Estimation of Time distribution of occurrence of Heat and cold stresses in Tehran City (Case Study: Area 9)

Climatic conditions is one of the most important factors affecting different aspects of human life, especially in urban areas and affects social and economic sectors. Establishing a thermal balance between the human body and the environment is one of the primary needs for health and comfort. Human thermal comfort conditions can be evaluated by using various indices based either on simple empirical approaches or on more complex and reliable human-biometeorological approaches. Human thermal comfort conditions with a single parameter or thermal indices derived from experimental equations, cannot fully evaluate thermal comfort conditions. In this study, we tried to investigate the temporal distribution of heat and cold stress events based on the physiological equivalent temperature (PET) index of Tehran in 9th area.

Determining suitable weather conditions for outdoor presence (in terms of heat and cold stresses) can be an important factor in reducing mortality and providing human comfort in these areas. Therefore, the focus of this paper is to examine the weather conditions in Tehran 9 area. The establishment of Tehran's Mehrabad Airport in this area as well as the presence of the western terminus on the northwest side of Azadi Square has made it one of the busiest areas in Tehran. The evaluation was based on statistics and information from Tehran's Mehrabad Airport synoptic station located on the southwestern side of zone 9. In this paper, to determine the time range of occurrence of heat and cold stresses, temperature data, relative humidity, water vapor pressure, wind speed, and cloudiness on a scale of hourly during 2008 to 2017 were obtained from the Meteorological Organization  and PET index was calculated using RayMan Model and analyzed on annual, seasonal, monthly, daily and hourly scales.

The PET index for Tehran 9 area was calculated from January 2008 to December 2017 using data from Tehran Mehrabad Synoptic Station. The results showed that in this decade, the thermal comfort, frequency of PET index accounted for only 12.91% of the whole period. Much of the analyzed data belong to cold stress classes of less or higher, with 54.62% being the most abundant. The rest of the data are in the range that according to the PET index classification, are slightly warm to hot and 27% of the whole period is affected by different heat stresses. The results showed that extreme cold stresses began in the second decade of October and continued until the first decade of April. This class of cold stresses completely disappears between the second decade of April to the first decade of November. Initial surveys showed that the highest percentage of very severe cold stress was distributed in the first 10 days of January. This percentage is slowly declining as temperatures rise in the coming days, reaching their lowest level in the first decade of April. The percentage of severe, moderate and mild cold stresses decreased by 11% over the whole period from 23% to 12%. The range of severe cold stresses started from the second decade of October and was steadily stable in the study area until the third decade of April. Maximum and minimum percentages of severe cold stress were in the second decade of March (35%) and the first decade of October (2.5%). Moderate cold stresses began 20 days earlier than severe cold stresses and continued until the second decade of May. According to the figures obtained from the PET index, it peaked at 46% in the second decade of April. Slight cold stresses that started in the third decade of April and continued until the third decade of June. Extreme heat stress is distributed only in June, July, August and September. Frequency of heat stress increases significantly in July, especially in the second decade. The frequency of this stress in the first 10 days of June increased by 1.3% and increased to 37.5% in the second decade of July. The convergence frequency of the PET index is observed in different percentages between the third decade of March to the second decade of October.

Area 9 of Tehran, 86% of the total year was outside the range of thermal comfort conditions, with 59% related to cold stresses (PET <18 ° C) and 27% related to varying degrees of heat stress(PET>23C°). Extremely severe cold stresses in January, February and December from 6 pm to 6 am and severe to moderate heat stress from late May to late September between 9 am and 3 pm.