As heatwaves increase in both frequency and intensity globally, the need to develop tools to predict the human impact and develop a more comprehensive understanding of the impact mechanism at a population level is becoming more urgent. Our study provides a taxonomy of heatwaves based on identifying sub-threshold lethal heatwaves through physiological adaptation and vulnerability. We use a classification algorithm applied to a lethal heatwave dataset, comprising 125,411 events where the temperature exceeded the 90th percentile across 140 cities, with combined meteorology and sociodemographic inputs to label these events. The accuracy of our model outperforms classification that relies on wet bulb temperature thresholds with a factor of 11 improvement in imbalanced classification performance. Furthermore, we find that the majority of lethal heatwaves within our dataset occur below high wet bulb temperature thresholds and that accurate predictions for heatwave mortality can be obtained by combining thermo-temporal differentials and population health metrics instead of absolute climatic conditions. We thus propose classifying heatwaves as either: Shock Heatwaves, where aggressive thermo-temporal differentials from a local acclimation point trigger adverse stress effects, particularly among the vulnerable; or Threshold Heatwaves, where high temperature and humidity conditions do exceed the ability to dissipate heat effectively.

Abstract Research on heatwaves has gained significant impetus over the past decade due to a warming planet and rapid 21st century urbanization. This study examines driving factors influencing heatwave trends and interannual variability across Southern California (SoCal) from 1950–2020. Inland urban areas of Los Angeles county are the most susceptible to heatwaves with strong increasing trends in frequency, duration, and intensity that are closely tied to nighttime warming. Coastal and rural areas are less impacted but show a significant increase in heatwave frequency over the past two decades. Heatwave nighttime temperatures combined with high humidity have been increasing at a rapid rate of ~1°C/decade since the 1980s—elevating heat stress and mortality risk to vulnerable urban communities. The increased nighttime humidity is associated with an anomalous moisture source off the coast of Baja California that has intensified over the past decade and is linked to ocean warming trends and changes in the California current system. Heatwaves are starting earlier and ending later in the year for urban regions. This augments public health risks and sets the stage for more intense fall wildfires by enhancing the drying of fuels. Droughts and heatwaves are strongly linked, particularly in inland urban and rural areas that have a high statistical probability of heatwaves increasing in frequency (42%), duration (26%), and daily mean temperature (2.2%) during severe drought conditions. Better understanding of heatwave climate drivers and underlying physical processes could help with prediction skill, in addition to providing effective data‐driven recommendations for mitigation efforts in SoCal's vulnerable urban regions.

Climate change increases the frequency, intensity, and duration of heat waves. Here we demonstrate the application of satellite thermal-infrared sensing to monitor urban heat waves, observe the spatial variation of land surface temperature (LST) over the diurnal cycle, and estimate the risk of heat-related mortality for elderly people. The work is based on a time series of 84 NOAA-AVHRR satellite images sensed during the August 2003 heat wave, which caused 4867 excess deaths in Paris and 616 excess deaths in London. In both cities, the thermal images reveal: (1) contrast between the downtown heat island during the night and multiple hot spots in industrial areas during the day; (2) the contribution of the heat wave, which enhances the heat island and further delays night cooling; (3) the relationship between LST and surface land use/physical properties; and (4) the consistent cooling effect of vegetation. In Paris, the highest nighttime LSTs matched the highest excess mortality ratio. The LSTs were sampled at the addresses of 482 elderly people (deceased/controls) to produce thermal indicators that were integrated into a regression model. Results show the relative impact of heat exposure on the elderly population at given locations and the time lag between heat exposure and death. They indicate the predominance of nighttime temperature, with a 0.5°C increment that doubles the risk of death, in the temperature range of the heat-wave episode. The results underline the relevance of satellite monitoring of extreme heat events for contingency planning and public health decision making, and implementing climate change adaptation and mitigation strategies.

Climatic conditions that challenge human thermoregulatory capacity currently affect around a quarter of the world’s population annually. Such conditions are projected to increase in line with CO2 emissions particularly in the humid tropics. Climate change can increase the risk of conditions that exceed human thermoregulatory capacity1,2,3,4,5,6. Although numerous studies report increased mortality associated with extreme heat events1,2,3,4,5,6,7, quantifying the global risk of heat-related mortality remains challenging due to a lack of comparable data on heat-related deaths2,3,4,5. Here we conducted a global analysis of documented lethal heat events to identify the climatic conditions associated with human death and then quantified the current and projected occurrence of such deadly climatic conditions worldwide. We reviewed papers published between 1980 and 2014, and found 783 cases of excess human mortality associated with heat from 164 cities in 36 countries. Based on the climatic conditions of those lethal heat events, we identified a global threshold beyond which daily mean surface air temperature and relative humidity become deadly. Around 30% of the world’s population is currently exposed to climatic conditions exceeding this deadly threshold for at least 20 days a year. By 2100, this percentage is projected to increase to ∼48% under a scenario with drastic reductions of greenhouse gas emissions and ∼74% under a scenario of growing emissions. An increasing threat to human life from excess heat now seems almost inevitable, but will be greatly aggravated if greenhouse gases are not considerably reduced.

