A huge part of winterizing your Shih Tzu will be to use functional clothing that protects the dog from cold weather and offers a shield the dog from harsh winds that can chap the entire body. There are 2 things to keep in mind regarding the actual temperature:
1) The wind chill factor.
This places a crucial role. Here are some quick calculations to keep in mind: If the air is 45 F (7.2 C) with a 15 mph wind, the air temp will actually be 38 F (3.3 C). If the temp is 35 F (1.67C) with a 10 mph breeze, your Shih Tzu will feel the real temp of 27 F (-2 C).
2) The ground surface.
Cold air sinks, warmer air rises. It is surprising how much colder the ground can be than just 5 or so feet higher.
Toys sized dog breeds have more trouble regulating body heat than larger breeds. This is because of a higher ratio of body surface to body weight. Therefore, a Shih Tzu will lose his body heat much faster than his medium or large breed counterpart. This can lead to a case of uncontrolled shivering and for some, hypothermia. Lastly, when a dog feels cold and unprotected, he will be less agreeable to going outside.
When you bring your Shih Tzu outside, whether for bathroom needs or his daily walk (more ahead regarding exercise), you will want your dog to be just as comfortable outside as in.
Protective outer gear is also recommended to keep the coat dry. Sleet, snow and cold rains will be absorbed through the single layer coat and cause shivering and increase the threat of hypothermia.
Booties - There are reasons to put shoes or boots on a dog as well:
1) Dogs that do not feel as if they have good footing can tend to shy away from going outside. The right boots provide excellent traction on all surfaces, including mud, ice and snow.
2) Bare paws will have direct contact with freezing surfaces; this can make walking uncomfortable at best and put a dog at risk for frost bite in worst case scenarios.
3) The ice melt chemicals that many city municipal services spread on the road are quite terrible for dog paws. They can cause chemical burns.
4) The tiny rock pebbles often dispersed as roads are 'sanded' can become wedged in the sensitive area between the paw pads.