour.