The Iranian highlands have a continental climate. The summers there are warm and dry. In Tehran the temperatures in July and August are up to 40°C. In the areas below 1,000 m it is very hot. In the higher altitudes, however, it is also pleasant in summer. In winter, from November to February or March, it is usually very cold and there is heavy snowfall. The spring, in which it hardly rains, is only very short. Autumn is very pleasant in all areas. In the Khuzistan plain and the Persian Gulf the winters are very mild, the summers hot (partly over 50°C) and humid. The climate at the Caspian Sea is mild and relatively balanced in all seasons. However, there are always heavy rain showers.
In Tehran there is an extreme air pollution, which is mainly caused by car exhaust fumes.
For the Iranian highlands, the periods from March to May and from September to November are recommended.