Because of its size, topography and altitude, Iran experiences great climatic extremes. Winters (December to February) can be unpleasantly cold, especially in the north and west, and in most of the rest of the country the nights are very cold. In summer (June to August) temperatures as high as 50°C are nothing out of the ordinary along the Persian Gulf coast and southern provinces.
Regular rainfall is more or less restricted to the far north and west – the area north of the Alborz Mountains receives an annual average of about 1300mm of rain, but although year-round cloud helps keep summer temperatures manageable the high humidity makes summer pretty muggy on the Caspian coast. In western Iran winter temperatures are regularly well below zero and snow frequently remains until early spring, making some mountain routes impassable.
Unless you’re a mad dog or an Englishman, it’s best to avoid the Persian Gulf coast between early May and mid-October, when the double whammy of high temperatures and oppressive humidity take much of the fun out of travel. Further inland, summer temperatures are very warm indeed, but low humidity makes life more bearable.