When is it "Too Cold" for Outdoor Activities?

Never. According to a recent article in the New York Times.
At least that's what Dr. John Castellani says. He's an exercise physiologist and expert on environmental medicine. “The big question was, ‘Is it ever too cold?’” Dr. Castellani said. “The answer is no. People go to the poles, people are out there when it’s minus-50 [...]