First of all, it's never too cold to work out outside. It may be unpleasant at first, but if you wear proper layers and keep moving, the body heat you generate will easily keep you safe from hypothermia.
In fact, exercising in cold temperatures does a lot of good. Aerobic exercise keeps your heart pumping and boosts your level of virus-killing cells, just when you most need them to fight off seasonal colds and flus. Want a simpler, even more compelling reason? Colder external temperatures make our bodies expend more energy to keep warm, so you burn more fat.
3. Sunlight Combats Depression
Seasonal affective disorder, sometimes called the wintertime blues, is a form of depression most common in the winter. Symptoms include weight gain, lethargy and hypersomnia. The most common treatment is bright light therapy, which simulates the beneficial effects of natural sunlight.
Not to say that spending time outside will save you from depression, but it's clear that spending months at a time inside, surrounded by artificial light, just isn't healthy.
4. Winter Sports