Slippery When Wet

[alexxkay]

What was it about last night's rainfall in the Boston area that made footing so unusually treacherous? I nearly fell on a number of occasions. Was there some unusual chemical compound coming down with the rain, that was extra-slippy? Or was it because of the way the temperature was hovering right around the freezing mark? I didn't observe any actual *ice*, mind you -- just patches of wet sidewalk that were unexpectedly slippery. Or is the answer more obscure than either of those?