1. Where the Water in an Icicle Comes From: Melting Roof Snow
The water in an icicle comes from snow that has melted on the roof. It is explained that the inside of a building is warm due to heating, and this heat travels through the roof, slowly melting the snow from underneath. [1]
The melted water flows along the slope of the roof to the eaves, where it touches freezing outside air and turns back into ice. Another combination occurs when temperatures or sunlight melt the snow during the day, and the cold of the night or early morning freezes it again. The Japan Weather Association explains that icicles grow more easily when there is heavy snowfall and large differences between warm and cold temperatures. [2]
2. Why a Thin Film of Water on the Ice Surface Lets an Icicle Grow
The Nakaya Ukichiro Snow Science Museum states that for an icicle to grow, a thin film of water must always be present on the ice surface. [1] The view is not that water is poured onto a frozen stick, but that ice covered in a water film extends outward.
When water freezes, it releases heat called latent heat. If this heat accumulates around the ice, it cannot freeze any further. A research team from the United States summarized in a paper that the growth of an icicle is determined by the speed at which this heat escapes to the surrounding air through the thin water film. [3]
The film flows pulled by gravity, affected by viscosity and surface tension, and falls as a water droplet at the tip. [3]
3. Why the Tip of an Icicle Is a Hollow Tube Holding Water
The tip of an icicle is not a solid stick with contents inside, but is shaped like a tube. The Japan Weather Association explains that only the outside freezes, leaving a water droplet hanging. [2] In general explanations, when a water droplet tries to fall, the outer part that touches the cold air freezes, and the icicle extends little by little this way.
In research, a model published in 1988 by Finnish researcher Makkonen is well known. The idea is that an icicle grows as a hollow tube that traps liquid water at the tip. Growth is said to be sensitive to atmospheric conditions like temperature and wind, as well as the amount of water supplied, making it complex. [4] However, the author has only seen the abstract of the paper and has not read the full text.
Safety Note: Do not stand under an icicle. Observe from a distance. It is dangerous if it falls, so do not touch it. The Japan Weather Association explains the tip as being tube-like, with only the outside frozen and a water droplet hanging. [2]
4. A Theory of Why Icicles Get Thinner Toward the Tip
Regarding the shape of icicles, a research team from the United States built a theory based on water flow and how heat escapes. The theory predicts that the shape of an icicle becomes a shape similar to that of stalactites (stones hanging from cave ceilings), and the limiting shape, which narrows toward the tip, can be expressed by the formula ρ = (4/3)ζ^(3/4), using dimensionless radius ρ and distance from the tip ζ. [3]
When icicles were made and examined in a cooled room, some icicles matched this shape very well, it is reported. Not all icicles match. [3]
No material was found this time that states in one sentence directly "why the tip is thin." Reading it as a result of the theory—that the shape is determined by the amount of water and how heat escapes—is the understanding that follows the materials.
5. Why Wave Patterns Form on Icicles Made from Tap Water
Sometimes, horizontal wave patterns line up on the surface of an icicle. In experiments in the United States, regular wave patterns appeared on 48 out of 53 icicles made from tap water. On the other hand, of 23 icicles made from distilled water (water with almost no impurities), only 2 had waves. [3]
In this experiment, wave patterns were frequently seen on icicles made from water containing impurities. Inside icicles with waves, there are V-shaped structures connected to the surface waves, and crescent-shaped liquids with concentrated impurities have been observed. Some of the mechanism for how waves form is becoming clear, but the cause of the underlying instability and the factors that determine the spacing of the waves are not yet known. [5]
The blog of the National Museum of Emerging Science and Innovation (Miraikan) introduces an explanation that small bumps form on the surface due to changes in water amount and temperature, and protrusions cool easily so they grow preferentially. [6] However, this is one easy-to-understand explanation, and the cause is not settled.
6. How to Observe Icicles Safely from a Distance
If you see an icicle, do not stand under it; observe from a distance. It is important not to touch it because it is dangerous if it falls.
The points to look for are three: Is there a water droplet hanging at the tip? Does the thickness get thinner from top to tip? Are there horizontal wave patterns on the surface? Try to verify with your eyes how much the content of this article applies. You can also take photos and compare the icicles.