Remember, the amount of bright sun you get will also affect your blanketing needs. Extreme cold that is dry and sunny (for example, some high altitude mountain climates) may not be a hard on your horse as moderate cold with wind, wet snow, and cloudy skies. Also keep in mind your horses hair coat. Fuzzy ponies and drafts don't need as much winter blanket as thin coated Thoroughbreds and Arabs, while ALL clipped horses require a winter blanket.
Pick a winter horse blanket that is both waterproof and breathable - this will ensure that your horse stays dry from the outside in and that any perspiration he builds up underneath the blanket can pass through to the outside air, leaving your horse warm and dry near his skin.
TURNOUT HORSE BLANKETS by BREED
Here are some links for you to peruse as you make your horse blanket choices:
Recommended Blanketing for your Horse's Breed