Under Pressure

Pressure ridge on Lake Winnipeg by Heather Hinam

With the ‘polar vortex’ that held much of North America in its frigid grip last week, it was interesting for this ‘girl of the north’ listen to southerners goggle about phenomena that I’ve been experiencing for most of my life.

I found one event, in particular, rather interesting. Last week, the media and thus a large portion of the population, was introduced to the concept of ‘frost quakes’. Torontonians were rattled out of their beds by thunderous booms that shook parts of the city at random intervals. Soon the headlines were reading that it was so cold in Canada, the ground was cracking.

Lake Winnipeg Cliffs

Large crevasse in the rock, likely split apart by frost action

Having spent a number of winters on the shores of Lake Winnipeg, where  temperatures regularly dip below -30C, I’ve seen first-hand the power of ice and its ability to snap rock in two. Ice expands and contracts with temperature fluctuations. It also becomes less flexible as it becomes colder. Water that finds its way into fissures in the rock or soil can push so hard went it freezes – especially if the temperature drops quickly – that the substrate buckles under the pressure. Here, along the lake, the limestone cliffs are full of cracks forced open by winter’s icy push. Still, these earth-shattering events are extremely rare. You don’t usually see new cracks on a yearly basis.

However, there is another type of frost quake, or ice quake, as I prefer to call them that happens considerably more often. Based on where the events were reported last week along Lake Ontario, I’m willing to bet that it was this type of cryoseism  that residents heard for the most part. While the ground doesn’t crack very often, the ice on the lake does. On large lakes, like Lake Winnipeg or Ontario, a sudden snap of the ice can sound like a cannon shot, nearly knocking you off your feet and rattling windows in their panes. While it’s still not something you experience everyday, such quakes happen on Lake Winnipeg fairly regularly.

That’s because this 23,750 sq km lake freezes completely to a depth of at least a metre every year. That much surface area can’t solidify into one piece. So it freezes into floes that knit together much like the tectonic plates of the earth did when the crust first formed. Like the earth’s crust, the lake’s surface is full of fault lines, or pressure ridges.  These giant cracks can run for kilometres along the lake and usually form in about the same place every year.  Some ridges, known as stamukhi, are grounded along the shoreline, where ice that is held fast to the shore meets the free-flowing ice of deeper waters, while others run along over top of varying depths.

Even frozen, the lake is very much alive and pressure ridges are the sites where this is most noticeable. It’s along these lines that the ice floes move, sliding along, away from and into each other. A particularly violent collision is like a mini mountain building event and along with an ice quake, you will also see a ridge of ice has been pushed sometimes more than 2 meters into the air.  More often, however, the two floes simply press against each other, expanding and contracting like long, drawn-out breaths as the temperatures wax and wane. Eventually, the pressure overtakes the compressive strength of the ice and the ridge snaps in a startling bang that is often followed by the gentle whale-like ‘whoom’ sounds of the pressure waves dissipating through the rest of the ice.

As fascinating as they are, pressure ridges are also dangerous places to be. The ice floes can slide away from each other just as quickly as they can come together and loose plates of ice can trick the unwary into thinking they are still on solid ground. A number of commercial ice fishermen have been lost through shifting ridges over the last century on the lake.

Unless you live along a lake that freezes regularly, Ice quakes are truly something few people get to experience. So, I’m glad that our recent continental cold snap gave more people the chance to learn a bit more about this fascinating phenomena and remember just how powerful nature can be.

Suspended Animation

My world is getting quieter. As winter descends on the boreal forest, it’s like a veil of silence wraps around us, stilling all motion, save for the whisper of the wind across the newly-formed desert of snow.  One by one, the waterways stop, frozen in time, held captive by the solid grip of truly frigid temperatures.

Around here, the last body of water to succumb to this relentless creep of ice is the great Lake Winnipeg. Staring out over its endless expanse during the summer, it’s hard to believe that this inland ocean, the tenth largest freshwater lake in the world, finally gives in and stills beneath over a metre of ice.

