What's the difference between icicles made with distilled water and icicles made with tap water? A team of scientists from the University of Toronto spun freezing water like a record, to see how it plays out.
The icicle in the video above is not spinning just because it looks cool. (Although we can all agree it looks cool.) Rotation makes it more likely that every side of the icicle gets exposed to the same conditions. The icicle rotates once every four minutes over its ten hour growing time.
It can't hurt that, when the icicle is spinning, air is continuously moving around it. Researchers found that, if you want a nice conical icicle, you need to keep air moving over it. Keep an icicle in still air, and it grows little off-shoots and fingerlings. Outside, icicles usually get exposed to enough wind to keep them sleek, but icicles that grow in shelters areas often look mutated.
This particular experiment involves looking at how impurities in water affect the icicles. The video above shows an icicle being made from distilled water, without any impurities. Below, we can see what an icicle made with tap water looks like.
So now you know. If you come out on your porch and see knobbly icicles, there are a lot of impurities in the water. Either it's raining dirt, or your roof is filthy.