Using a proprietary range finding algorithm with a small (.5 mrad) laser beam divergence, the X1i performs extremely well in fog and rain. In foggy conditions, you are trying to overpower the mist in the air. With the X1i, if you can see it, you can range it. In rainy conditions, although it may appear like it is pouring, the speed of light is exponentially faster than rain drops coming down. Use of the rangefinder is similar to freezing the sky each time you take a range. It is unlikely rain drops will affect your ability to range at typical shooting distances. Any errant readings would also be removed from the average of all the ranges taken.