Tehran has a cold semi-arid climate with continental climate characteristics and a Mediterranean climate precipitation pattern. Tehran’s climate is largely defined by its geographic location, with the towering Alborz mountains to its north and the country’s central desert to the south. It can be generally described as mild in spring and autumn, hot and dry in summer, and cold and wet in winter.
As the city has a large area, with significant differences in elevation among various districts, the weather is often cooler in the hilly north than in the flat southern part of Tehran. For instance, the 17.3 km (10.7 mi) Valiasr Street runs from Tehran’s railway station at 1,117 m (3,665 ft) elevation above sea level in the south of the city to Tajrish Square at 1712.6 m (5612.3 ft) elevation above sea level in the north. However, the elevation can even rise up to 2,000 m (6,600 ft) at the end of Velenjak in northern Tehran.
Summer is long, hot, and dry with little rain, but relative humidity is generally low, making the heat tolerable. Average high temperatures are between 32 and 37 °C (90 and 99 °F), and it can occasionally drop to 14 °C (57 °F) in the mountainous north of the city at night. Most of the light annual precipitation occurs from late autumn to mid-spring, but no one month is particularly wet. The hottest month is July, with a mean minimum temperature of 26 °C (79 °F) and a mean maximum temperature of 34 °C (93 °F), and the coldest is January, with a mean minimum temperature of −5 °C (23 °F) and a mean maximum temperature of 1 °C (34 °F).
The weather of Tehran can sometimes be unpredictably harsh. The record high temperature is 43 °C (109 °F) and the record low is −20 °C (−4 °F). On January 5 and 6, 2008, a wave of heavy snow and low temperatures covered the city in a thick layer of snow and ice, forcing the Council of Ministers to officially declare a state of emergency and close down the capital from January 6 through January 7.
Tehran has seen an increase in relative humidity and annual precipitation since the beginning of the 21st century. This is most likely afforestation projects, which include expanding parks and lakes. The northern parts of Tehran are, still, more lush than the southern parts.