Many Spider Beam, Hexbeam or another wire yagi users noticed a high or changing SWR when the antenna became wet. The problem with the rain is resulting from the fiberglass spreaders are getting wet, and therefore somewhat conductive and will therefore affect the resonance frequency of the nearby driven elements. It will not affect the overall antenna performance, but it will shift the resonance of the driven elements. As soon as the spreaders are dry again, the problem is gone. Rotate the antenna back and forth to shake off the raindrops, and you will notice an improvement!