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In my world I have a region of desert, bordered on the east by the steep cliffs of a tableland or plateau. The plateau slopes gradually down to the east coast of the continent, a distance comparable to the width of Europe. The prevailing winds come from the east, off the ocean, and blow across these tablelands, being gradually pushed upward by the rising land.

If they're high enough, could these these tablelands create a rain shadow that would explain the desert? If so, how high off the desert floor would be high enough?

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Sure.

When an airmass moves upslope, it cools and its relative humidity increases. If it is humid enough (or cools enough) it will reach 100% relative humidity and drop some of its moisture content as rain. When it passes over the obstacle it just climbed and goes down the other side, it warms and its relative humidity decreases and you get dry air and little rain.

What the barrier is doesn't really matter. (Well, as long as it doesn't have a huge lake on top to add new moisture to the air!)

The Wikipedia article on Orographic Lift has some good material and a list of places with rain shadows, some of which appear to be tablelands.

Note that the height of the obstacle is very important, while its width much less so.

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  • $\begingroup$ So it doesn't have anything to do with how quickly the air is forced up? A steep mountain range and a gently sloping rise both work? $\endgroup$ – realityChemist Aug 9 '18 at 16:03
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    $\begingroup$ @realityChemist Pretty much any realistic slope would work, though I suppose if you had a rise that was shallow enough and a wind speed which was slow enough you might start seeing some other effects. The basic phenomenon is adiabatic, meaning that the airmass neither gains nor loses energy during the process. (en.wikipedia.org/wiki/Adiabatic_process) If it went slowly enough that it took many hours or even days, I'm sure the process would be different, but I still expect that the rise would squeeze out some of the water. $\endgroup$ – Mark Olson Aug 9 '18 at 16:18
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I don't know if a tableland, as you call it, would be enough.

A couple factors you can play around with.

  • cold ocean current.
    • This will allow dryer air to be blown inland.
  • an escarpment that causes the rain to fall before the desert.
    • this doesn't necessarily have to be 'high', so much as wide.
    • possibly have a range of low mountains running in parallel to your desert escarpment/tableland.
    • see the Sierra Nevada mountains. There isn't much rain above 2400m as most of the rain has already fallen due to orographic rainfall. Any air blown beyond this height will now be dry.
  • a semi-permanent high pressure system over the desert.
    • this will help prevent the winds from blowing moist air into the desert.
    • see the Kalahairi High. A semi permanent anticyclone situated over the interior of southern Africa.
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  • $\begingroup$ The high pressure system is interest, I'll have to think about that one. And thanks for the note on what height up the mountains is needed to stop most rain $\endgroup$ – realityChemist Aug 9 '18 at 16:02

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