What are the benefits of exercising outdoors once the mercury drops?

​Your body will work hard to accommodate you, no matter what you do - that's it's job - so, following the initial swear and involuntary shiver do your job too - get your base layers on and enjoy the benefits of cold weather work.
Boost your immune system, through regular exercise and you're likely to feel better over the winter period.
You'll burn more calories as your body works hard to maintain it's core temperature.
You'll strengthen your heart as it works harder to distribute blood.
You'll get a good dose of Vitamin D - need to get it where we can over winter.
You'll feel happier - cold weather exercise can boost your mood as your body works hard to stay warm, the amount of endorphins also increases, leaving you smiling.
You'll build tolerance to the cold but only if you get out there regularly, so as the winter develops, prepare well, layer up and get going with whatever it is that you enjoy doing.