Health studies have repeatedly used air temperature (T-a), sometimes adjusted for humidity, when analyzing the impact of weather on mortality. The aim of this study is to highlight the importance of mean radiant temperature (T-mrt) and its impact on heat related mortality. T-mrt is an essential meteorological parameter that influences the thermal comfort (heat load) of humans. It is useful when assessing the impact of weather, especially heat, on people's health. T-mrt is directly influenced by urban geometry and surface material, which also makes it a good measure to identify urban hot spots. The performance of models using T-a and Tmrt for daily mortality is compared for Stockholm County, Sweden. It is demonstrated that T-mrt models fit heat related mortality better than T-a models, which implies that health studies should consider using T-mrt rather than T-a. The use of T-mrt models allows us to determine more accurate thresholds for increased risks of heat related mortality, and thus to better identify adverse weather conditions and heat prone urban geometries. Such information is needed to implement heat-warning systems and mitigate harmful effects of heat stress. (C) 2014 Elsevier B.V. All rights reserved.

Background: Heat waves have a drastic impact on urban populations, which could increase with climate change.Objectives: We evaluated new indicators of elderly people’s exposure to heat in Paris, from a public health prevention perspective, using satellite thermal images.Methods: We used a time series of 61 images from the satellites of the National Oceanic and Atmospheric Administration’s (NOAA) Advanced Very High Resolution Radiometer (AVHRR) taken from 1 to 13 August 2003 to produce thermal indicators of minimum, maximum, and mean surface temperatures and diurnal temperature amplitude, with different lags between the meteorological data and the health impact. Health data came from a case–control study involving 241 people ≥ 65 years of age who died in the city of Paris or the nearby suburban area of Val-de-Marne during the August 2003 heat wave, and 241 controls who were matched to cases on age, sex, and residential zone. For each person, we integrated the thermal indicators in a conditional logistic regression model, adjusted for age and other potential confounders. We computed odds ratios (ORs) comparing the 90th and 50th percentiles of the temperature differences between cases and controls for various indicators.Results: Mortality risk was significantly associated with exposure for two indicators: minimum temperatures averaged for 1–13 August [for a 0.41°C increase, OR = 2.17; 95% confidence interval (CI): 1.14, 4.16] and minimum temperature averaged on the day of death and the 6 preceding days (for a 0.51°C increase: OR = 2.24; 95% CI: 1.03, 4.87).Conclusions: Our results support the influence of night temperatures on the health impact of heat waves in urban areas. Urban heat exposure indicators based on satellite imagery have the potential to identify areas with higher risk of death, which could inform intervention decisions by key stakeholders.

PP-30-084 Background/Aims: Heat waves are particularly deadly in large cities where the distribution of surface heat fluxes is altered compared with natural areas. This could be reinforced by climate change. The objectives were to analyze the urban surface temperatures, and to build a new risk indicator of exposure according to the residence location. Methods: The study is based on 61 thermal images at 1-km resolution, sensed by the NOAA-AVRR satellites during the 1–13 August 2003 heat wave, and a case-control study concerning 482 persons aged 65 or more, living in the Paris region (France) at that time. For each person, minimal, maximal, and mean temperature indices were built for different periods, and integrated into a conditional logistic regression model to test their use as exposure indicator and their effect on mortality. The model was adjusted on other risk factors such as age, sex, socioeconomic conditions, autonomy, behavior of heat adaptation, health problems, housing, and geographical district. Results: The observed surface temperature amplitude ranged from 12.18°C to 45.41°C, with a median at 21.4°C at night and 34.2°C during day. The differences of surface temperatures between cases and controls ranged from −6.1°C to 8.4°C. The results of the analysis are statistically significant for minimal temperatures computed from 1st to 13th August, and for minimal temperature averaged on the period going from the day of the death to the sixth preceding day (OR of 2.57 and 2.22, respectively). Conclusion: The results confirm the significant health effect of night-time high temperature, and point out the location and time of heat islands. Such data could be used for long-term prevention, by targeting the districts where intervention is a priority. Studying the links between night temperatures and urban characteristics should help public health authorities and planning agencies to determine the better actions for reducing heat islands.

