Freshly Baked Science
Icicle Formation (& Underwater Icicles?!)
7th December 2018
During the winter, it’s not unusual to see ice and snow, but icicles are a rarer occurrence. They’re well characterised by their cone shape and a beautiful glass-like finish. But how do these icy daggers come about?
Icicles only form in very specific weather conditions. It needs to be a sunny day or an alternative heat source, but below freezing. In these conditions, the ice or snow can melt and then refreeze as it drips. The icicle starts off as a few frozen droplets, then once it gets large enough, the water begins to run down the side of the frozen droplets and freezes as a thin film all the way down the icicle.
The thin film of water that coats the icicle gives off heat, warming the air around it, by a process called conduction. Warm air is less dense than cool air, so the warm air will rise. When the warm air rises, the heat is removed from the liquid layer, causing it to freeze.
If you’ve ever looked closely at an icicle, you may have also noticed that the surface has ripples. Two scientists, Professor Morris and Mr Chen, discovered that this surface is caused by impurities, such as salts, in the water. When icicles are created from distilled water, the surface is smooth and as soon as the salts are added back, the ripples return.
Icicles don’t only form overground. They can also form underwater and are called brinicles, but are also known by a less desirable nickname; ‘icicles of death’. These are formed in a slightly different way to normal icicles.
When sea ice is formed in the Arctic and Antarctic the impurities, like salts, are forced out. This means that if you were to eat sea ice (for whatever reason), it wouldn’t be salty like sea water. It also makes the surrounding water saltier. When water gets saltier, the freezing point lowers and the density increases. This prevents the water from freezing and causes it to sink. When this cold, salty water reaches the warmer water below, it causes the water surrounding it to freeze.
The brinicle grows as the salt continues to be forced out of the ice and sinks even further. Once the brinicle reaches the seabed, a web of ice forms and spreads across it, freezing everything it touches (hence the name, ‘icicle of death’).
If you see any icicles around this winter, please send us a picture via email (firstname.lastname@example.org) or social media (@wonkscience) with the hashtag #STEMintoChristmas and we’ll get you featured on our website!