Cats are warm-blooded animals and need to maintain a body temperature between 100.5 and 102.5 degrees Fahrenheit to stay healthy. Below this range, they can develop hypothermia, which can be fatal. If you're transporting your cat in a carrier, there are several things you can do to keep them warm.
First, consider the carrier material. Some materials insulate better than others. For example, hard plastic carriers don't retain heat as well as carriers made from soft materials like fabric or fleece. If possible, choose a carrier with good insulation properties.
Second, add bedding material to the carrier to help your cat retain heat. A thick towel or blanket will work well. You can also buy special cat cozy products that are designed to fit inside carriers and help keep cats warm.
Finally, make sure the carrier is placed in a location where it won't be exposed to drafts of cold air. A good spot would be on the floor of your car behind the front seats (away from any open windows). You may also want to place a heating pad underneath the carrier (set on low) to help keep your cat warm during transport.