Warmth Factor is the number on each garment's product page that tells you how warm it is, and the higher the number, the warmer the garment.
Sleeveless tees are a 1, our lightest layer, and long sleeve tees are a 2. Tweaters, sweaters and Long Johns are warmer again, so check the number on the product page you're looking at. Our sweaters are the warmest thing we make.
As a rough guide, at 15°C (60°F) or below it's time to pop a tee on your hound, and if you need a long sleeve or a jumper, they probably do too. On colder days, layer a sweater over a tee.
For more help choosing, read our blog post on picking the right Warmth Factor.