As air moves up and over a mountain, the temperature drops 5.5 degrees for every 1000 feet of rise. The water vapor in the air condenses at the lower temperature and precipitates on the mountain and notch below. As the air comes back down the other side, the air warms 5.5 degrees per 1000 feet and has an increased capacity to hold onto it's moisture. Sort of like squeezing moisture out of a damp towel. The towel that was just dripping while squeezing is now drier and not likely to drip.
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