Why does my street look white?

January 22, 2007 6:14 PM

Why do roads take on a whitish-grey color in winter? And I don't mean because of snowfall.

I grew up in warmer climates. But in my time in the northeast, I've noticed that, during the winter, city roads turn white in places when there is no snow accumulation, and I've long wondered why. Is it due to moisture (or lack thereof)? A chemical reaction? An optical illusion?

posted by padjet1 to Science & Nature (20 answers total)

Perhaps salt from the roads when it snows, crushed and spread out by all the traffic?
posted by true at 6:16 PM on January 22, 2007

Best answer: It's road salt. It dissolves in snow or rain, then precipitates in a fine white film as the water evaporates.
posted by brianogilvie at 6:17 PM on January 22, 2007

Salt. If your city's department of transportation is doing its job, they'll have the salt trucks out spreading salt at the first sign of frost. That lowers the freezing point of water, keeping the roads ice-free. You'll notice the salt buildup on your shoes, on cars, etc.
posted by orangemiles at 6:18 PM on January 22, 2007

I really wanted to answer this question, but everyone else did =/

Salt, dammit. Salt.
posted by The God Complex at 6:20 PM on January 22, 2007

I understand that it might have something to do with salt.
posted by Kraftmatic Adjustable Cheese at 6:21 PM on January 22, 2007

Response by poster: How can it be salt when there's been no appreciable snowfall so far this year? I live in NYC.
posted by padjet1 at 6:23 PM on January 22, 2007

Because people bought a shitload of salt for fear of upcoming storms, which, though never materialized, give everyone a great reason to spread the stuff whenever the lightest flurry passes through.
posted by Civil_Disobedient at 6:25 PM on January 22, 2007

The salt isn't in the snow. The salt is something the city puts down on the streets/sidewalks, usually before a big freeze, in ordre to keep the roads safe. So if, for example, meteorologists are predicting sub-zero temperatures, the city puts salt down everywhere in order to keep ice from forming.
posted by The God Complex at 6:26 PM on January 22, 2007

Lick the ground. That'll let you know whether or not it's salt.

Seriously, though: no appreciable snowfall? Maybe it's the whole "Western New York lake-effect snow" thing, but we've gotten a bunch.
posted by Verdandi at 6:26 PM on January 22, 2007

Doesn't have to be snowy before they salt the streets, just cold enough to freeze. Also, salt salt salt salt salt.
posted by sonofslim at 6:27 PM on January 22, 2007

Response by poster: The salt isn't in the snow.

Fortunately, I'm not quite that dense. That would be a cool thing though -- self-melting snow.

I guess it hadn't occurred to me that people are so trigger-happy with the ground salting absent precipitation. I was secretly hoping for a more exotic explanation.
posted by padjet1 at 6:35 PM on January 22, 2007

Well, the city will spread salt if it's raining and the forecast says that it might get below freezing. That explains why they've been using it.
posted by Dipsomaniac at 6:41 PM on January 22, 2007

Sorry, I mean the salt isn't in the snow in the sense that they don't usually wait for the snow to be on the ground before they salt (which would lead to horrific road conditions). Generally it's pre-emptive. Or, if you live on the west coast of Canada, it's assuredly not pre-emptive. In fact, six inches of snow will shut down major cities for one, two, even three days, given the poor anti-snow assault teams ;)
posted by The God Complex at 6:57 PM on January 22, 2007

The answer is salt. Can't believe no one got that.
posted by Krrrlson at 7:09 PM on January 22, 2007

You guys are all wrong. The answer is salt.
posted by raconteur at 9:17 PM on January 22, 2007

NaCl.
posted by donpardo at 4:12 AM on January 23, 2007

"That would be a cool thing though -- self-melting snow."
umm...
posted by Cat Pie Hurts at 7:28 AM on January 23, 2007

I'm going to go against the grain here, and say that salt is only part of it. In my experience in Wisconsin, in the winter, it seems we get very few non-cloudy days. The lack of direct sun light seems to cast a grayish pall over everything, especially making the streets and anything concrete look exceptionally drab. I know for a fact that it looked like this, this year before we had any snow.
posted by drezdn at 9:56 AM on January 23, 2007

That, and maybe salt.
posted by flabdablet at 2:14 PM on January 23, 2007

Road salt and gravel/rock. These get spread even when it MIGHT be icy, especially in NYC.
posted by PetiePal at 9:35 AM on January 25, 2007