This spring in Colorado there was a pattern with the kinds of avalanches people were getting involved with: dry snow avalanches in a spring context. There are a couple nuances with this specific setup that can help us operate more effectively in the spring.
First let’s bring it back to the fundamentals of new snow settlement.
When new snow falls it has inherently unstable grain form. The stellar dendrite—the perfect snowflake—it is predisposed to settlement. The large surface area to volume relationship in the shape of a stellar is fundamentally unstable and immediately it begins to deteriorate.
Soon as it falls the stellars begin to decompose into a more stable shape with a better surface area to volume relationship on its path a ‘rounded grain’. As they deteriorate, they begin to form bonds and loose pore space becoming more dense. This process generally referred to as ‘settlement’. It’s fundamental to how new snow to begin taking on the slab like properties that allow fracture to propagate and avalanche conditions to evolve as well as that problem to eventually bond out.
As the new snow is settling amongst itself becoming a slab, it’s also starting to bond to the underlaying old snow. This new/old interface is the key to why dry snow avalanches extra hazardous spring.
Snow likes to bond to like snow. The more similar the character of a grain the more effectively it can bond. This means that new dry snow can bond out very effectively on a old surface of relatively new snow–and thus similar snow grains. Sometimes this process of settling out can be only a matter of hours. In a very maritime area with lots of snow and some relatively warm temps it might only take a few hours for the life cycle of a storm slab to run its course. At 8am you arrive at the TH to find 2 inches of new snow–and low danger. By noon there is 18 inches and all that new snow has bonded into a slab and is very reactive–call it considerable danger. By 3pm when you head home the new slab has bonded to the old snow surface and is no longer reacting–were back toward low danger.
In the case of spring snow surfaces in Colorado, especially with a solar tilt, we have old snow surfaces composed of crusts and melt forms. These are very different in character to the new dry snow and it does not bond well at all.
So not only do we have old snow interfaces that don’t allow new snow to bond well, but we are dealing with a much higher and hotter sun in spring. This heat leads to rapid settlement in the new snow. Couple this with poor bonding to the old snow surface and slopes easily and rapidly shed large snow volumes. Even if its not a ‘slab’ it can be high volume entrainments.
This combination makes it easy for snow volumes to grow rapidly as every small avalanches immediately entrain all the new snow available below them as they run.
On solar aspects with seemingly small snow volumes can turn into fast ticking time bombs as the sun warms them up. It’s easy to find parties booting up a solar couloir at around 10 am as the snow quickly transforms from a few inches of low density snow into a large loose/small slab avalanche as the sun hits the slopes. We also see the sun making snow shed off rocks and create a ready supply of natural triggers.
In these conditions I tend to prefer shaded aspects as the new snow can bond faster as well as there are less natural melt related triggers to get large snow volumes moving above me.
The other options is to be entering slopes from the top, as these problems can be very manageable before they grow in size during their flow. Managing a six inches of snow in this character, from the top can be very reasonable, but that same avalanche will quickly have an unmanageable volume after a short flow.