This article is from the Chihuahua Breed FAQ, by email@example.com (Melinda 'Bob' Casino) with numerous contributions by others.
Owners must be very sensitive to the fact that short-haired
Chihuahuas, and even long-haired ones, are vulnerable to the cold. In
temperatures of 35-40 degrees Farenheit, I would recommend dressing
your Chihuahua in a special dog sweater for brief walks. Walking your
Chihuahua in temperatures below 35 degrees Farenheit is STRONGLY
discouraged, especially when there is a wind-chill factor. In this,
as in other aspects of dog maintenance, keep your dogs' health top
priority, not your own desires. In fact, in some cold regions, an
indoor heater will be necessary.