The Subtle Sublimation of Snow
Where's the melt?
Snow slows down life. It snarls traffic, cancels schools, and causes a run for French toast. Even simply watching falling snow can be relaxing, soothing, and therapeutic.
Snowpacks also melt slowly, which is important for the environment. A steady release of water over time provides a consistent water supply to rivers, lakes, and reservoirs throughout the spring and summer months. This is essential for drinking water, agriculture, and industrial use. Additionally, that slow melt allows more water to percolate into the ground instead of running off, replenishing aquifers and maintaining groundwater levels.
Predicting and planning for snowmelt is an important challenge for agriculture, energy production, and flood prevention. But there’s a complication lying right there in the word itself - melt - the process that turns the snow into water. It may stink for skiers, but without snowmelt you wouldn’t even get snow in the first place.
However, some snow skips the liquid phase and transitions straight into water vapor. In doing so, it cheats the earth of a vital drink of water. In fact, this process, known as sublimation, can remove up to 90% of water from a system.
Sublimation
Recall from elementary science class that there are three main phases of matter: solid, liquid, and gas (plus plasma, but like an uncle’s awkward joke, we’ll ignore that for now). Typically, to get from solid to gas, the substance has to melt into a liquid. But that liquid state is a nice-to-have, not a must-have. In some conditions, the molecule goes straight from solid to gas.
Snowflakes are made of crystallized water molecules, and some of these molecules will sublimate into the air. At the same time, some water vapor in the air will directly freeze onto the crystal (the opposite of sublimation, known as deposition). The water vapor pressure gradient between the snowflake surface and the ambient air heavily influences this exchange of water molecules. Higher pressure leads to more exchange of water molecules because more molecules are in contact with the crystal.
Many variables impact this exchange, but wind is one of the most important. The process of blowing snow into the air and keeping it suspended there limits the movement of heat vertically. This creates a strong temperature gradient above the snowpack, supporting a relatively stable boundary layer of colder air. When the air temperature just above the snow surface is colder, it typically has a lower capacity to hold moisture, resulting in lower vapor pressure, which increases sublimation.
A research team led by Dr. Jessica D. Lundquist recently studied snow sublimation rates to better understand its role in hydrology. They collected data from November 2022 to June 2023 in the East River watershed, Colorado with an array of four towers equipped with over 100 instruments from NCAR’s Integrated Surface Flux System. That’s more instruments than a symphony orchestra.
The researchers observed the highest rate of sublimation on December 22, a day of intense wind. The wind created drifts and altered the topology of the snowpack, further influencing sublimation rates. Increased sublimation was also measured during the spring melting season, likely due to patches in the snowpack that locally altered temperatures above it, generating more air turbulence.
Snowpacks are cold
With so many instruments, they collected a large amount of data. One notable observation they made is that when the boundary layer is stable, such as during nighttime (and enhanced in areas of blowing snow), the air temperature directly above the snow has a rapid temperature gradient, increasing by nearly 5°C within just 2 cm of the surface.
Below the surface, the soil was at thermal equilibrium once the snowpack reached a depth of 20 cm. At that point, the snowpack began acting like a blanket, with relatively little diurnal variability in soil temperature, and the soil received more heat from below than from above.
Dusty Snow
Heat also significantly influences sublimation. A snowstorm in early April carried dust from the Arizona deserts. The dust was initially covered by freshly fallen snow. But by April 7, the dust became exposed, reducing the snow's albedo (its reflectivity) by over 20%. The exposed dust absorbed and redistributed heat, leading to increased snowmelt and sublimation.
So the lesson for Frosty, Olaf, and similarly structured crystalline creatures that want to persist beyond the end of the story, is to stay out of the wind and avoid dust (and, of course, Summer Wheeze).
Forecasting snow is hard. The beleaguered Colorado River receives about 80% of its water from snowmelt. Recently, the amount of snow runoff has decreased, increasing pressure on the vast region the river supports. We know sublimation plays a role in this, but it hasn’t been very well studied. Model predictions for the amount of water available in the Colorado River basin have been all over the place. This data, taken over 8 months in the Colorado Rocky Mountains, will help improve those models and predictions by adding another component they can consider.
And Now for Something Completely Different
Redlining is the practice of preventing some people from getting housing mortgages in neighborhoods in which they would be a minority. It still happens today but was more openly prevalent in large urban areas of the United States in the early to mid-20th century. The term comes from a Federal government policy of outlining certain neighborhoods on a map and suggesting to banks that they not to provide mortgages in those areas because they were “risky.”
A study by J. Hoffman, V. Shandas, and N. Pendleton found that areas that were redlined in the past are now on average 5 degrees warmer in the summer than surrounding areas.
Lack of homeownership and municipal investments left these areas comparatively neglected in terms of the development of green space. That dramatically increases the urban heat island effect. Even though redlining was outlawed in the 1970s, its impact is having an effect over a century later.
We acknowledge and welcome a new scientific reviewer to our team: Dr. Milind Sharma.
Thanks for sharing this. The study regarding "redlined" communities is very interesting. I guess not too surprising since environmental racism has lasting effects, especially in urban neighborhoods. As our climate warms, our concrete and glass cities will, unfortunately, bear the brunt.
I would guess that the "roughness" of the surface of a snowfield would effect the sublimation rate too. Ceteris paribus, the smoother the snowfield the slower the sublimation? Wind sculpting would definitely be a culprit, but could ice crystal size be an even greater driver? Two snowfields might look identical to the eye, but one might have a rougher surface, generating wind resistance, surface turbulence, and increased micro-sculpting at low temps of course. Just guessing!