When water makes needles

An ice needle is a particular kind of ice crystal that forms in cold air (especially in clouds or fog). 

It has a long, thin, linear shape
may resemble a tiny “sliver” or “thread” of ice.

It is often observed in phenomena such as freezing fog, very thin icy precipitation or low temperature in Russian lakes or northen lakes.

Why water don’t always form classic “flake-like” crystals

Water molecules naturally tend to organize themselves into a hexagonal structure when they freeze.

That’s the standard shape of ice that we all know.

However, the final shape depends on environmental conditions, especially:

• temperature
• humidity
• growth rate

Ice needles form when crystal growth is strongly directional (called anisotropic growth – a growth that is not equal in all directions)

In this kind of situation, water molecules stick more easily to certain faces of the crystal
much less so to others.

Therefore, the crystal grows faster in one direction → it becomes “elongated”.

Ice needles often form around a specific range of temperature: -3°C to -8°C

In this range vertical growth (following the vertical c-axis) is favored, while lateral growth is slower, that’s why they produce a thin and long structure.

Ice grows selectively.

In general ice has a hexagonal structure, with two main directions:

• c-axis (vertical) → upward growth → needles/columns
• a-axis (horizontal) → lateral growth → plates/flakes

NOTE:

The normal ice is Ice Ih (hexagonal ice) but in specific conditions the ice exist like Ice Ic (cubic), high-pressure ice (Ice II, III, V, etc.).

The structure depends on physical conditions but on Earth: under normal conditions the water crystal  creates an hexagonal form.

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So the surfaces are NOT all the same: some are easier for molecules to attach to, others are more difficult.

When the temperature is in the range between -3°C / -8°C something very specific happens:

The lateral (basal) surfaces become stable, they are smoother and more orderly, so the incoming new water molecules have difficulty sticking together and the lateral growth slows. Furthermore they also have less cinetic energy, so they are unable to break the smooth surface and cannot create new growth points. The lateral growth is almost completely blocked.

Edge-shaped (prismatic)  surfaces instead are more active, they have more irregularities so there are more sites for molecules to attach and the growth along the c-axis accelerates.

We can imagine the crystal as a building, where the horizontal sides are like a smooth floor where is very difficult to build on, while the vertical sides are like walls with handholds where is more easy to build. So you can build only in height, not in width, and a needle is born.

Another image: we can imagine the crystal also as a hexagonal prism. Think of a hexagonal pencil: it has two plain bases (top and bottom), and it has 6 vertical sides. In ice, it’s the same.

 There are two types of surfaces.

1. Basal surfaces are the bases above and below. Flat, horizontal, like the “cap” of the prism.
2. Prismatic surfaces (edge-shaped) are the vertical sides that connect the top and bottom and form the edges of the crystal.

Edge-shaped = surfaces that define the edges of the hexagonal shape.

In crystal physics, we speak of:

kink sites (attachment points)
steps (steps)
terraces (flat areas)

Basal surfaces have few kink sites → slow growth

Prismatic surfaces have: many kink sites → fast growth

The crystal grows along the direction that passes through the bases so it elongates using the prismatic surfaces.

At around -15°C: the basal surfaces become more active, so the it starts a rapid lateral growth and the moleculas create branched tassels. 

At around -5°C: the prismatic surfaces are predominant, so the growth is vertical and needles are formed.

If the air is very humid: many water molecules arrive all together and they attach themselves where it’s easiest, so there is a fast dominant direction that further strengthens the needle growth.