Hair ice -- a type of ice shaped like fine, silky hairs that resembles white cotton candy -- grows on the rotten branches of certain trees when the weather conditions are just right, usually during humid winter nights when the air temperature drops slightly below the freezing point.
Scientists have now discovered exactly what gives “hair ice” its strange shape. It's caused by a fungus called Exidiopsis effusa. When the fungus is not present, ice still forms but in a crust-like structure instead. The fungus allows the ice to form thin hairs – with a diameter of about .01mm, and can keep this shape over many hours when the temperature is close to zero.