The rain shadow occurs when moist air rises on the windward slope, cools, condenses, and precipitates (orographic precipitation). Crossing the summit and descending, the now-dry air warms by foehn effect, creating arid conditions on the lee side.
In Spain, the Ebro Valley is a rain shadow of the Iberian System and Pyrenees (300 mm/year vs. 1,500 mm on the northern slope). Almería (200 mm/year) sits in the shadow of the Betic ranges.