Snow Science for Skiers
Author: Matt Silverman
Published Date: 05/10/2024
Read Time: 15 Minutes
Hey skiers and riders, it's your snow reporter, Matt. You might recognize me from the daily Mountain Report, but today, we're digging a little deeper. Before coming to Snowbird, I participated in snow science research as a student at Colorado College. I mainly focused on how atmospheric aerosols affect snow grain metamorphosis and worked on a machine that could fit on a normal lab bench and consistently make nature-identical snowflakes. I’ve spent a lot of time thinking and learning about snow, and wanted to share some of what I’ve learned with you.
If you’re reading this, there's a good chance you love skiing or snowboarding, and by extension, snow. But how often do you stop to think about the snow crystals as you ski over them? What about the variety of minuscule changes in the atmosphere that change snow and make it better or worse for skiing? You may have heard the term “lake effect” when referring to storms here in the Wasatch, but do you really know what it is? If not, you're in luck, because we're exploring the basics of snow science to give you a taste of why it all matters to you as a skier or snowboarder.
Water and Why it Matters
To really understand snow, we have to go back to the basics: water. Water is fundamental for life on Earth and the basic building block of snow. Water is the only natural substance that can exist in all 3 phases of matter (solid, liquid and gas) under normal atmospheric conditions—an essential trait in the creation of snow. If you’ve ever taken a chemistry class, you're probably familiar with this diagram.
What you’re looking at is a visual representation of how water exists in all 3 phases of matter. If you change pressure or temperature (or both) enough, you’ll move between phases. For snow crystals to form, you need vaporous water to become a solid, crossing that line in the red section. If the water molecules transition to solid from the liquid state, you get ice crystals instead of snow crystals, which, while similar, have very different characteristics. If you’ve ever noticed a difference between how natural snow and man-made snow feel, you have experienced this first hand. Snowmaking creates its “snow” from water mist rather than water vapor, meaning it actually produces small ice crystals. While we don’t rely heavily on snowmaking here at Snowbird, it’s incredibly useful in building a strong base for skiing/riding.
Snow Water Equivalent and Snow Liquid Ratio
A significant factor in the quality of snow is how much water it holds, which may vary greatly. You may have heard the terms “snow water equivalent” and “snow liquid ratio” tossed around when discussing the snowpack. These concepts are essential to understanding how new snow will ski, as well as how our snowpack will translate to water in the spring and summer, which is critical to our water supply here in the West. Let's dive deeper into these terms.
The first of these terms to be aware of is the snow water equivalent or SWE. SWE is the amount of water you would be left with if you were to melt down a given volume of snow. We calculate this by utilizing the known density of water as well as the weight and volume of the snow. While this can be done manually using a tool called a density wedge, you can also just use a handy tool called SNOTEL to see how much water we've gotten throughout the water year (October 1 – September 30). SNOTEL is a network of automated stations across the US, run by the Department of Agriculture, that monitor snowpack, precipitation, temperature and other climatic conditions to aid in water resource management. Our record-breaking season last year was reflected by the SNOTEL site here at Snowbird breaking 170% of our peak median water. In years past, when there wasn't such a robust snowpack, this network of SNOTEL stations has been an essential tool for allocating water resources around the West.
There is a reason Utah license plates boast about our snow, and that is in part due to the snow liquid ratio, or SLR, of our typical snowpack. The SLR, which is simply the ratio of how much snow fell to how much water the snowfall contained, is the number that will affect you most as a skier, as it measures how light the snow is. Common SLRs range from about 4:1, or 4 inches of snow for every inch of water, all the way up to 20:1. The lower the number, the heavier and denser the snow will be since there is more water in it.
According to a 2010 article by Dr. Trevor Alcott and Dr. Jim Steenburgh, the average SLR here in Little Cottonwood Canyon is around 14:1. This ratio of 14:1 is phenomenal for skiing, providing the right amount of float while still being light and playful. It’s hard to overstate the importance of the right water content when it comes to powder skiing, and there is a common misconception that lighter is always better. Have you ever gotten up on a pow day only to feel disappointment as you sink through all the fresh snow and hit the hard pack beneath it? This is an example of snow being too light, lacking the structure to support the weight of a skier before it consolidates. This translates to an SLR of 12:1 to 15:1 in order to hit the sweet spot for flowy and playful powder skiing.
