Now is the time for "Hey, Science"—America's primary source of scientific education—in which we enlist real live scientific experts to answer your most most interesting and/ or inane scientific questions. Today: does snow make a city cleaner, or dirtier?

The question: Much of America has been deluged by record levels of snow this winter. When it all finally melts, will cities be left cleaner than before, their dirt washed away? Or does the grime and pollution that accumulates in snow actually leave cities dirtier than before? Is all this damn snow a net positive for urban cleanliness, when it's all said and done? To the snow experts!

Nolan Doesken, state climatologist for Colorado at the Colorado Climate Center:

As I'm sure you already realize, there is more to this question than meets the eye. Snow is great at cleansing particulate matter from the air as it falls, but once it's on the ground that's another story. Rain quickly washes off roads and sidewalks — the heavier it rains, the better the scrubbing. But snow melts quite slowly so doesn't have the flushing force of a good rain. And then there's the issue of road treatments — salt, sand etc. The more it snows and the longer it stays cold, the more salt and sand are applied to make roads and sidwalks "safe" for transportation. The sand, salt and other gritty materials effectively make things dirtier than they would be anyway — and you can get fairly exact numbers from the city streets depts to learn just how much materials they apply and how that varies from year to year. To a lesser extent there is also the issue of temperature inversions that trap cold air over snowcovered areas in stagnant "cold pools". Air pollution can build up in these conditions — and they are more likely to occur in cold and snowy winters.

Overall, though, I would say it is the road surface treatments that are the deciding factor.

One more thing — when it comes to snow — it makes it so easy to see the dirt. And it falls and melts at the time of year when there is not the lovely green patches of grass to help mop up dirt too.

Tim Garrett, professor of atmospheric sciences at the University of Utah:

I can only speculate on whether cities are cleaner or dirtier after snow or rain.

I can say this however. Snow easily gives the appearance of getting dirty in cities for the same reason that clean snow is white. Like rain, snow grains are transparent, but unlike a layer of liquid rain, snow is white because there is such a high density of individual snow grains on the ground. By working together, each of these snow grains scatters light many, many times (I can give a more precise answer on how many times tomorrow if you are interested). Light is bounced around repeatedly within the snow interior. And, because snow is also "deep", almost all light that is incident on the snowpack gets reflected back to the observer (i.e. it doesn't go through to the ground). Accordingly snow is bright and white (with a slight bias towards blue).

Now, because snow is so efficient at scattering, even the tiniest amount of soot, even in the parts per billion, is enough to produce a visible darkening of snow brightness. A particle of soot in the air might only get one chance to absorb observed light, but the same soot within snow has many, many chances to absorb light (again I can give the precise number to times later). This is due to the repeated bouncing of light back and forth between the snow grains. Before light can escape the snow it has many chances to be absorbed by the dark soot.

The result is that snow readily shows off slightest bit of city pollution. Because rain is transparent, it does not produce the same effect. So the higher dirtiness you suggest may be only an illusion with some interesting physics as its explanation.

Matthew J. Labovites, assistant commissioner of operations with the department of public works in Worcester, Massachusetts, one of America's snowiest cities:

Well, the thing is we spread sand on residential streets (salt on arterials), so naturally we are adding "dirt" or sand to the gutters. The snowbanks do tend to hold litter, etc., so whenever the snow does melt, things look a bit unsightly until street sweeping can occur.

One can argue either way [comparing snow to rain, in terms of cleanliness]: The incremental rainstorm-by-rainstorm flushing of street gutters that occurs vs. the unveiling of all the sand and litter when snowbanks melt. Certainly the snow scenario is more unsightly, if aesthetics are a factor. From a stormwater quality perspective, I would say it's close to a draw.

The verdict: A deceptively complex question. Dirty snow is, at least, probably not as dirty as it appears. That said, when you take into consideration the salt and sand put on the streets for snow treatment, combined with the dirt that snow accumulates when falling and the slow melting rate of snow, it seems that heavy snowfalls do not leave a city cleaner than it would have been otherwise.

Fucking snow.


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[Image by Jim Cooke. If you have a question for "Hey, Science," email]