
The temperature in caves (which are typically a few hundred feet deep) is close to the temperature of land, averaged over many years. In the northeast, caves are 50-55F, while in the southwest they are typically in the 60s and 70s. The part that Alexander was in is a very deep part of the cave; it's not a matter of "open air to the surface" that the temperature is that warm. It's simply how the cave is in southeastern New Mexico, where it gets frickin' hot in the summer, but never very cold in the winter.
From where he was, it's not possible that the cave served as a waveguide for the RF. And as best as I know, there's no metal in that part of the caves- electrical lines or metal handrails, as there are in the tourist parts of the cave- that would allow the transfer of RF energy.
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