How Does Cold Weather Affect A Heart Patient?

My Dad had a mild heart attack several months ago. He had 3 stents put in and has been doing great all summer, he is very active. He just saw his heart doctor and he told my dad that with winter coming he is going to have to be very careful. He said that the cold blowing snow could cause him to have another heart attack. He said that he does not recommend even pushing snow with his four wheeler. What does the cold, blowing snow have to do with his stents? What if the temperature is 36 degrees, does that change things? Thank You.