If you split open a guard hair and look at it through a microscope, the inside looks like the honeycomb in a bee's nest. Thousands of tiny air pockets fill each guard hair. That makes them waterproof and warm. This warm winter coat is so thick that it can keep snow from melting on an elk's back. This is the same cue that causes leaves to fall in autumn.
Elk have other tricks for staying warm in winter.
For example, elk can make their hair stand on end, trapping more air and creating a thicker coat. They can also tuck their legs beneath them when they lie down so they lose less heat through their legs, chest and belly. There they bed beneath thick trees. The trees hold warmer air near the earth, catch snow before it hits the ground, and break the wind. On sunny days, elk move to open south and west slopes.