I’ve lived in the Chicago area all my life, and every winter this question has popped into my mind at least once, and was immediately forgotten. This time, it stayed in my brain long enough for me to get here. I’ve noticed that my least favorite days are those bitter cold days (like today) when the sidewalks and streets turn white. (No, it’s not snowing - I’m a bit of an idiot, but not THAT much of one.)
I have this theory that it’s so cold and dry and windy that the moisture on the ground has completely evaporated, leaving a white residue from rock salt. (I came up with this theory after swimming in the ocean a couple summers ago.)
I know what you are talking about. I think you are basically right that it is caused by residual salt. Here in Boston this year, we have gotten practically no snow at all but it has gotten very cold a few times in the past few weeks. The pavement looked normal until this week when we had just enough overnight snow to dictate salting the parking lot at work. By mid-day, the moisture was gone but the pavement had that chalky look.
It could also be frost. Water can crystallize with a fairly porous structure with reflective properties similar to snow. Is it mostly a morning phenomenon? Frost tends to melt by mid-morning (least in these parts), while salt residue could stick around all day. If it continues to perplex you, you could always give it a lick.
Another way to tell is by looking for rings. I’m familiar with the salt-residue-phenomenon you speak of, but it seems to be concentrated in bands, if you will. The residue is concentrated in small amounts of water, and then deposited where that water evaporates. If its not a uniform layer, I’d vouch for your theory.
Nope, it’s not frost, and it’s not in bands. This is a fairly uniform white surface, and it’s not only in parking lots and sidewalks, but also on the roads. The normally black asphalt is white.
It’s salt. Same as the residue on my car that gets onto my coat and jeans if I lean against the side panel while loading the kid into her car seat. The kid, meanwhile, tries to lick it off my sleeve (I don’t let her, obviously). Salt frickin’ everywhere. It’s amazing we haven’t turned Lake Michigan into an inland sea with all the salt runoff.