Climate changes may result in summer warming trends increasing the incidence, intensity and duration of heat waves, particularly in western and central Europe, Mediterranean region and western and southern USA. According to climate models for the XXI century, temperatures of the summers 2003, 2006 or 2007 might become the norm in a few decades. In August 2003, the Paris region experienced nine consecutive days with air temperature maximum daytime reaching 38ºC and minimum nighttime steadily increasing from 20 to 25ºC, resulting in 4,867 heat-related fatalities and a mortality increase of 60%. Following this extreme climate event, the French institute of public health attempted to assess the social and environmental factors contributing to heat stress and mortality. Such study implies the knowledge of high resolution meteorological observations that are unavailable, since the weather station of reference for Paris is located in a park. Surface radiant temperature, its diurnal variation and the distribution of its amplitude during the heat wave, were best resolved from a time series of 50 NOAA-AVHRR satellite thermal images, at 1km resolution. Results indicate large surface temperature gradients and contrasted daytime/nighttime heat islands patterns. The relatively small temperature amplitude in reference to normal summers confirms the impact of high minimum temperatures on heat wave process, the lack of nighttime relief and subsequent heat-stress and mortality. Temperature thresholds and areas vulnerable to heat stress were delineated. The data was then applied to a case study of local conditions for 500 persons aged 65 and over. For each location, a thermal index was produced from the satellite images and integrated into a conditional logistic regression model to estimate the spatial variability of risk factors and to implement the health alert system. Based on summertime satellite surveillance, a near-real time interactive website is being designed to inform the public and the local authorities as to extreme surface temperatures and related heat stress at the local scale, for the cities of Paris, Lyon and Marseille.

Keywords: Extreme climate event, heat wave, heat stress and mortality, thermal index, satellite thermal imaging, urban risk factors, health alert system.