Heat wave: definition, causes, impacts, and response

Causes and how they are measured

Heat waves commonly form when a persistent high-pressure system stalls over a region, reducing cloud cover and trapping warm air — a pattern sometimes described as a heat dome. Urban heat islands, drought, and reduced soil moisture can intensify warming. Climate change has increased the frequency and severity of many heat waves, shifting baseline temperatures upward. Researchers measure heat waves using absolute thresholds (e.g., consecutive days above a set temperature) or relative thresholds (e.g., days above the 90th or 95th percentile for that location). See regional criteria at regional definitions and discussion of humidity effects at heat index resources.

Impacts

Prolonged extreme heat affects public health, ecosystems, agriculture and infrastructure. Health risks include heat exhaustion, heatstroke, and exacerbation of chronic conditions, especially among older adults, infants, outdoor workers and those without adequate cooling. Infrastructure impacts include higher electricity demand, stress on power grids, road and rail damage, and increased wildfire risk. Historic events have demonstrated how severe heat can strain emergency services and food systems; see examples and case studies at case studies.

Adaptation and response

Responses combine short-term emergency actions and long-term planning. Emergency measures include heat-health warnings, cooling centers, public outreach on hydration and shaded rest, and protective rules for outdoor labor. Longer-term strategies cover urban design (more green space, cool roofs), building standards for passive cooling, resilient power systems, and early-warning systems. Policy and community measures are described in practical guides and planning documents such as adaptation guides.

Distinctions and notable facts

• Heat wave vs. heat burst: a heat burst is a short, abrupt warming event often at night; heat waves are prolonged.
• Wet-bulb temperature and heat index account for humidity's role in human heat stress.
• Because definitions vary, comparisons of heat-wave frequency across regions require care.

Understanding local thresholds, preparing vulnerable populations, and integrating cooling into urban planning are key to reducing the health and economic toll of heat waves as global temperatures continue to change.