Study of the urban heat island in a coastal Mediterranean City: The case study of Thessaloniki, Greece

The urban heat island in the coastal city of Thessaloniki, Greece, is investigated using near-surface temperature data measured at 7 sites in the greater Thessaloniki area for the 1-year period from March 2008 to February 2009. The urban heat island in Thessaloniki is stronger in the nighttime than in the daytime and decreases with increasing wind speed, while there are indications that it is more pronounced during the warm half of the year. Observations of the maximum urban heat island intensity range from 2°C to 4°C and from 1°C to 3°C during the warm and the cold part of the year, respectively, showing a smaller variability during the summer moths than in the winter. Greatest values are more usually observed following sunset, whereas minimum values are detected during solar peak hours. A regression analysis is carried out to investigate the impact of moisture availability and wind speed on the development of the urban heat island in Thessaloniki. It is found that the nocturnal heat island results to increased nighttime water vapor pressure in the urban areas, whereas during the day the heat island intensity and urban/semi-rural water vapor pressure differences are found to be anti-correlated. Furthermore, the intensity of Thessaloniki's heat island appears to decrease significantly when wind speed exceeds 4m/s. The impact of the urban heat island on human thermal comfort in Thessaloniki is also investigated, computing hourly values of the discomfort index and the approximated wet bulb globe temperature. The center of the city is found to exhibit the highest discomfort index and approximated wet bulb globe temperature values. In addition, the urban heat island is found to have a negative impact on thermal comfort on most of the observed occasions. In particular, a 1.5°C increase in the urban heat island intensity appears to result to an average 1°C increase in discomfort index and 1.4°C increase in approximated wet bulb globe temperature of the urban area on about 50% and 75% of the cases, respectively.