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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

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  • $\begingroup$ Another reason may be that one side of the mountain gets a different amount of sunlight. $\endgroup$ – Gimelist Oct 22 '15 at 0:35
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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.

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    $\begingroup$ Nice answer, though a DEM or topo map would drive home the point. Not everyone is familiar with the terrain in that area. $\endgroup$ – farrenthorpe Oct 22 '15 at 4:58
  • $\begingroup$ This is a great answer! In case you want to read more about this form of rainfall, it's known as orographic precipitation: en.wikipedia.org/wiki/Precipitation_types#Orographic $\endgroup$ – Gaurav Oct 22 '15 at 6:48
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    $\begingroup$ So you had it backwards, the rain doesn't allow the air to rise - the rising allows it to rain. As you go up the mountain slope, the rising allows it to rain. Just like they taught us way back in geometry class in high school. Slope: rise over rain! $\endgroup$ – Mason Wheeler Oct 22 '15 at 14:52
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I'll augment the other answer with an example. Consider a theoretical north-south oriented mountain range that rises 2000 m above sea level and the land on either side of the range is at sea level. A little ways west of the mountain is ocean. The location is the mid-latitudes and the prevailing wind is from the west.

Air along the surface travels above water, picking up moisture. Lets assume a surface temperature of 28 °C and a dewpoint of 23 °C once the air reaches the base of the mountain on the west side. This parcel now rises the 2000 m vertical height of the mountain. At around 500 m the parcel has cooled to saturation and now has a temperature of 22 °C and a dew point of 22 °C. This height will also be the altitude where clouds form. At and above 500 m the mountain will be cloudy. The parcel will continue to rise another 1500 m to the top of the mountain at 2000 m where its temperature has now cooled to about 16.5 °C.

As the temperature dropped from 22 °C to 16.5 °C water condensed to contribute to cloud water and rain. In total, for every kg of air flowing up the mountain in this state, 3 g of liquid water is produced via condensation. This windward side of the mountain with the moist upslope flow is going to be lush from frequent and abundant rain showers.

Once the parcel tops the mountain and begins to flow down the lee slope it will begin to warm and relative humidity will drop. Any cloud blown over the mountain top will evaporate as it moves downslope. By the time the parcel reaches the ground at sea level east of the mountain it has a temperature of 36 C and a dewpoint of 19.5 °C - hot and dry. The less slope and the region to the east will be arid. Regions like this are referred to as being in the "rain shadow" of a mountain.

These numbers are ballpark back-of-the-envelope estimates from a Skew-T diagram and while not exact they are representative.

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