Here is an interesting article about FLIR (Forward Looking Infra-Red Imagery) conducted on Polar Bears. Notice that there has to be a contrast between temp and dew point. Sun light also interferes as does snow, fog, and I would think rain. It says some dens will never be detectable. I wonder why? Are they deeper perhaps?

http://alaska.usgs.gov/science/biology/polar_bears/denning.html
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