Realistically, 14 Farenheit (or -10 Celsius) is the last range that can be comfortable, although
with a Balaclava, double pants, double coat and double hat, you won' t be cold, but will
it be worth your while to commute in this manner, to work, for example? Nope!
Regarding a MTB, although I often dispise it in dry or good weather, when it's rainy,
or when there's 6 inches of snow on the ground, an aluminum frame MTB literally flies
over the snow and bad surface conditions, surprisingly, which is impossible to do with
a hybid or road bike.