I have stood on Mt Shasta under a clear sky and watched wind do the work of a storm that ended the day before, packing snow into a stiff slab on the Leeward aspects while the slope I had was standing on stayed scoured and hard. Nothing was falling from the sky, yet the snowpack was still changing. It’s one of the first things to understand about weather in avalanche terrain. Weather does not wait for a storm to matter, and it does not stop when the storm passes.
In the last piece I called the avalanche forecast a hypothesis, an assessment a forecaster writes after gathering weather data, field observations, and snowpack information. You do not need to have read that piece to follow this one, because the forecast is simply the forecaster’s statement of what problems are expected, and where they are expected to live in the terrain that day. This piece is about the force that builds and rebuilds the snowpack behind that forecast, day by day, hour by hour, and sometimes within minutes, so that every layer in the snowpack carries the signature of the weather that made it. Weather is that force, and it deserves a article of its own, because everything that follows in this series comes back to what the sky has been doing.
Weather never stops
In a Recreational Level 1 course we draw a simple triangle with terrain, snowpack, and weather at its three corners, and the triangle earns its place on the whiteboard because an avalanche needs all three sides at once. Terrain on its own is only a slope, snow on its own is only a blanket, and weather on its own is only sky, and it takes all three together to make a slope able to slide. Terrain does not change from day to day, since the slope you skied last week has the same steepness and the same aspect today, while the snowpack changes in large part because of how weather acts on it. That leaves weather as the side of the triangle that never seems to sit still, a constant variable working on the snowpack whether a storm is raging or the sky is clear.
That constancy is easy to miss, because we tend to watch weather only when it is dramatic, and a storm gets our attention in a way a blue sky does not, even though a blue sky is still weather, with sun warming the surface by day and cold air cooling it again at night. A calm, clear night can write its own layer on the snow surface, and the next storm buries it, so that a day which looked uneventful from the parking lot can still leave a record in the snowpack that matters later. Learning to read that record, rather than assuming a quiet day meant a quiet snowpack, is part of the work.
Temperature sets the snowpack
Temperature decides whether snow grains bond together or stay weak, and you can feel that change as the thermometer moves. Gentle warming lets snow grains round and bond, the way snow packs into a snowball, and the pack settles into a stronger structure, while steady cold preserves whatever is already there, including weak layers, which is why a problem can outlast weeks of clear weather. Rapid warming does the opposite, because a fast rise in temperature loosens the bonds between grains by adding water between them, and wet avalanches can follow the same day, which is why a warming trend deserves as much of your attention as a storm total.
Temperature also works inside the pack, where you cannot see it, and snow is a good insulator, so on a clear cold night the surface can sit far below freezing while the snow near the ground stays near the freezing point. Where temperature changes fast with depth, heat moves upward through the pack and water vapor moves with it, and when that gradient exceeds about one degree Celsius across ten centimeters, the crystals start to change shape, growing into angular, faceted grains that bond poorly to their neighbors, much like dry sugar that will not stick together. A weak layer can be born inside the snowpack in a single cold snap, and the whole story of the snowpack is a contest between rounding, which builds strength, and faceting, which takes it away, with temperature deciding which one wins.
New snow: how heavy, and where
New snow is weight, and weight is load, so a storm that drops a foot of snow has added a foot of slab material sitting on whatever weak layers are already in place, with a slab meaning a cohesive layer of snow that can slide as a unit once the layer beneath it fails. A foot of snow is not always a foot of the same thing, because what matters is not just depth, but how much water that snow holds, and how heavy new snow is depends on the temperature of the storm that made it. A cold storm lays down light, fluffy powder of low density, and a warm storm lays down dense, heavy snow, so the same depth can mean very different loads, and the forecast snowfall total alone does not tell you which one you got until you also know the temperature behind the total.
Where new snow falls matters just as much as how much falls, because a storm does not spread its snow evenly, with elevation deciding whether a slope gets snow or rain, and the track of the storm deciding which ranges get loaded and which stay thin. Then wind moves snow a second time, picking it up from windward slopes and depositing it on lee slopes, in gullies, and behind ridges, so that a slope which received a modest amount of new snow can carry a wind slab far deeper on its lee side, built in a matter of hours, while the slope across the ridge gets scoured down to a hard surface. Snow does not have to be falling for a slope to be loading, since the sky can be blue and the storm over since yesterday while wind still packs snow into stiff slabs downwind of ridges and rollovers, the way wind piles a drift against a fence long after the snow stopped falling.
Rate matters as much as total, because new snow takes time to bond, and until it does, it is a slab waiting for a trigger, which makes a storm slab problem most sensitive while the storm is still loading the slope, usually fading in days as the new snow bonds. Fast loading overwhelms weak layers, slow loading gives the pack time to adjust, and two storms with the same total can behave very differently for that reason alone, which is why forecast centers often treat around thirty centimeters of new snow in a day as a threshold worth paying attention to, not as a rule, but as a sign that the pack is being asked to adjust faster than it comfortably can.
Sun
Sun is the quickest change most of us ever see at work in a day, because direct sun on a slope can start softening the snow surface within minutes, which sounds like an exaggeration until you have watched a cold, shaded slope catch the sun and loosen in front of you. Sun warms the surface first, and the surface is where the bonds start to give, so a slope that was holding together in the shade can be a different slope after the sun hits it, with the change driven by warmth alone and no new snow required.
