Inter-annual variations and trends of the urban warming in Tehran

• Inter-annual temperature changes in the urban city of Tehran are examined
• A warming trend of the order 0.37 °C/decade has been observed, with the stronger nighttime warming compared to the daytime
• A seasonal cycle in the changes of the diurnal temperature range was identified, with a stronger decrease rate in winter.

Urbanization is an extreme case of land-use change which amplifies in most cases the regional warming, and highly impacts a number of sensitive sectors, particularly human health. Using near-surface daily temperature records for a 63-yr period from 1951 to 2013, temperature changes over the urban city of Tehran were examined. Statistically significant warming trend of the order 0.37 °C/decade has been observed, leading to 2.3 °C rise over the entire 63-yr period, and the warming trend has been intensified in recent years. The observed warming in the urban city of Tehran has been more than triple the rate of change in the global average temperature, indicating that urbanization has had a significant warming effect. The nighttime warming (0.62 °C/decade) in the urban city of Tehran was found to be more than three times greater than the daytime warming (0.17 °C/decade), resulted in a decreasing trend in the diurnal temperature range (DTR; i.e. the difference between the daytime maximum and nighttime minimum temperatures). In a similar manner, a decreasing trend in the number of cold nights was identified, and the rate was more than twice as high as the rate of increase in the number of warm days. From a seasonal perspective, warming trends of the urban city of Tehran have been found to be nearly identical in winter and summer, with the rates of 0.35 °C/decade and 0.33 °C/decade, respectively. However, a seasonal cycle in the changes of the DTR was identified, with a stronger decrease rate in winter than summer because while the nighttime warming (which has been greater than the daytime warming) has not changed considerably from winter to summer, the daytime warming has been stronger in summer.