Why One Side of Mountain Range is Lush I'm new to this site and certainly not someone with any real background in atmospheric science, so please forgive me if this question is a little stupid.

I recall learning in High School about mountain ranges frequently being dry on one side and lush on the other. The explanation was that as a cloud approaches the mountain from the lush side, it releases any rain so it can rise over the range. This always seemed like a silly explanation to me, because in the way it's phrased, it seems to imply that the cloud is cognizant. Also, this would mean that clouds can only (for the most part) approach the mountain from the lush side, which may or may not be true, but I don't know why. So I'm sure there's a better explanation as to why this happens, but I'm hoping someone can explain.

Thanks
The Cascade Mountain Range in the US Pacific Northwest is a good example to use to explain this. The predominant wind direction is from the West - over the Pacific Ocean. The air over the ocean picks up moisture from evaporation. After it passes the coast, the mountains cause the air to rise. As it does so it cools. Colder air can hold less water vapour than warm air so that the air becomes saturated with water and it condenses out into clouds. With enough condensation you get rain - a lot of it on the western side of the mountains. So you had it backwards, the rain doesn't allow the air to rise - the rising allows it to rain. Clouds mainly approach from the lush side because that is the prevailing wind direction but it is better say that it is the lush side because clouds mainly approach that way. When wind comes from the other direction is travelling across land so it contains less water and the cooling generally isn't enough to cause rain.

Once the air crosses to the other side of the mountain range it warms and becomes undersaturated with water. Clouds often disappear and even if they remain you aren't getting further condensation to produce a lot of rain. So those areas are much drier.