A City plow moves snow on Foster Street on Feb. 23, when the temperature reached 49 degrees. Photo by Heidi Randhava

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Sunshine and temperatures reaching near 50 degrees on Feb. 23 revealed the mysterious nature of snow mounds – soft and hard, frozen and melting, but remarkably persistent.

The lingering snow resulted in a delay in the start of the City’s 2021 street cleaning season. Street cleaning operations and related parking restrictions have been canceled until March 15.

Even though warm temperatures are expected to continue into the late week and weekend, the unwieldy mounds of white and gray ice crystals are not likely to melt as quickly as one might expect, or hope, especially in cold-weather cities like Chicago, ranked as America’s 49th coldest city in a 2019 report by Thomas C. Frohlich.

Three days of temperatures at 50 degrees can melt two to four inches of snow, according to CBS 2 meteorologist Ed Currran. But the science is different for snow mounds, especially those in more densely populated areas, created after snow is pushed with plows and re-melting occurs, making it very dense.

Michael Dietze, who leads the Ecological Forecasting Laboratory at Boston University, explained why snow piles can take so long to liquefy, or in some cases vaporize, in “The science behind why those huge snow piles just won’t melt,” an article written by Eric Levenson, published in the Boston Globe.

Three factors determine how long it will take a snow mound to melt: energy, rain, and density.

The answer boils down to three main factors: the incredible energy needed to change a solid to a liquid, the lack of rain, and the thickness of the piles.

“Ice doesn’t immediately turn into water as soon as it warms to the melting point. Although the temperature for both ice and water can be 32 degrees Fahrenheit, transitioning from one to another takes a massive amount of energy. To turn ice into water, that’s called latent heat of fusion. When liquid turns into vapor, that’s called latent heat of vaporization,” wrote Mr. Levenson in his article.

Although ice warms up as temperatures warm up, the latent heat – the energy needed to change ice to water – is a major factor in the melting process that cannot be measured by a thermometer.

The second reason that snow mounds linger is lack of rain. Without rain, pockets of air that act as insulators in snow piles are protected, making the piles less likely to melt, even when air temperature increases. Rain, and the heat it brings with it, pierce the pockets of air that helped to insulate the snow mounds, and they melt quickly.

“Wind also carries away moisture in the air that can speed up the melting process,” Mr. Curran told CBS Chicago reporter John Dodge.

If the air is dry, and especially if it is sunny, snow can turn from a solid directly to a gas, bypassing the liquid watery stage. Sublimation is often used to describe the process of snow and ice changing into water vapor in the air without first melting into water, according to “Science for a changing world” at usgs.gov.

On a cold, sunny day, huge snow piles pushed into parking lots “sometimes look as if they’re steaming. This is one sign that sublimation is underway,” writes Bob Berman in “How Snow Disappears Without Melting,” published in 2020 in The Farmer’s Almanac.

Even densely packed snow can go away all by itself because water molecules will go from their liquid or solid phase into their gaseous phase when they are moving fast enough. Dry air and sunlight accelerate sublimation.

“Each molecule jiggles at a particular speed and in a huge mass of them, some are always moving fast enough to escape the water or ice and join their gaseous buddies in the atmosphere,” wrote Mr. Berman.

Finally, dense, high snow mounds created when snow is pushed into one area to clear streets and walkways contain many layers of insulating ice and snow, and often debris. The inside layers do not get exposed to sunlight. Sun speeds up the melting of snow even when the temperature is below freezing. This is especially true when the snow is spread thinly over a larger surface.

In an excerpt of his newly released book, published in the Philadelphia Inquirer, “Snow: A History of the World’s Most Fascinating Flake,” author Anthony R. Wood writes, “The nation has a unique and complex relationship with the six-sided crystals that Emerson called nature’s masterpiece. One obvious reason is the fact that no other country has so many densely populated areas prone to frequently snowy winters and mega storms…I learned that snow entrances precisely because it is so hard to pin down.”