At work, we discussed when to change to winter tires for bicycles (and cars). We assumed that temperature was the key factor and I used a program (Mathematica) to check the mean temperature in Stockholm (see this question Probability of ice in Stockholm).
My question is: besides the risk of ice, what are key factors for deciding when to change from summer tires to winter tires? E.g. is the rubber of summer tires negatively impacted when it gets colder.