As per usual, jimboalee has a formula for it.
Its a bit complicated and is centred around the Canadian National Weather Service's frostbite chart ( on which is the formula ). On top of that, there is the Siple Windchill formula for calorific expenditure at reduced skin temperatures.
In a nutshell - If the Siple result is more than the actual forward propulsion, the bike stays in the shed.
And there's more.... Thermoconductivity of common clothing textiles. Dry and damp.