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Sunny mornings with freezing temperatures are an odd pair in winter. The effects of this pair are even weirder. Icicles are pointy ice shards that form on branches and wires or any edge when the sun melts ice or snow. Water drips down from an edge, and when it is cold enough these droplets refreeze, forming icicles.
If you have ever held an icicle, you will find it to be quite slippery. This is because every icicle is covered in a thin film of liquid water. Why does that film of water not also freeze? To understand this, you must know that heat is transferred from a region of higher temperature to lower temperature. When you hold a piece of ice in your palm, the ice melts, while your palm, which is hotter than ice, gets colder. Similarly, the dripping water freezes (becomes colder) and in the process releases heat to the surrounding air, which gets warmer. Warm air is light, rises upwards, and gives the icicle its characteristic shape.
Pretty straightforward, right? Well, not with the bumps in icicles. If you made icicles with distilled water, they would be smooth and pointy. However, the icicles that form in tree branches or on the edge of roofs contain impurities, like dust. These icicles have bumps on them, which are always a centimeter apart (that’s right, you can use an icicle as a meter ruler!). Icicles appear hazy, and it was thought that trapped air bubbles were responsible. New studies at the University of Toronto not only disprove this notion, but also show that there are hardly any air bubbles in an icicle.
Then why are icicles hazy? The scientists mixed a dye with the water that they used to form icicles. They found that these dyes were densely present in pockets of ice inside the icicles, which contain dollops of water inside. The scientists deduced that these water pockets make icicles hazy. And, like rings of a tree trunk, these water pockets can trace the formation of the icicles.