Heat wave
A heat wave is a period of unusually hot weather that lasts long enough to strain bodies, buildings, power systems, roads, farms, and ecosystems. The danger depends on local climate, humidity, nighttime temperatures, exposure, health, housing, and access to cooling, which is why the same temperature can be routine in one place and hazardous in another.
What a heat wave is
A heat wave is not defined by one universal temperature. It means a stretch of unusually hot weather for a particular location and time of year, usually lasting two or more days. Local thresholds matter because people, buildings, crops, and infrastructure are adapted to different normal climates. A temperature that is ordinary in one region may be dangerous in another.
Why duration matters
One very hot afternoon can be risky, but several hot days in a row create a different problem. Buildings store heat, roads and rails expand, electricity demand rises, and people have less time to recover. Warm nights are especially important because they prevent bodies and homes from cooling down before the next day begins.
Heat index and humidity
Air temperature alone does not describe heat stress. Humidity slows sweat evaporation, so the body loses one of its main cooling tools. Heat index combines temperature and humidity to estimate how hot conditions feel in the shade. Direct sun, low wind, heavy clothing, hard work, and radiant heat from pavement or machinery can make actual exposure worse.
What happens in the body
The human body tries to keep its core temperature within a narrow range by moving blood toward the skin and sweating. During extreme heat, dehydration, heart strain, medications, alcohol, illness, age, pregnancy, and disability can reduce the body's ability to cope. Heat exhaustion can progress to heat stroke, a medical emergency in which the body can no longer regulate temperature.
Who is most exposed
Anyone can be harmed by extreme heat, but risk is uneven. Older adults, infants and children, pregnant people, outdoor workers, athletes, people with chronic conditions, people without air conditioning, people experiencing homelessness, and socially isolated residents face higher danger. Low-income neighborhoods and historically marginalized communities often have fewer trees, more pavement, and less access to cooling.
Cities and heat islands
Urban heat islands can make heat waves more intense and longer-lasting in cities. Asphalt, dark roofs, concrete, dense buildings, traffic, and waste heat store and release energy, while limited vegetation reduces shade and evaporative cooling. Cool roofs, green roofs, trees, parks, reflective pavements, and shaded streets can lower local temperatures when they are planned and maintained well.
Infrastructure strain
Heat waves stress more than human health. Power demand can surge as air conditioners run, raising outage risk if grids are already strained. Rail tracks, roads, bridges, water systems, vehicles, and communication equipment can all be affected by high temperatures. Farms and ecosystems may face heat stress, drought interactions, water demand spikes, and wildfire risk.
Warnings and response
Heat response plans use forecasts, health data, local thresholds, and public communication to warn people before conditions become dangerous. Common actions include opening cooling centers, checking on high-risk neighbors, shifting outdoor work hours, providing shade and water, reducing electricity load, and sharing clear instructions about heat illness symptoms and when to seek help.
Why it matters
Heat waves are sometimes treated as ordinary summer weather, but they can be deadly because they build quietly across days and nights. Climate change increases the odds of more severe heat, while urban design and inequality decide who feels the worst impacts. Good heat planning can save lives quickly, and long-term cooling strategies can make neighborhoods safer before the next event arrives.