How can I keep a PC dry outdoors?

The best way to avoid getting moisture in your computers is to use a desiccant pack. Probably the best material to use for this purpose is a moisture absorber like that found in a modern diaper. As long as you protect the computer so that no water can directly run into the case, any condensation that occurs will absorbed by the diaper. Placing this item inside the case and covering the CPU enclosure with a plastic tarp should do the trick. I have operated equipment outside like this (in a rainstorm no less!) for several hours without any trouble.

"Condensation is formed when the temperature of the object is below the dewpoint of the air.

So just keep those computers warm....turn the fans off and put space heaters in front of them. No don't do that.

Maybe group them all next to each other."