Spring is the season when many Yukoners head out to the coastal passes in search of the perfect snowslope. For skiers, snowmobilers and snowboarders, the mountains in spring offer an ideal playground.

Unfortunately, this playground is not certified safe; and snow -- the same substance that offers so much delight -- can also form avalanches capable of levelling everything in their paths.

Taken down to its very simplest form, an avalanche is just snow sliding down a slope. But predicting which slopes pose a danger is no easy matter, primarily because snow itself is such a complex substance.

"Snow is the most unique substance in the entire world; no two crystals are alike. And as soon as it falls from the sky it starts to change," says Kirstie Simpson, who teaches avalanche courses for the Canadian Ski Patrol and trains avalanche rescue dogs.

Starting with the first snowfall of the season, complex changes and interactions take place within the snowpack. Temperature, pressure, and the movement of water vapour change the forms of individual crystals, a process called metamorphism in the field of snow science.

For example, consider the fate of a picture-perfect snowflake with six fragile arms after it falls to the ground. It might be bashed by wind, rounded by higher temperatures and crushed by the weight of millions of other flakes falling on top of it.

The original flake can end up as a small rounded crystal that will pack together tightly with its neighbours, or a large, angular crystal that will not bond very well. Whatever its shape, it will be part of one of the numerous layers of snow deposited through the winter and spring.

It is these layers -- and the bonding between them -- that will determine the stability of the snowpack. If the bonds between the layers are weak, the snowpack is less stable and the seeds of an avalanche may be planted.

When people are travelling in the mountains, they should never forget about the multitude of layers that lie beneath the deceptively smooth and uniform snow surface. An avalanche hazard can begin to build when a weak layer of snow forms on a slope.

For example loose crystals of frozen dew, known as surface hoar, are not dangerous when deposited on top of the snowpack. But if more snow is deposited on top of the surface hoar, the surface hoar can form a potentially weak and dangerous sliding surface.

Simpson encourages her students to keep two general concepts in mind when evaluating a snowpack. "A shallow snowpack and cold temperatures encourage facetting, which weakens the snowpack, while a thick snowpack and warm temperatures encourage rounding, which can strengthen the snowpack," she says.

Simpson describes facets as hollow six-sided crystals shaped like Dixie cups. At high school presentations she sometimes demonstrates what happens within a snowpack by piling books on top of the Dixie cup until it eventually collapses.

Just as the final book can be the weight that flattens the hollow cup, the weight of a skier on a slope can collapse a weak layer of snow buried in the snowpack and trigger an avalanche.

Facet crystals, also known as Thermal-Gradient (TG) snow, often form early in the winter after the first snowstorms of the season have dumped their loads. Since the temperature of the ground is usually close to 0°C, there can be a considerable difference between the temperature of the snow surface and that of the ground surface in cold climates.

This temperature gradient drives the movement of water molecules within the snowpack, with vapour flowing from high- to low-temperature regions in the snowpack. As water vapour is deposited on the grains, they grow larger, and can eventually form the large crystals known as depth hoar.

Depth hoar is common in interior areas like Whitehorse where the snowpack is not deep and temperatures are very low. Also known as sugar snow, depth hoar is the bottomless stuff in which you just sink when trying to walk across it.

While TG snow tends to be inherently unstable, Equi-Temperature Metamorphism strengthens the bonds between snow by rounding the crystals and forming necks between these grains. ET-Metamorphism occurs when temperatures are moderate or the snowpack is deep.

And then there is Melt-freeze Metamorphism, when the sun melts the upper layers of the snowpack during the day, but freezing still takes place at night. With MF Metamorphism, larger grains grow at the expense of smaller ones, helping to strengthen the snowpack in most cases.

But in the field of avalanche prediction, there is always more than one factor at work, and too much meltwater can also set off a slide. That is why caution is advised on south-facing slopes on warm spring afternoons.

While snow physics and the changes in snow crystals are just one part of avalanche prediction, Simpson says she personally loves the complexity of snow science.

"One of the things I like to focus on in the high school programs is how the concepts you learn in grade 9 physics -- like the solid, liquid and vapour stages of water -- can be turned around and used to help keep you safe while snowboarding in the mountains. It gives the 'application of science' a whole new meaning to the students," she says.

Travelling safely in the mountains requires paying attention to a host of factors, including the layers of the snowpack, the steepness of the terrain, wind direction, and whether you are on a sunny south-facing slope or a steep north-facing one.

There are few hard and fast rules. The one constant is that snow will continue changing until it completely melts away.

On the web, general avalanche information is available through the Cyberspace Snow and Avalanche Center and the Canadian Avalanche Association.