Don't any of you guys running in the deep freeze get a little concerned when you can't feel your feet anymore? How do you know frostbite isn't setting in?
Everything Broad Arrows says. Lemme just add that for me, there are two kinds of numbing: gradual and immediate. I call these "Numb Plunge I" and "Numb Plunge II."
With gradual numbing, or Numb Plunge I, it takes up to a mile or two for the numbing to level off, and only the toes and a bit of the side of the foot gets numb, although the metatarsals can feel stiffer as well. The numb bits are not completely numb and remain nice and red.
With Numb Plunge II, the numbing is fast and there's no leveling off. My feet will keep getting number and number and then turn white. This is when frostnip or frostbite occurs.
Like BA says, everyone has to discover their own tolerances, both mental and physiological. For me, I experience Numb Plunge I down to about 5-10F in dry conditions, and down to about freezing in wet. I experience Numb Plunge II in wet conditions below freezing, and sometimes a little above it as well, and in dry conditions in the single digits (F) or below.
Yesterday, for example, the temp was 44, the windchill 40, and the pavement was consistently wet from the melting snow, but my feet didn't get very numb at all, and after a mile they started to warm up a bit. And after two weeks of frozen snow, it was fantastic to feel the pavement again with my soles. For some of us, the plantar stimulation is truly addicting, like spicy food or something.