It’s not a quick process and it actually starts a lot earlier than most of us realize. As the days get shorter and the air gets cooler, heat from the lake is slowly released into the atmosphere, often creating lake-effect precipitation, but that’s a story for another day.

As the water cools down, it becomes denser because the molecules’ natural vibrations slow and they cluster together. This denser water sinks deeper into the lake, letting warmer water rise to the surface and release its energy to the skies. This cycle continues until all the water in the lake reaches the same temperature. For freshwater, the magic number is 4 degrees Celsius, not zero, like you may have guessed. The water still freezes at zero degrees, but from 4 degrees on, the water molecules start to form the lattice work that ultimately becomes ice and at this point, this water is less dense than the rest of the layers below it, keeping it at the surface, where it starts to freeze.

At over 23,000 square kilometres, Lake Winnipeg doesn’t go quietly. November is a restless month, with flinty skies and often driving winds. Wild weather churns up this giant, but relatively shallow cauldron, breaking apart the fragile skin that forms on the surface, forcing it to start over.  Still, the cold eventually wins. The ice thickens and sheets knit together, sealing off the water below as it creaks and moans like a giant humpback whale trapped below the surface.

By the end of December, the ice will usually be thick enough to hold the weight of snowmobiles and tank-like Bombardiers used by the commercial fishermen to get to their harvesting grounds. By January, it’s strong enough to hold the weight of fully-loaded semi-trailers charging across the barren ice roads to deliver goods and supplies to towns that in summer can only be reached by air or by boat.

All the while, the lake is still very much alive beneath its frozen shell, reminding us of its presence with rattles and muted groans rising up from the depths. It’s a sound I never tire of hearing because it reminds me that the lake I love is still there, restless and waiting to be released with the warmth of spring.

Double Rainbow All the Way

Double rainbow over Lake WinnipegThere’s just something about rainbows. They’ve been immortalized in endless songs, myths, stories, movies and even cellphone commercials based on the painfully hilarious mushroom-induced exaltations of an overly-enthusiastic youtube star.

Rainbows just capture the imagination. For the vikings of old, they were the Bifrost Bridge between Asgard, the home of the gods and our world of Midgard. In ancient Rome, they were the path of a messenger between Earth and the heavens and of course we all know they’re where leprechauns store their pots of gold.

But what are they really? Why can you never find the rainbow’s end? I think Kermit the Frog had it right when he labelled them ‘only illusions’.

Rainbows are the product of an observer standing in the right place at just the right time. What is that right place? It’s about 42 degrees from the direction opposite the sun. You can never get to the end of the rainbow because it will keep moving with you as you walk towards it. Stray off the bearing and the image will vanish into the mist.

What you’re seeing is sunlight being refracted, dispersed and reflected back at you through millions of water droplets suspended in the air. You usually only get enough water hanging around after a storm has passed, hence the name ‘rainbow’. Of course, if you’re standing next to a waterfall, fountain or someone’s sprinkler, you can often get the same effect if you’re in that magical optical sweet spot.

I’ll try and keep the physics simple, but here’s how it works. The white sunlight enters the water droplet and is dispersed into the full spectrum of colours. Then, it’s reflected off the back of the raindrop, just like the inside of a camera. On the way back out of the drop, each wavelength is refracted (their direction of travel is changed) as it passes from the water back out into the air.  How much each wave is refracted depends on the wavelength (colour) of the light. Red light (short wavelengths) are refracted less than blue (long wavelengths). The result is what was once a beam of ‘white’ light is now spread out into an arc of continuous colour. Double rainbows appear when the light is reflected off the back of the droplet lens twice. This second fan of light comes out at a slightly different angle and the spectrum of colour is inverted.

To us, the viewer, we see that colour in bands of red, orange, yellow, green, blue, indigo and violet because of the way photopigments in our eyes receive the light that is then interpreted by our brains. Take a black and white photo of a rainbow and you won’t see any bands, just a continuous gradation in intensity.  Animals whose brains can interpret wavelengths we can’t, like ultraviolet or infrared, would see a completely different rainbow than we do.

I think that’s what I find so fascinating about rainbows. Their beauty is truly in the eye of the beholder.