Which slopes change, and when, is a matter of aspect and time of day, since south facing slopes get the most sun, east facing slopes warm in the morning, west facing slopes warm in the afternoon, and north facing slopes stay cold and shady through midwinter, which preserves weak layers there long after they have healed elsewhere. Note the time each change starts, because a slope that was fine at 9 a.m. can be a different slope at 2 p.m., and the difference is often just the sun moving across it.
Rain
Rain works on the snowpack two ways at once, because it adds weight and water at the same time, so rain can start changing the snowpack within hours, as the water weakens bonds between grains and wet slabs and loose wet avalanches can follow the same day. That is why the freezing level matters so much, since the freezing level is the elevation where rain turns to snow, and whether that line is rising toward your elevation tells you whether the storm above you is building a slab or soaking one.
The same rain does different work depending on how much water the snowpack already holds, because a dry pack takes time to wet through, and the water has somewhere to go before it reaches a weak layer, while a pack that is already holding water is closer to the point where bonds give way, so a little more rain moves it a long way in a short time. The rain did not change, and the snowpack it landed on did, which makes rain a variable twice over, the rain itself and the condition of the snow it meets.
How widely problems spread
Weather also decides how widely problems get spread out, since a weak layer that formed during a long, calm, cold stretch can lie across a whole range, and when the next storm buries it everywhere, the problem is everywhere with it. A wind event that loads only certain aspects leaves a problem only on those aspects, and a rain event that reaches only lower elevations leaves a problem only below a certain line, so the pattern of the problem follows the pattern of the weather that made it. When you look at the aspect and elevation roses in the forecast, you are looking at a map of where weather did its work, and the roses are not abstract, since they are the record of which slopes got loaded, which got warmed, and which got rained on.
What we can measure, and what we cannot
Here is the paradox of weather, which is that it seems fairly simple, because we have reliable resources to monitor it, with a mountain weather forecast every morning, remote stations reporting temperature, wind, and snow depth through the night, and our own eyes to watch the sky and note the time each change starts. Compared to digging into the snowpack, watching weather feels straightforward, and for the most part it is, since the tools are good and the habit is easy to build.
It is not the whole story though, because a measurement is taken at a point and a mountain is not that, with a weather station reporting one elevation and one aspect, and a forecast describing a zone, while we travel across all aspects and all elevations, weather can vary across every one of them. We cannot take exact and precise measurements at all times, in all places. That same limit holds true at the ski resort or in the backcountry, since we know the temperature at the station, and we do not know the temperature on the slope we are standing on, or the one around the corner, or the one a thousand feet higher, except by observation and judgment. It is a little like checking the temperature at the trailhead and assuming it will hold on the summit. Weather is easy to see and hard to fully know, and that gap is where good decisions live, because you leave margin for what you cannot measure.
No two storms are the same
Put the variables together and the possibilities are endless, since temperature, precipitation type, precipitation amount, rate, wind speed, wind direction, sun, timing, and the condition of the snowpack all of it lands on, and changing any one of them changes the outcome. A cold storm followed by wind writes one story, the same storm followed by sun writes another, and rain on a dry pack writes a different story again from rain on a wet pack, so no two storms are ever quite the same, because the snowpack they meet is never quite the same.
That is why experience in avalanche terrain looks less like memorizing rules and more like recognizing patterns across many days, as you learn what a warming trend feels like on a south aspect in the afternoon, and how fast wind can load a lee slope after a storm ends, and how a forecast is a starting point that your own observations keep honest. The forecast was an assessment about this place, and your eyes are in this place, so if the wind at the ridge is twice what was forecast, or snow is falling harder than predicted, you update your picture, and that updating is the skill itself.
The sky is still writing
You do not need a windsock to notice wind direction, or a classroom to notice that the last three clear nights were calm and cold, or that the freezing level is climbing toward the ridge you planned to ski. That attention is the beginning of judgment, and it is also the habit that keeps you honest later, when you are standing on a slope deciding whether the morning forecast still holds up under your skis. Weather is the constant variable, never stopping, touching everything in the snowpack, and combining in endless ways, and learning to watch it means learning to watch the thing that builds every avalanche problem you will ever meet. You can learn a good deal of that watching on your own, and an avalanche course is where you get to practice it in the field, with someone checking your work and giving you feedback on your decisions.
Next time in this series: weak layers. The fragile layers this weather creates and buries, why some of them linger for weeks while others fade in days, and how to think about a layer you cannot see. See you in a couple weeks.
This is the second of a series walking through the pieces of a Level 1 avalanche course, one at a time. If you want the next installment, follow us on social media, or check back on our website weekly.
Written By Caleb Burns
SWS Mountain Guides runs avalanche courses all winter in Mount Shasta, Bear Valley, and Mount Charleston.
A note on what this series is and is not: These articles are not a replacement for an avalanche course. There is no substitute for time in a classroom and time in the field with a qualified instructor, learning in real conditions with feedback on your decisions. These pieces are only meant to share information that might be useful as you start paying attention. If you travel in avalanche terrain, take a course from a recognized provider of the American Avalanche Association.