Iran has a variable climate; because of its size, topography and altitude, Iran experiences great climatic extremes. It ranges from arid or semiarid to subtropical along the southern Caspian coast. On the northern edge of the country, the Caspian coastal plain and the northern forests, temperatures rarely fall below freezing and the area remains humid for the most of the year. In the summer temperatures rarely exceed 30 ℃. In the eastern part of the plain the annual precipitation is 680 mm and more than 1,700 mm in the western part.
In the northwest, winters are cold with heavy snowfall and subfreezing temperatures during the months of December and January. Relatively mild temperatures are in spring and fall, while summers are dry and hot.
To the west, the Zagros higher mountain valleys experience lower temperatures, severe winters with below zero average daily temperatures and heavy snowfall during winters. But the summers are hot and dry.
In the south, winters are mild with an average temperature of 18 ℃, and the summers are very hot, having average daily temperatures in June and July exceeding 41 ℃. On the Khuzestan Plain, summer heat is accompanied by very high humidity. The coastal plains of the Persian Gulf and Gulf of Oman in southern Iran have mild winters, and very humid and hot summers. The annual precipitation ranges from 135 to 355 mm.
The eastern and central basins are arid, with less than 200 mm of rain, and average summer temperatures 39 ℃.
In general, Iran has an arid climate in which most of the relatively scant annual precipitation falls from October through April. In most of the country, the annual precipitation average is 250 mm or less. The major exceptions are the higher mountain valleys of the Zagros and the Caspian coastal plain, where precipitation averages at least 500 mm annually.