Summer warming trends in Western Europe are increasing the incidence, intensity and duration of heat waves. They are especially deadly in large cities owing to population density, physical surface properties, anthropogenic heat and pollutants. In August 2003, for 9 consecutive days, the Paris metropolitan area experienced an extreme heat wave that caused 4867 estimated heat‐related deaths. A set of 61 NOAA‐AVHRR (advanced very high‐resolution radiometer) images and one SPOT‐high resolution visible (HRV) image were used to analyse the spatial variations of land surface temperature (LST) over the diurnal cycle during the heat wave. The LST patterns were markedly different between daytime and night‐time. A heat island was centred downtown at night, whereas multiple temperature anomalies were scattered in the industrial suburbs during the day. The heat wave corresponded to elevated nocturnal LST compared to normal summers. The highest mortality ratios matched the spatial distribution of the highest night‐time LSTs, but were not related to the highest daytime LSTs. LSTs were sampled from images at the addresses of 482 elderly people (half were deceased persons and half were control ones) to produce daily and cumulative minimal, maximal and mean thermal indicators, over various periods of time. These indicators were integrated into a conditional logistic regression model to test their use as heat exposure indicators, based on risk factors. Over the period 1–13 August, thermal indicators taking into account minimum nocturnal temperatures averaged over 7 days or over the whole period were significantly linked to mortality. These results show the extent of the spatial variability in urban climate variables and the impact of night‐time temperatures on excess mortality. These results should be used to inform policy and contingency planning in relation to heat waves, and highlight the role that satellite remote sensing can play in documenting and preventing heat‐related mortality. Copyright © 2010 Royal Meteorological Society

This article was submitted without an abstract, please refer to the full-text PDF file.

Scatterometer data at 25-km resolution are used to investigate the effects of the Hawaiian and Cabo Verde islands on the mean atmospheric flow. A wake of weak winds, flanked by accelerated winds, appears for each major island of both archipelagos. The resulting wind stress curl displays dipole-like structures, with positive values on the northern side and negative values on the southern side of the lee, extending several island diameters downwind. These curl anomalies reach a magnitude of 2 10(-6) Pa(.)m(-1) and correspond to Ekman pumping velocities of 3 m(.)day(-1) for Hawaii and 4 m(.)day(-1) for Cabo Verde. They spin up cyclonic eddies on the north side and anticyclonic eddies on the south side of the lee of each island. The response of the ocean circulation is investigated using a simple Sverdrup balance. Two counter-rotating Sverdrup gyres are spun up west of the island of Hawaii and extend to the western boundary of the Pacific Ocean. They result in an eastward zonal transport confined between 19degrees and 20degreesN. East of 170degreesW, the surface expression of this transport coincides with the Hawaiian Lee Counter Current. Similar gyres are anticipated to form in the Atlantic Ocean, but remain to be observed. These results suggest that strong mesoscale patterns in the wind field occurring in the lee of high-topography features must be resolved to force global ocean circulation models.

Large variations of backscatter intensities were observed in between and within ERS-SAR images of the Los Angeles metropolitan area. Low intensities backscatter were found over smooth pavement and asphalt, and higher ones over industrial and commercial areas, indentified using a SPOT-HRV image classified for land-use. Highest backscatter anisotropy occurred when the flight direction of the SAR was parallel to the alignment of streets. These contaminations mask the backscatter variations due to surface roughness and to soil moisture, and indicate that the retrieval of moisture availability from ERS-SAR images, is contingent upon eliminating backscatter contamination from processes inherent to the urban structure.

A set of SAR images of the Los Angeles basin was analyzed to assess their potential to derive soil moisture, an important component of the surface energy balance in urban areas. Large variations of backscatter intensities were observed for different land uses and different SAR images. Low intensities were found over smooth pavement and asphalt. High intensities were found over industrial and commercial areas, with a marked illumination azimuth dependence for the latter; and maximum intensities were found when the flight direction was parallel to building alignments. A similar anisotropy occurred over residential areas, albeit at lower intensities. These contaminations mask the backscatter variations resulting from other processes, and may limit the estimation of soil moisture to undeveloped blocks and parks, unless a correction is applied

Hundreds of fires were set in Los Angeles following the verdict of Rodney G. King versus the Los Angeles Police Department on April 29, 1992. These fires were of sufficient intensity and extent to be imaged at 1‐km resolution by the Advanced Very High Resolution Radiometer (AVHRR) aboard the polar‐orbiting satellites operated by the National Oceanic and Atmospheric Administration (see VanWoert et al. [1992] for an overview of the characteristics of the AVHRR instrument). Here we present a thermal infrared image taken the first night of the riots, compare it with an image taken several years earlier but typical of this area, and discuss the relationships with the land cover observed in a 20‐m resolution image from the French satellite SPOT (Systeme Probatoire pour l'Observation de la Terre).