Rain shadow

Evaporated moisture from water bodies (such as oceans and large lakes) is carried by the prevailing onshore breezes towards the drier and hotter inland areas.

When encountering elevated landforms, the moist air is driven upslope towards the peak, where it expands, cools, and its moisture condenses and starts to precipitate.

As the air descends the leeward side of the landforms, it is compressed and heated, producing foehn winds that absorb moisture downslope and cast a broad "shadow" of dry climate region behind the mountain crests.

Typically, descending air also gets warmer because of adiabatic compression (as with foehn winds) down the leeward side of the mountain, which increases the amount of moisture that it can absorb and creates an arid region.

When low pressure systems skirt the Rocky Mountains and approach from the south, they can generate high precipitation on the eastern side and little or none on the western slope.

Effect of a rain shadow
The Tibetan Plateau (center), perhaps the best example of a rain shadow. Rainfalls from the southern South Asian monsoon do not make it far past the Himalayas (seen by the snow line at the bottom), leading to an arid climate on the leeward (north) side of the mountain range and the desertification of the Tarim Basin (top).
The Atlas Mountains ' (top) rain shadow effect makes the Sahara even drier.
The mountain ranges on the eastern side of Madagascar provide a rain shadow for the country's western portion.
The eastern regions of the Western Ghats lie in a rain shadow, receiving far less rainfall.
Most of Iran is rain-shadowed by the Alborz mountains in the north (just south of the Caspian Sea ), hence the country's mostly (semi) arid climate.
Lake Urmia (centre) and surrounds rain-shadowed by the snowy Zagros mountains to the west.
Cantabrian Mountains in the north, which rain-shadow most of Spain
The Cascade Range to the north and the California Coast Ranges and the Sierra Nevada to the south provide a significant rain-shadow for the inland North American deserts .
The Atherton Tableland rain-shadowing the dry Tablelands Region in Queensland (bottom-right).
The Southern Alps in New Zealand rain shadow the eastern side of the South Island .
The Andes mountains block rain and moisture from the Amazon basin to the west ( Bolivia ).