Definitions of heat wave focused on temperatures that exceed a specific threshold?
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Definitions of Heat Waves Focused on Temperatures Exceeding Specific Thresholds
Introduction to Heat Wave Definitions
Heat waves are acute periods of extreme warmth that significantly impact human health, activities, and natural environments. However, defining a heat wave is complex due to the variability in threshold values, duration, and ancillary variables used by different researchers and meteorological agencies . This article synthesizes various approaches to defining heat waves, focusing on temperature thresholds.
Temperature Thresholds in Heat Wave Definitions
Percentile-Based Thresholds
One common method to define heat waves is using percentile-based thresholds. For instance, heat waves can be defined by daily mean temperatures exceeding the 90th, 92.5th, 95th, or 97.5th percentiles for durations of two or more consecutive days. This approach allows for community-specific definitions, accommodating local climate variations. Studies have shown that higher temperature thresholds correlate with stronger associations between heat waves and mortality.
Absolute Temperature Thresholds
Another approach involves setting absolute temperature thresholds. For example, in a multi-city study in China, heat waves were defined as periods when daily mean temperatures exceeded specific values, such as 30.2°C in Beijing and 34.5°C in Changsha, for two or more consecutive days. This method provides clear, location-specific criteria that can be easily communicated to the public.
Combined Temperature and Humidity Indices
Some definitions incorporate both temperature and humidity to better represent the thermal stress experienced by humans. The National Weather Service (NWS) in the United States uses the heat index, which combines ambient temperature and humidity, to issue heat wave warnings when thresholds are exceeded for at least two consecutive days. Similarly, the wet-bulb globe temperature (WBGT) index, which also accounts for humidity, has been shown to identify longer heat wave durations and higher health risks compared to air temperature alone.
Variability in Heat Wave Definitions
Moving Reference Periods
The selection of reference periods can significantly affect the reported frequency of heat waves. Using a moving reference period, which adjusts the temperature threshold for each calendar date, can provide a more accurate assessment of thermal discomfort and better capture anomalies. This method has been shown to identify heat waves throughout the year, not just in the summer months.
Compound Heat Waves
Heat waves can also have a compound structure, featuring sequences of hot days interspersed with cooler breaks. These compound events are expected to become more common with global warming and pose additional health risks. Definitions that account for these breaks can provide a more comprehensive understanding of heat wave hazards.
Impact on Health Outcomes
The choice of heat wave definition can significantly influence the assessment of health risks. Studies have shown that definitions based on higher temperature thresholds or combined indices like WBGT result in stronger associations with adverse health outcomes, such as increased mortality and hospitalization rates . Therefore, selecting appropriate thresholds is crucial for effective heat wave warnings and public health interventions.
Conclusion
Defining heat waves using specific temperature thresholds is essential for understanding their impacts and implementing effective response strategies. While percentile-based and absolute temperature thresholds are commonly used, incorporating humidity and considering compound heat wave structures can provide a more accurate assessment of health risks. Harmonizing these definitions internationally could improve the comparability of studies and the effectiveness of heat wave warnings.
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