In summer, one of the lowest pressure centres in the world prevails in the south. Warm Arabian winds bring heavy moisture from the Persian Gulf. The gulf area, where the heat and humidity are unbearable, stands in sharp contrast to the Caspian coastal region, where moist air from the sea mingles with the dry air currants from the Alborz to create a soft nightly breeze. Precipitation also varies greatly, ranging from less than two inches in the south-east to about 78 in the Caspian region. The annual average is about 14 inches.
Winter is normally the rainy season for the whole country.
Frequent spring thunderstorms occur, especially in the mountains, where destructive hailstones also fall. The coastal region presents a sharp contrast to the rest of the country. The high Alborz mountains, which seal off the narrow Caspian Plain, wring moisture from the clouds, trap humidity from the air, and crete a fertile densely populated semitropical region with think forests, swamps, and rice paddies. Frost is rare. In Iran, the change from one season to the next is fairly abrupt.