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It’s quite an ice day out there

12 mins read

I’m sure many people in Scotland are aware of how the weather has acted in the last couple of days. Indeed, the temperature plummeted 23’C below freezing in Braemar, which recorded the lowest temperature in Scotland since 1995.

But something else has recently been catching the attention of various social media users on Twitter. Ice.

You see, ice has amazing characteristics that allow it to manifest itself in many different formations. These can vary from shape to size, and are dependent on other factors such as environment, temperature, location and humidity.

Really, ice is just frozen water. But the beautiful thing about ice is that rather than contracting in size like most solids do, ice expands. This means it has a unique characteristic independent of that from other solids, although bismuth and antimony are other examples of this process.

When water freezes, the molecules do not slow down and form tighter bonds like in other solids. Instead, ice takes the form of a six sided crystalline structure. Due to the space between each bond, ice is less dense than water, and therefore it floats on the water surface.

Okay, so I told you the basic bit. But, you ask, what’s going on with the social media posts? Why are there spikes on birdbaths? Why does the ice look like pancakes in the River Kelvin?

Let’s go through some of these, where I try to explain how each ice formation occurs.

Ice spikes

Have you ever wondered why you’ve gone outside one day, and seen what looks like a huge long spike penetrating out of a frozen water surface in your birdbath or pet bowl?

Does it look something like this?

New Scientist Default Image
Image credit: Andrew Bradbeer: New Scientist

This is an ice spike. It is a natural phenomena and happens in small bodies of water such as birdbaths and pet bowls. All of the water in the bowl or bath tends to freeze except for a small hole. The small hole allows the ice to further expand, pushing water up, which then freezes around the edge.

Let’s have a quick geometry lesson. The simplest formation of an ice crystal is a hexagonal prism.

You can see from this image the top face of the prism. In an ice crystal, this face is called the basal plane. There are two basal planes: one at the top, and one at the bottom. Now imagine a line that starts from the bottom basal plane, and finishes at the top. This is the c-axis. In an ice crystal, the c-axis influences the formation of a spike.

This is the simplest possible scenario. Now if you turn the prism to the side, the c-axis will become horizontal.

If the first ice crystal to form does not have a vertical c-axis (like in the image) the basal plane will intersect the water surface along a line that is perpendicular (at a right angle to) the c-axis. Ice crystals begin to form along this line.

At the same time this is happening, a curtain of ice grows down into the supercooled water along the basal plane. Crystals merge, becoming rapidly fixed in place as the film of ice grows across the water surface. The sheet of ice continues to freeze until only a small hole remains unfrozen.

This hole may be triangular in shape as the crystallite curtains tend to form at 60 degrees. The continuing expansion of the ice downward squeezes remaining water up through the hole, forming a meniscus that is convex.

Image result for convex miniscus
Meniscus shapes. Credit: Socratic

Surface tension of water usually results in a concave meniscus. But here, the meniscus is convex because the force of gravity is pushing down on the water as it moves upwards. The edges of the meniscus freeze, forming a dam. This pressure forces the water to rise. The process continues as the spike slowly forms and reaches upward.

The process continues as long as the rate of water expansion is the same as the freezing rate at the lip of the hole. Successive layers form a tube of ice that protrudes out either vertically or at an angle.

Eventually all the water is frozen, or the tip seals over, and the spike stops growing. If the spike grows from a crystallite formed below the water surface, it will project at a steep angle rather than perpendicular to it.

This process can only happen in supercooled water. Supercooled means the water has been lowered below freezing point without it becoming a solid. This happens in small confined areas such as pet bowls and birdbaths. On a larger water surface, spikes cannot form in this way because the water cannot expand upward if it is already frozen on the surface.

As a result, you won’t see ice spikes at temperatures far below freezing. You will only see them at or a few degrees below freezing. This explains the media reports which documented ice spikes in the UK and which baffled citizens of much colder countries like Canada.

Pancake ice

Pancake ice is an incredibly rare ice formation that only forms in sub zero temperatures. In Scotland, it has been spotted in the River Kelvin, close to Glasgow, by passers by. Pancake ice is exactly what the name says. It looks like this:

Image
Pancake ice on the River Kelvin, near Glasgow. Credit: Colin McCredie

They resemble lily pads found in ponds. But the way they form is through wave action.

You see, when waves jostle pieces of sheet ice against each other, rounding occurs. This is similar to the processes of abrasion and attrition you might have learned in a geography class at one point.

Basically, as the sheet ice needle like crystals (frazil ice) collide against one another, they start to round the edges, forming a disk like shape. But the real indication that you have pancake ice is when the rims of the discs elevate upward.

Random bumping against one another, and periodic compressions at wave troughs force the splashing freezing water, frazil ice or slush to pile up around the edges of the discs. Slush is a combination of snow and water.

Pancake ice can also form on grease ice. This is a thin layer of ice that has gathered on the surface of agitated water, such as the swell in seas. The grease ice is made up of floating pieces called rinds. When rinds break up, the constituent pieces are affected by wave action.

Another type of ice that forms pancakes is called shuga. These are ice lumps a few centimetres across and have a spongy texture. Shuga, grease and rinds are categorised into new ice. Shortly afterward, the ice becomes stronger and evolves into nilas which is just a term for stronger ice crusts. After that, ice is categorised as first-year ice, and so forth.

The following specific conditions are required for pancake ice:

  • Supercooled water
  • Turbulence, but which is not too small or too big
  • Attenutation of wave action

Supercooled water is needed in order for grease ice and the needle like structures of frazil ice to form. There needs to be enough turbulence in the water in order for wave action to occur. Eddies within turbulent water help to form the circular shape of the pancakes. Finally the attenuation (or the reduction of force) of the wave action is taken into account.

Attenuation is determined by the location. In open water, attenuation has a large effect. Let’s first consider a sheet of sea ice above the surface made up of frazils (needle like structures). The bottom of the ice sheet is underwater. Congelation ice is a term used to describe a fresh stable sheet of very smooth bottom ice below the surface. It forms when nilas slide other each other in a process called rafting, and then thickens into a more stable sheet.

In open water away from the edge of a large ice sheet, congelation ice increases with distance. Frazil ice does not form here because the conditions are calmer below the surface. In other words there is not much interaction with the air and wind (although there may be ocean currents and light winds that create a bit of movement in order for the rafting to occur). As a result there is an increased attenuation.

The lack of wave action here simply means pancakes cannot form. On the other hand, in supercooled turbulent water, the attenuation of the waves is much lower. Frazil crystals form, the wind and wave action force frazil ice against each other and the pancakes result.

What is quite ironic about the recent formations of pancake ice in Scotland, is simply the fact it is almost Pancake Day.

So, it’s an ice time to be talking about recent ice formations in the UK then.

Feature image credit: Pexels

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