Urban structure and street canyon geometry in the microscale urban climate zone greatly change the urban surface thermal environment, increasing heat storage 21. Additionally, while rural temperatures might be lower than urban they do not always result in decreased heat stress during heatwaves 20. While UHI is used to quantify urban heat it does not necessarily quantify the thermal comfort 19. The UHI effect refers to the temperature differences between urban and rural areas caused by a lack of vegetation and increased man-made in highly urbanized areas 18. In addition to heatwaves, urban heat is one of the most severe heat-related issues urban residents face, often quantified as an urban heat island (UHI). Heatwave formation and duration are complex in the western U.S., especially over coastal and mountain areas where heatwave characteristics are strongly affected by marine meteorology and mountainous terrain 16, 17. Heatwaves and their response to vegetation changes were studied in London (U.K.) and central France where heatwave intensity was exacerbated due to vegetation losses 14, 15. For example, a connection between cyclones/anticyclones and heatwaves was found in Melbourne (Australia), and reinforcing effects were found as a result of the pressure dipole 13. revealed the dynamic mechanisms and synoptic responses that lead to intense heatwaves. Studies in Australia, Europe and the U.S. Many studies have been conducted to scientifically investigate the formation of this natural hazard, especially under the changing climate. Heatwaves have been attributed to 0.3–0.5% of the European gross domestic product damages and will become five times worse by 2060 if no actions are taken to mitigate climate change impacts 12. It is anticipated that global heat-related mortality will increase from 92,207 to 255,486 from 2030 to 2050 due to climate change 11. At least one-third of the heat-associated mortalities in the past decades have been attributed to climate change 9, 10. Excessive heat stress subsequently induces additional risks to both human health and ecosystems 4, 5, 6, 7, 8. Global climate change is enhancing the intensity and frequency of extreme weather events including heatwaves, which are natural hazards with prolonged periods of excessive heat 1, 2, 3. Our findings demonstrate the importance of scale interactions impacting extreme heat and the need for holistic approaches in heat mitigation strategies. We discuss the temperature impacts associated with processes across scales: climate or long-term change, the El Niño–Southern Oscillation, synoptic high-pressure systems, mesoscale ocean/lake breezes, and urban climate (i.e., urban heat islands). In 2021, daytime maximum temperatures during heat events in eight major cities were 10–20 ☌ higher than the 10-year average maximum temperature. We show the atmospheric scale interactions and spatiotemporal dynamics that contribute to increased temperatures across the region for both urban and rural environments. Here we investigate the extreme heatwaves in the western U.S. The urban environment can also exacerbate heat stress because of man-made materials and increased population density. They result in increased heat stress to populations causing human health impacts and heat-related deaths. Extreme heat events are occurring more frequently and with greater intensity due to climate change.
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