Tom,<br><br>A frozen filter is bad juju. The ceramics will crack, I've also heard of problems with others. You'll note that the user manual for Katydan includes warnings to that effect. They must be pumped out after use. A damp filter isn't a problem, to best of my knowledge, only one filled with water. If the damp filter "freezes," any water warm enough to be liquid will quickly melt any ice particlestrapped in the filter, may take a bit of extra time/pressure for the first few pumps, but in my experience it's not really much of an issue. <br><br>I might as well take this opportunity to remind folks that flters that are meant to be cleaned (as opposed to replaced) should be cleaned after every trip and dried according to instructions.<br>