Why Is Spring Snow So Sticky?
The science of spring snow – what it means (depending on who you are) and other explorations in the science of warm(ish) snow.
Photo Credit: Tom Black
From Kerry McClay, WWA National SnowSchool Director (5/3/2023)
Recently, our SnowSchool Weather Station in Boise, Idaho, was equipped with a new infrared radiometer capable of measuring the surface temperature of snow without physical contact.
The weather station, originally installed in 2015, is a three way collaboration between the US Forest Service (USFS) Rocky Mountain Research Station, Winter Wildlands Alliance, and Bogus Basin for the purposes of education, scientific advancement and nordic skier beta. This winter, shortly after installing the new sensor in February, the entire station was buried by unprecedented snowpack depths. At times, only the anemometer (wind gauge) remained in view and served mostly as a tripping hazard for SnowSchool instructors and their students.
The irony of a weather station and snow surface temperature sensor buried deep underneath the snowpack did not go unnoticed by those close to the project. But it did highlight, in dramatic fashion, the challenges facing snow scientists who conduct research in rugged mountain environments. Much like a SnowSchool field trip postponed by a blizzard (which also happened this season) things returned to normal operations once the snow storm(s) subsided.
As the historically deep snowpack has finally begun to decline, the weather station and its sensors have re-emerged as a valuable tool for those interested in spring snow. But what does it tell us?
The answer depends partially on who you are.
Are you a snow slider?
The huge snowpack and onslaught of spring snow has led to extended ski seasons in many places this year. While this sounds like a no-lose scenario for those who love to slide on snow, I was reminded of one of the classic spring-specific challenges a few weeks back. As I stood at the top of the local bunny hill with my three year old, I suddenly realized (to my dismay) that the warm snow and lack of proper spring wax meant that we could only move downhill by means of an incredibly awkward shuffling maneuver. Thwarted by wet warm, sticky snow once again! It was a good reminder that picking the right wax for spring conditions is essential to anyone interested in enjoying the sensation of sliding on snow.
I asked one of our partners at the NASA SnowEx team, Matthew Sturm, about the physical processes behind skis sliding (or not sliding) on snow at different temperatures. He referenced a handful of scientific papers (here, here, and here) that examined the phenomenon and pointed to a thin layer of water that is produced on the surface of the snow by the frictional heating of the skis. This thin layer of water makes the coveted slide and glide skier experience possible. It appears that at very cold temperatures, too much dry friction (not enough of a water layer) can slow down a skier; however, in warm spring conditions, the presence of too much water can create “capillary drag” and bring skiers to a soggy halt. In theory, ski wax can help mitigate these different types of unwanted friction.
And while many wax makers – like our friends at mountainFLOW eco-wax – provide skiers with a general color-coded waxing guide (for example, use Red wax for warmer [20 to 36 degree] temps and Blue wax for cooler [10 to 25 degree] temps), often skiers are left to make their choices based on ambient air temperatures. But the reality is that the temperature of the snowpack’s surface changes much slower than air temperature. So, while a warm air temperature reading might tell you a bit about what the temperature of the snow might be later that day, it doesn’t necessarily tell you what the snow temperature is right now.
For a nordic skier, whose enjoyment (or competitive racing advantage) depends on the right wax, the preference is for more sophisticated metrics such as the previous night’s air temperature lows and the current snow surface temperature. That’s why when the original snow surface temperature sensor on the SnowSchool Weather Station stopped working in the fall of 2022 (due to the consistently harsh mountain conditions), the Bogus Basin Nordic Center staff rallied in collaboration with the USFS to find a replacement.
“The snow surface temp sensor has been an amazing tool for our staff to pick more accurate daily wax recommendations to help Nordic skiers find just the right kick and glide each day,” said Paul McNeil, Bogus Basin Nordic Center Director.
These sensors are somewhat of a rarity, but if you are interested in finding out if any of your local mountain weather stations have surface temperatures readings, you can utilize the MesoWest map to search all weather stations and networks in your region.
Are you a SnowSchool student?
Not every visitor to America’s snowscape is focused on sliding down snow on wax-covered skis. One of the classic SnowSchool activities involving spring snow temperatures has K-12 students on snowshoes examining temperatures deep within the snowpack. Kids are challenged to shovel out a deep snow pit (to the ground when possible) and use a hand-held thermometer to assess the difference between the temperature at the top of the snowpack and the bottom temperatures of the snowpack.
When asked, the majority of first time participants (whether they are students, parents, or teachers) tend to predict that a 4 foot deep snowpack will be the very coldest at the bottom (right near the soil) of a 4 foot deep (for example) snow pit. On a cold day, they usually discover that the opposite is true.
Snow is an insulator, and like a white winter blanket, it holds in heat from the earth’s surface. But on a warm spring day in the mountains, students often discover that the snowpack is 1 temperature all the way through (usually around 31-32 degrees F). This phenomenon of an isothermal snowpack is an important indicator snow hydrologists use to help determine key windows of time when the snowpack is going to start shedding water.
When SnowSchool students return to their classrooms, they use current conditions and historical data like this from weather stations to study and better understand the connection between mountain snow and water. In a paper published in the proceedings of the Western Snow Conference, Winter Wildlands Alliance and the USFS showed that students’ understanding of these snow hydrology connections were greatly enhanced when they analyzed data from the SnowSchool Weather Station.
Are you a hydrologist?
With 80% of our water coming from mountain snow, understanding how much water is in the snow and when it will enter our streams, rivers and waterways is critical information.
This is especially important for water managers following winters such as this, as snowpack levels in some watersheds have reached historic highs (see photo).
But when closely examined, the internal thermal gradient of the snowpack can be a complex and fluctuating phenomena.
A first-of-its kind temperature ladder was originally designed and installed by the USFS Rocky Mountain Research Station in 2016 (see photo).
The ladder measures the snowpack temp every 5 centimeters through the entire depth of snow. This new invention allows hydrologists to better understand and model the energy stored in the snow pack as snow accumulates and eventually melts. A cable is attached between the two posts every 5cm vertically. Each line supports a thermocouple wire that can measure the temperature at that exact location in the snowpack/air.
This new groundbreaking instrument combined with air temperature, surface radiation and snow surface temperature data has been providing a fresh perspective on how fast the internal temperatures of the snowpack can change.
After looking at the 2023 data from a cooler than average March, Tom Black (USFS Hydrologist) commented, “The interesting thing to me is that the snow surface temperatures drop steeply overnight (on clear nights) due to surface radiation while the deeper snow temperature does not change much.” As these new instruments lead to new data the patterns that emerge find will help guide the future of snow hydrology and water management in the Western US.