The Mayor of NYC, whom I respect but frequently disagree with, has predicted riots due to the jobs situation. I agree with his assessment but the factor isn't jobs, it is the belief that the situation is not going to improve any time soon.
The mayor looks to London and witnessed days of rioting. Most but not all of the rioters fit the traditional stereotype of the poor. There were also a few art majors and social science types, but thus far no known accounting majors.
Riots tend to take place in warmer weather. Thus as the temperature gets colder we in the North are given a respite from rioting. No doubt some clever blogger will find some riot that took place in the dead of winter. However, even the draft riots in NYC were a summer event. There were riots in Baltimore but those riots took place in April which is not cold weather. The civil war riots may or may not have been provoked by Copperheads and may be an unrelated phenomena.
The Boston Tea Party was a political act and does not count as a spontaneous riot. No doubt some white people decided to dress like Dr. Yeagley and the rest is history. There was one in Cincinnati that appears in the 1800 and the Thompkins Square riots but most appear to be a warm weather activity.
For those that hate the winter, at least you have some reason to be thankful.
Remind me when I visit Cincinnati to drink only bottled water. How that city gets on the list so many times is unknown.