How do the Alborz Mountains affect the climate of Iran's Caspian plain?

The Alborz Mountains play a critical role in shaping the climate of Iran’s narrow Caspian plain by acting as a physical barrier that separates the region from the rest of the country.1

As northerly air movements enriched with moisture from the Caspian Sea collide with the steep northern faces of the mountains, the range wrings moisture from the clouds and traps humidity. This process creates a distinctively humid, semitropical climate on the Caspian side, characterized by luxuriant forests, swamps, and rice paddies.12

Because the mountains effectively seal off the plain, the region experiences high humidity—sometimes reaching nearly 100 percent—and temperatures that can soar to 100 °F (38 °C), while frosts remain extremely rare. This environment stands in sharp contrast to the arid conditions found on the southern, inland slopes of the Alborz, which share the dry, steppelike climate of the Iranian plateau.12