How Polar Bears Stay Warm–and Nearly Invisible
Monday, February 10th, 2014February 10, 2014
Polar bears live in some of the coldest climates on Earth. With enough food, polar bears can stay warm and cozy in temperatures that would quickly freeze a human to death. In a recent paper in the scientific journal Optics Express, researchers revealed a trick polar bears use to stay warm–their shiny white fur reflects waves of heat energy back into their bodies. Their research also helps solve the mystery of why polar bears are nearly invisible on infrared cameras.
For polar bears, white fur can be a blessing and a curse. Their pale color helps them blend into snow, making it easy for them to ambush prey. On the other hand, white fur—like a white t-shirt on a sunny day—scatters more of the sun’s heat than it absorbs. One might think polar bears would be better off with dark fur that absorbs more of the sun’s heat. (Scientists have disproven the idea that hollows within the strands of fur somehow channel solar radiation into the bear’s body.)
However, the sun is not the only object that radiates heat energy. All warm objects give off thermal radiation. This radiation is largely in the infrared range of the electromagnetic spectrum, beyond visible red light. Warm-blooded polar bears stand to lose a great deal of their body’s warmth via thermal radiation.
To learn how this bear-exiting radiation is affected by the bear’s fur, scientists used computer models. They looked at layers of furlike material of varying densities and transparencies. They found that polar bear fur acts like a “gray body”—somewhere between an all-absorbing “black body” and a heat-scattering “white body.” As thermal radiation leaves the polar bear’s body, some gets scattered through the fur, and some gets absorbed. The fur works much like the reflective coating on the inside of insulated vacuum bottles that can keep coffee and soup warm for hours.
Additional World Book articles: