You need strong calf muscles for trudging through snow! Your calves are made up of two major muscles; the gastrocnemius and the soleus. Both of these muscles are very important for simple daily activities such as walking. A well-trained soleus can help prevent ankle rolling.
Images from WikiHow: http://www.wikihow.com/Do-Calf-Raises
Exercise
Stand up straight, grasping a chair or wall for balance.
Position your feet hip-width apart.
Slowly raise your heels until you’re on your tiptoes. Balance your body weight on the balls of your feet.
Slowly lower yourself. Repeat.
Start with 5 repetitions.
Need an extra challenge? Try balancing your body weight on the ball of only one foot. Tuck your left foot behind your right heel and repeat the above steps. Then switch sides.