Spring (April–May) is the best time to visit Tehran. The air pollution improves, the Alborz mountains above the city are still snowcapped, the parks are green, and the whole country is in the mood for Nowruz (Persian New Year, 21 March) and the weeks that follow. Temperatures are mild — 15–25°C in the city.
Autumn (September–October) is nearly as good: cooler than summer, clear skies, and the mountain slopes turning gold.
Summer (June–August) is brutal in the lower parts of the city — 35–40°C with heavy pollution, though the northern suburbs (above 1,500 metres) are significantly cooler. This is when wealthy Tehranis escape to the Caspian coast or abroad; the city empties noticeably in August.
Winter (December–February) is cold — heavy snowfall is not unusual — and the ski resorts operate. The city's pollution reaches its worst in windless winter weeks when smog settles over the basin. But the Golestan Palace in snow is beautiful, and the tea houses of Darband are warm and full.
Nowruz (around 21 March) is a national holiday of two weeks — museums, bazaars, and restaurants close or run reduced hours.