How does the cone form?
The photographs show an unusual ice structure that formed on the filter of an environmental air sampler. The sampler is a low volume pump, which pulls outdoor air through a filter, which sits horizontally on top of the pump, under a mushroom shaped rain cap. On Friday, Feb. 18, 2000, the weather was very foggy and cold. There was frost on the grass and black ice on the roads. The filter, which is changed every three to five days, was removed shortly before 10 a.m. Ice cones have been seen on the filters before, but this was the first time with a crystal balanced on the point of the cone. We believe due to the weather, the water in the air is very near the triple point (the temperature, pressure, and concentration combination where water can exist as a solid, liquid, or vapor). Because of the shape of the rain cap, the air flow changes direction as it goes through on the way to the filter, a low pressure zone forms at the center of the filter, and water vapor and/or droplets condense ou