This SLR range is only part of the reason why Utah has such legendary power—the other part is due to our location and a concept known as orographic lift. The storms we get come off of the Pacific Ocean and pick up lots of moisture before hitting the West Coast. As the storm cloud moves eastward, it rises over mountain ranges. Every time it rises over a range, the change in elevation causes both the temperature and pressure to drop, and the water vapor stored in the cloud turns into snow. Each time this cloud lifts, the amount of water vapor decreases and the snow it produces in its next drop gets drier and lighter. Here in the Wasatch, we are situated in the ideal location—where storms have reliably passed over enough mountain ranges to drop moisture—to receive much lighter snow. To the west, the snow is much denser, and to the east, it’s too light.
From Liquid to Solid
The other factor that influences water’s impact on the quality of snow we ski is how its composition changes throughout the snowpack. The water content of snow changes throughout storms and between storms—an ideal storm will form a “right side up” snowpack with heavier snow on the bottom and lighter snow on top. This allows you to ski lighter pow without falling through it, because there is structure beneath.
If we go back to the phase equilibrium diagram that we looked at when we first started talking about water, it’s not just the movement of the water molecules from vapor to solid that matters—the exact pressure and temperature of the water vapor as it passes over the barrier is essential in the formation of different snow crystals. As the water moves over the vapor pressure barrier, it becomes supersaturated, which, in everyday terms, is essentially when the humidity exceeds 100%. The amount of supersaturation and the temperature conditions that allow the snowflake to form dictate the crystal form it takes. The Nakaya diagram is a visual representation of how different conditions (pressure and temperature) form different crystals.
Different snow crystals have different characteristics that make them better or worse for skiing. The optimal crystal shape for powder skiing is the stellar dendrite, circled on the chart above, which is the lightest and most playful. Looking at the diagram, it’s easy to see why dendrites make the best crystals for powder skiing, as they are the “fluffiest” and airiest among the main precipitation types. It's easy to imagine sliding or rolling over columns or prisms, whereas dendrites really make you float.
Rounding & Faceting in a Dynamic Snowpack
Once the snow falls, it starts forming our snowpack. The snowpack forms when all of the different crystals settle together, creating a big mass. A snowpack is an ever-changing object—every time a new snowflake falls on it, a person skis or walks over it or the temperature changes even slightly, the snowpack reacts. The main method of change through the snowpack is snow grain metamorphosis. From the second a snow crystal lands on the snowpack, a transformation begins. This transformation is driven by temperature changes and mainly occurs in 2 ways: faceting and rounding. Both processes begin with the crystal decomposing to create a smaller and simpler form than it was before, but each process has unique characteristics.
When talking about the temperature in a snowpack, we refer to a temperature gradient. Rather than taking the temperature at a specific point, we take the temperature at various points throughout the snowpack and map how it changes with depth. The ground under the snow is always 0 degrees Celsius, so if a snowpack went from 0 at the base to 9 degrees Celsius at the surface and it is 90 cm deep, it would have a gradient of 1 degree Celsius per 10 centimeters. This is the equilibrium state of the snowpack, the state the snowpack wants to be in, where no change is occurring to the grains.
If the snowpack gets cold and the temperature gradient decreases to less than 1 degree over 10 cm, grains will go through a process called rounding, where the grains become more circular. Round is the most stable and least chaotic shape, and if there were no external factors influencing most of the things we interact with on a daily basis, nature would round them over—similar to a rock in a stream. The same is true for our snowpack. Rounded crystals have the most surface area touching each other, allowing them to bond or sinter with each other best.
The other process commonly seen is faceting, the opposite of rounding. When the snowpack warms up and the temperature gradient exceeds 1 degree per 10 cm, faceting occurs, and the grains become angular and sharp. These crystals are super light, often referred to as “sugary snow,” and do not bond to each other very well due to their angular nature and the lack of surface area touching other crystals. Facets are commonly talked about in the Wasatch in conversations about persistent weak layers. Persistent weak layers often form in instances where we experience early-season snowfall followed by an extended dry period. That thin layer of snow on the ground will have a high-temperature gradient due to how thin the snow is and how cold the air is. This temperature gradient will cause the snow to facet, and once a new layer of snow settles on top, the weak, unconsolidated facet layer will lack the structure to support any weight on it, which typically causes stability problems until there are conditions that allow it to heal, it gets buried deep enough that it is no longer reactive or until the layer fails and an avalanche occurs.
If you are a frequent reader of our Mountain Report—or any mountain report, for that matter—you may notice that settled mid-mountain depths shrink during dry periods. While some of this may be due to sublimation or melting, most of the changes we see come from our snowpack consolidating through rounding or faceting and gravity rearranging the crystals. As this process, known as settlement, occurs, the snowpack shrinks and becomes more of a uniform, solid structure rather than a series of individual layers.
