Adequate shelter means your cat is kept warm, comfortable and dry. If you have outdoor cats or indoor-outdoor cats, make sure that your cat has a shelter of its own. The shelter should be raised off the ground, and have dry bedding that is insulated or heated.
Also, make sure also that your cat has a constant source of fresh clean water – not snow. Thermal heaters are available to make sure the water source doesn't freeze.
If your cat spends a lot of time outdoors feed him extra calories because it takes more energy in winter to keep body temperature regulated. Without proper care, cold weather can be dangerous to your cat.
For more information, please read the story Winter Cat Care.