Remember when horse owners only had one style and color to choose from for their horse's blanket? Those days are over. Technology has modernized the blanket-making industry, creating hundreds of new styles, fabrics and colors for horse owners to choose from. Even an experienced equine expert can be a little overwhelmed when it comes to purchasing horse blankets. Finding the best blanket is all about fit, purpose, price and quality.
(Found this handy blanket resource center that acts as a pretty good one stop guide)
Horse blankets typically come in three different weights - lightweight or sheet, medium weight for cold weather and heavy weight for very cold weather. Medium and heavy weight blankets are filled to provide warmth; sheets do not have any filling since their basic function is to keep the horse clean of mud. This fill is known as the "denier" and it indicates the size or warmth. The higher the denier then the heavier and warmer the blanket is.
If your horse needs to be protected from the cold but will be spending time in the pasture, use a turn out blanket. These blankets are made with waterproof materials and are designed so that it is difficult for horses to become twisted around if the horse decides to roll around on the ground. Except for these added features, the turn out blanket is quite similar to any other stable blanket.
Obviously the weight needed for a winter horse blanket depends on where you live and how cold it gets. Blanket manufacturers will typically tell you what temperatures their blankets are capable of handling. Place your hand between the blanket and the horse to see if the horse feels warm. If its body temperature feels cool, then you may want to move to a heavier weight. In general, I use a sheet for weather in the 50's, a medium weight blanket in the 30s and 40s, and a heavy weight blanket temperatures 20 and below. I might also use a sheet if it is in the 60s and raining. Wind chill can warrant the need for a warmer blanket.
Every horse is different. Older horses or horses that are clipped may require warmer blanketing. So, you will want to gauge what is best for your particular climate and your horse's needs.