In an ideal world, every snowpack would be moderately rounded throughout its entire depth and would be stable, weight-bearing and cohesive, with a nice layer of powder on top. Unfortunately, that is almost never the case. Grain type, grain metamorphosis and density are highly variable throughout the snowpack. This can be due to a variety of factors, including the conditions under which it formed and the surrounding weather variability. Oftentimes, when looking at the profile of a snowpack, you can isolate individual layers as products of specific weather events.
If you've ever been skiing on a warm spring day when the snow was slushy, you may have experienced a warm, isothermal snowpack. This means that the snowpack lacks a temperature gradient and the snow has reached a uniform grain type. In other words, the layers cease to exist.
One of the most recognizable examples of metamorphosis we see as skiers (mostly in the spring) is corn skiing. It skis smoothly, it’s elusive and it's fun, but what is it really? Corn snow is a melt-freeze formation that occurs when there are cold nighttime temperatures and warm daytime temperatures. When talking to dedicated corn harvesters, you’ll often hear them talking about the “corn cycle,” referring to the multiple stages of the melt/freeze cycle. Starting in the early morning, the snow has a nice, firm surface that has formed overnight. As the sun comes out and the snow begins to “corn up,” it softens into a buttery and playful surface. If you wait too long, however, it continues to melt to the point where it gets too heavy and slushy. As the sun sets, colder temperatures return and the corn refreezes—starting the cycle all over again.
The final stage of snow grain metamorphism, which is all too familiar to the avid spring skier, is when the snow moves back from being a solid to either being a liquid or a gas through melting or sublimation, signifying the end of the ski season and the beginning of boating season.
The Great Salt Lake Effect
One of the defining characteristics of skiing in the Wasatch is our legendary Great Salt Lake Effect (GSLE) storms. The GSLE is the cherry on top for skiing in the Wasatch and another phenomenal example of how our geographic location is optimal for skiing. Lake effect snow is not specific to the Wasatch—in fact, it is more common in the Great Lakes region—but it is less commonly seen over smaller bodies of water such as the Great Salt Lake. The proximity of the Wasatch Front to the Great Salt Lake, combined with the weather patterns that pass over the lake, gives us a unique topographical advantage over other popular skiing destinations. Lake effect snow occurs when a cold mass of air passes over a warm body of water and absorbs some of the water vapor that is evaporating from the lake. This vapor further supersaturates the cloud, meaning the next time it hits a mountain range, it will deliver large snowfall totals. It is not uncommon for lake effect storms to reach 3-4 inches of snowfall per hour.
Unfortunately, there needs to be a very specific set of parameters in place to achieve a GSLE storm. The relatively small size of the Great Salt Lake makes lake effect storms rare in the Wasatch, but when they do hit, they bring lots of snow with them. For a GSLE storm to “turn on,” the temperature gradient over a several thousand-foot section above the lake has to be just right, and the wind direction has to be flowing within a 60-degree range for the reaction to occur. While all of these factors need to align perfectly, we still benefit from at least several of these events a season, seeing them most often from October to February. While there have been some attempts to model GSLE and predict when it will occur, it remains a rather sudden and unpredictable event.
What Does All This Mean as a Skier?
So, how can you use this information during your next ski day? You could use the temperature forecasts to try and time your skiing to coincide with perfect dendrites, or if it’s a warmer day or a day where snow is wet, slushy and sticks to your jacket as you ride the lift, you’ll know to expect heavier snow that’s less playful and perhaps opt to stick to the groomers. While you may have known all this in practice before, it’s nice to know the why behind the what.
Interested in learning more? There are many incredible books on snow science, some of which are local to the Wasatch. Two of the best books on the topic that maintain a high level of scientific accuracy while being approachable and easy to read are Secrets of the Greatest Snow on Earth by Jim Steenburgh, a professor of Atmospheric Science at the University of Utah and A Field Guide to Snow by Matthew Strum, a snow scientist at the University of Alaska Fairbanks. If you’re interested in learning more about how snow science applies to skiing, specifically in the backcountry, check out an avalanche education course with Snowbird Mountain Guides. These courses are a great way to learn some more about what’s going on in the snowpack and how to make educated decisions in the backcountry.
About the Author
Originally hailing from California, Matt grew up with the Sierra Nevada as his playground. After earning his degree at Colorado College, he came to the Wasatch to check out the rumors of amazing powder skiing. It's safe to say he wasn't disappointed. He is passionate about snow science, photography, SCUBA diving and pretty much any outdoor activity you can dream up. He is currently Snowbird's Snow Reporter and Social Media Assistant.
Author - Matt Silverman