1. Does Pressure Melting Really Explain Slippery Ice?
The blade of a skate is very thin, so your weight presses down on a small area. When ice is pushed hard, it can melt even at temperatures below 0°C. This led to the idea that "the ice under the blade melts into a water film, making it slippery." This pressure-melting explanation was proposed in the 19th century and believed for a long time [2].
However, when scientists do the calculations, they find that the temperature at which ice starts to melt drops only slightly. In an example calculation published in Physics Today, even if a pressure of 466 atmospheres is applied under the blade, the melting point of ice only falls to about -3.5°C [2]. Note that 466 atmospheres is just one calculated example; real skating does not always create this exact pressure.
In this calculation, pressure can only melt ice when it is warmer than -3.5°C. So, how do people skate on ice colder than that?
2. Why Pressure Melting Fails for Ice at -30°C
The ice temperatures preferred by skaters are about -5.5°C for figure skating and about -9°C for ice hockey [2]. Both of these are colder than the -3.5°C limit from the calculation above. Furthermore, people can skate even in climates as cold as -30°C [2].
If you can skate on ice that is colder than the range where pressure causes melting, then pressure cannot be the only reason for slipping. The next clue came from a different phenomenon: not sliding, but ice fragments sticking together.
3. Faraday's Idea: A Water Film Always Covers Ice Surfaces
In 1850, Faraday noticed that when you press two pieces of ice together, they stick. From this, he thought that the surface of ice might always be covered by a thin water film, even below 0°C [1]. It is explained that ice sticking together and snowballs forming happens because the trapped water layer freezes again [1].
This idea was directly confirmed by experiment in 1987. A team led by Yoshinori Furukawa at the Institute of Low Temperature Science, Hokkaido University, used a method called polarization analysis using laser reflection to examine the ice surface. They confirmed that a water layer exists even below the melting point. Depending on the state of the ice, water layers have been found even at -100°C [1].
The thickness of the water layer changes with temperature; it gets thinner as it gets colder [1]. Measurements using very fine instruments showed values ranging from about 12 nanometers (at -24°C) to about 70 nanometers (at -0.7°C) [2]. One nanometer is one-millionth of a millimeter. If a human hair is 0.1 mm thick, 70 nanometers is about 1/1400th of that width. However, estimates of the thickness can differ by more than two orders of magnitude depending on the measurement method and interpretation, so there is no fixed value [6].
One reason it is hard to make snowballs with powder snow is that the air temperature is low, making the surface water layer thin [1]. Also, this surface melting phenomenon is not unique to ice. The same explanation states that metals also form a thin liquid film on their surfaces near their melting points and should become slippery like ice [1].
The existence of the water layer has now been confirmed. The remaining question is: how does this layer form?
4. Does Friction Heat Create the Water Film When We Slide?
Is the water layer there from the start, or is it created by sliding? Another strong theory is friction heat. When an object moves quickly over ice, the contact points warm up locally. In experiments by Colbeck and others, the rise in ice temperature with increased speed matched the heating caused by local friction [2].
In 2018, a team from the University of Amsterdam and the Max Planck Institute for Polymer Research investigated friction on ice. They reported that the ease of movement of surface water molecules is related to slipperiness [3]. A striking finding in this study was that friction is lowest at -7°C. It is said that speed skating rinks sometimes set their ice temperature to this same -7°C [3].
What happens with warmer ice? Between -7°C and 0°C, the ice becomes softer, so objects sink in deeper, making it harder to slide instead [3].
This research focuses less on a "film" of water and more on the mobility of surface water molecules. Surface molecules connected by only two hydrogen bonds can move like small rolling balls due to thermal vibration. This mobility is said to be the cause of ice slipperiness [3].
However, in 2025, another answer emerged.
5. The 2025 Saarland University Theory and the Unanswered Questions
In 2025, researchers at Saarland University announced that pressure and friction are not very important [4]. Water molecules are electrically polarized and have a direction (dipole). In the research team's simulations, the dipoles of the touching object's surface attract the dipoles of the ice molecules. As sliding occurs, the bonds between molecules rearrange, disturbing the crystal structure and creating a liquid-like layer. This is a new explanation, but it is still at the hypothesis stage [4].
Does this settle the matter? Not quite. According to a Quanta Magazine article from December 2025, researchers agree that "there is a thin water-like layer," but there are multiple hypotheses about how it forms. Other researchers have raised objections or offered different views on the new theory [5].
Pressure, friction heat, surface molecular motion, and electrical orientation. Even as the answers have changed over time, many researchers agree that there is a water-like layer on the surface of ice near its melting point. What has changed is the explanation for "why" this layer exists.
It is interesting that even with familiar ice, questions remain unresolved.
6. A Home Experiment: Pressing Two Ice Cubes Together
You can try Faraday's clue at home. Take two ice cubes and place them on a plate. Gently press their flat sides together, wait for about 10 seconds, and then gently separate them to see if they stick.
If they stick, it is explained that the trapped water froze again [1]. Results vary depending on the ice temperature and how wet the surface is, so not sticking does not mean failure. You can also compare ice taken straight out of storage with ice left out for a while until the surface gets wet.
Safety Note: Ice is cold, so do not hold it for too long.
If you find books or articles in a library about why ice is slippery, check what year they were written and which theory they introduce: pressure melting, surface water layer, friction heat, molecular motion, or electrical orientation. This will let you see with your own eyes the history of how the answers have changed.