Buzz
The Icehotel in Jukkasjärvi, Sweden, is renowned for sourcing massive ice blocks directly from the Arctic Torne River. The ice is incredibly clear, a result of the area's prolonged cold seasons, snowfall, and winds. Ben 'Big Ben' NilssonKey Points to Remember
- Ice becomes cloudy because of impurities and air bubbles trapped in the water, which get pushed to the center as it freezes from the outside in.
- Clear ice, such as black ice or ice produced in specialized equipment, forms gradually with fewer impurities, allowing it to freeze without cloudiness.
- Boiling water before freezing it helps eliminate dissolved gases, leading to clearer ice cubes. This is an easy technique to get clearer ice at home.
As the Northern Hemisphere cools down, the season of autumn settles in, bringing with it the impending return of winter on December 21, 2018. Along with these seasonal changes come the usual obstacles. One term, in particular, causes car owners to panic—"black ice." The National Weather Service defines this as "patchy ice on roadways or other transportation surfaces that is difficult to spot."
To the naked eye, it's nearly invisible. Black ice isn't really black; it's transparent. The dark appearance only occurs when it covers a layer of pitch-black pavement. Accidents happen when drivers, unaware of the hazard, drive over it and lose control of their vehicles.
Other types of ice are more visible. You've probably noticed that homemade ice cubes tend to be cloudy and opaque in the center, resembling blocks of cotton candy. This contrasts sharply with the sheer transparency of black ice. Why is black ice see-through while the cubes in your standard ice trays are not?
Feeling Transparent
Liquid water appears to be clear. This happens because water doesn’t absorb most of the visible light that passes through it. However, there's a trick to water: no matter how fresh it looks, it's never entirely pure. Water samples often contain microscopic particles of organic matter—algae, plant fragments, etc.—along with suspended dust, sediments, and minerals like calcium and lime. Additionally, water may hold various dissolved gases, like oxygen, which fish rely on to breathe. These dissolved gases and physical impurities explain why the ice cubes in your lemonade jug are often cloudy.
When water freezes, the internal debris and air bubbles that get trapped within it can accumulate in clusters. These clusters obstruct the passage of light, causing the visible light spectrum's various frequencies to scatter. This scattering effect makes the ice appear white and cloudy. However, this doesn't always occur.
Walking on Water
In 2014, Tomas Nunuk, a YouTuber, recorded a video of himself and a companion walking on Velke Hincovo Pleso, a deep freshwater lake located in the Slovakian mountains. The video quickly went viral because it captured the stunning clarity of the ice, allowing the rocks on the lake's bottom to be clearly visible through the transparent water. Some viewers even suspected the footage had been faked due to how clear the ice appeared. But Velke Hincovo Pleso is entirely real, and when the lake freezes (which happens frequently), the surface ice can become nearly crystal clear.
For ice to achieve such transparency, it needs to be nearly free of air bubbles and debris. There are also other essential factors. Ice forms as crystals, and when H2O freezes quickly, the crystals tend to be smaller and more numerous. This results in a whiter appearance since crystals have reflective surfaces. More crystals mean more light scattering, which makes the ice look whiter. On the contrary, slowly frozen ice forms larger and fewer crystals, which aids in its transparency.
In summary, ice only becomes clear and see-through if it freezes slowly and is relatively free of impurities or crystals. Tomas Nunuk's viral YouTube video demonstrates what happens when all these conditions align. On the day of his trek across the Slovakian lake, Velke Hincovo Pleso had a thin layer of ice about 2 centimeters (0.78 inches) thick. The ice likely formed gradually during a calm period, with no strong winds to stir up the water or introduce debris into the lake.
Black Ice and Tray Trouble
A similar phenomenon occurs with black ice on the roadways. While the sources of black ice can vary (it may form from fog, mist, drizzle, or melting snow), it always freezes slowly in calm, low-wind conditions, is uniformly thin, and contains very few impurities. This is why black ice is so clear—and such a hazard for drivers.
Despite the problems it causes on the roads, transparent ice is actually rarer in nature compared to the more common white, cloudy ice. There are many reasons why ice may not be clear. Some ice forms from fallen snowflakes, whose jagged, angular shapes and irregular crystals scatter light. In other cases, water freezes quickly, resulting in ice with smaller, less transparent crystals.
The same principles apply to artificial ice. Most household ice trays are designed to freeze water in blocks from the outside in, which has the unintended effect of pushing impurities to the center of each cube. As a result, the center of the cubes tends to look white and cloudy. To create clearer, more appealing ice cubes, some high-end restaurants use machines that freeze the water very slowly from one side, allowing the impurities to move outwards.
Fortunately, there are several inexpensive methods for making transparent ice cubes. A popular method involves boiling the water beforehand to remove dissolved gases before the freezing process begins.
Water may appear colorless, but technically it isn't. It actually has a subtle bluish tint, which occurs because it absorbs red, yellow, and orange light more efficiently than blue light. However, our eyes only detect this faint blue hue in larger bodies of water. This explains why the ocean appears blue to us, while a glass of water looks clear. It also sheds light on why large glaciers often appear to have a royal blue shade.
