A heat wave is a period of unusually hot weather that lasts for several days or longer relative to the typical conditions in a particular location. It can develop during summer in any part of the United States, from the humid Southeast to the dry Southwest and the normally cooler northern states. The exact definition varies by region because temperatures considered extreme in one area may be common in another.
Heat waves are more than a temporary inconvenience. Prolonged extreme heat can strain the human body’s cooling system, worsen existing health conditions, damage crops, increase energy demand, and place pressure on water supplies and public infrastructure. Understanding how heat waves form, why they are dangerous, and how to respond to them can help people protect themselves and their communities.
What defines a heat wave?
There is no single temperature threshold that defines every heat wave. Meteorologists and public health agencies consider factors such as local climate, how far temperatures exceed normal levels, how long the heat persists, and whether nighttime temperatures remain unusually high.
A temperature that is manageable in a region accustomed to hot summers may pose a greater risk in a place where residents, homes, and infrastructure are less adapted to extreme heat. For this reason, heat waves are assessed relative to local conditions rather than by using one nationwide temperature cutoff.
Duration matters as much as peak temperature. A single hot afternoon may allow people to recover overnight, but several consecutive days of extreme heat can cause the body to accumulate heat stress. When nights remain warm, the body has fewer opportunities to cool down, and buildings may retain heat well into the following day.
Humidity also affects how dangerous a heat wave becomes. Air temperature alone does not fully describe the conditions a person experiences outdoors. High humidity can make it substantially harder for the body to release excess heat, increasing the risk of heat-related illness even when the air temperature is lower than it would be during a dry heat wave.
How heat waves form
Heat waves typically develop when atmospheric conditions allow heat to accumulate over a broad area for an extended period. Several processes can contribute, and more than one may operate at the same time.
Persistent high-pressure systems
One of the most common causes is a strong, slow-moving high-pressure system. High pressure is associated with sinking air, which tends to warm as it descends and compresses. This process can discourage cloud formation and help produce clear skies.
With fewer clouds to reflect or block incoming sunlight, the ground absorbs more solar energy during the day. The heated land then transfers energy to the air above it, raising temperatures near the surface. If the high-pressure system remains in place, this cycle can continue for days.
These systems can also redirect weather patterns around them, limiting the arrival of cooler air. As a result, a region may experience persistent sunshine, weak winds, and little relief from passing weather systems.
Soil moisture and dry conditions
The amount of moisture in the soil influences how energy from sunlight is distributed. When the ground is moist, some of the available energy goes toward evaporating water. This process, known as evaporative cooling, limits how much the surface warms.
When soils become dry, less energy is used for evaporation. More energy can instead heat the ground and the air immediately above it. This can intensify hot conditions, particularly in areas experiencing drought.
The relationship can become self-reinforcing: hot weather dries out the soil, and dry soil then contributes to higher temperatures. However, the strength of this effect depends on local conditions, vegetation, soil properties, and the availability of moisture.
Warm air masses and regional weather patterns
Heat waves can also occur when warm air spreads into an area from another region. Winds may transport hot air from deserts or other warm environments, while persistent atmospheric circulation patterns can keep that air in place.
In some situations, winds descending from elevated terrain warm as they move toward lower elevations. These downslope winds can produce unusually high temperatures in nearby valleys and plains.
Local geography affects the severity of a heat wave. Elevation, distance from the ocean, prevailing winds, vegetation, and urban development can all influence how hot a particular area becomes.
Climate change and increasing heat extremes
Human-caused climate change is making heat waves more frequent and intense in many parts of the world. The primary mechanism is the increase in greenhouse gases, such as carbon dioxide and methane, which trap more heat in the climate system.
As the average global temperature rises, the range of temperatures experienced in many regions shifts toward warmer conditions. An event that was once exceptionally hot can therefore become more common, while the hottest events can reach even higher temperatures.
Climate change does not mean every heat wave has the same cause or that every location warms at the same rate. Individual events still depend on atmospheric circulation, soil moisture, geography, and other factors. Natural climate variability also influences year-to-year conditions.
The important distinction is that climate change raises the baseline on which these weather patterns occur. When a persistent high-pressure system produces hot weather in an already warmer climate, the resulting heat wave can be more severe than a similar weather pattern would have produced in the past.
Why heat waves are becoming a major public health concern
Extreme heat affects health because the human body must maintain a relatively stable internal temperature despite changes in the surrounding environment. Under ordinary conditions, several physiological processes help regulate body temperature. During prolonged extreme heat, those processes may become insufficient, especially when humidity is high, physical activity is strenuous, or access to cooling is limited.
Heat also affects people differently. Age, health, medications, fitness, acclimatization, housing conditions, and access to water or air conditioning can influence a person’s ability to tolerate high temperatures. Even people who are generally healthy can become seriously ill when exposure is intense enough.
Heat-related illness is not limited to outdoor workers or people spending time in direct sunlight. Indoor temperatures can become dangerous when buildings absorb and retain heat, ventilation is inadequate, or air conditioning is unavailable. People who spend most of their time indoors may therefore face substantial risks during a prolonged heat wave.
How the body responds to extreme heat
The body maintains its internal temperature through thermoregulation, the process of balancing heat production, heat gain, and heat loss. When the surrounding environment becomes hot, the body relies primarily on two mechanisms: increasing blood flow to the skin and producing sweat.
Blood vessels near the skin widen, a process called vasodilation. This allows more warm blood to reach the body’s surface, where heat can transfer to the surrounding air. The heart must work harder to circulate blood while maintaining adequate blood flow to vital organs.
Sweating provides another way to lose heat. As sweat evaporates from the skin, it carries energy away from the body. Evaporation is most effective when the surrounding air is relatively dry and moving. In humid conditions, the air already contains substantial water vapor, so sweat evaporates more slowly.
When evaporation becomes inefficient, sweat may drip from the skin without providing much cooling. The body can continue losing water and electrolytes, which are minerals involved in fluid balance, nerve signaling, and muscle function.
If heat exposure continues, dehydration and cardiovascular strain can worsen. The body may struggle to maintain blood pressure and deliver enough oxygenated blood to the brain, muscles, and other organs. At the most severe end of the spectrum, internal temperature regulation can fail, resulting in heat stroke and potentially life-threatening organ damage.
The role of the heat index
The heat index estimates how hot conditions feel to the human body by combining air temperature and relative humidity. Relative humidity describes how much water vapor is in the air compared with the maximum amount the air can hold at that temperature.
At high humidity, sweat evaporates less efficiently, making it harder for the body to cool itself. Consequently, a humid day can create greater heat stress than a dry day at the same air temperature.
The heat index is useful, but it has limitations. It generally represents shaded conditions with light winds and may not fully capture the additional heat experienced in direct sunlight, during strenuous activity, or near hot surfaces. Wind, clothing, physical condition, and the duration of exposure also influence actual risk.
Another important measure is the wet-bulb temperature, which reflects the combined effects of heat and moisture by considering cooling through evaporation. It helps scientists assess how effectively evaporation can cool a surface, including the human body.
No single environmental measurement predicts exactly when a person will become dangerously overheated. Individual vulnerability and the intensity and duration of exposure matter. Public heat alerts, local forecasts, and symptoms should therefore be considered together rather than relying on one number alone.
Health risks associated with heat waves
Extreme heat can affect nearly every major system in the body. The severity of illness depends on the conditions, the person’s health, and how quickly the person can reduce exposure and receive appropriate care.
Dehydration and heat cramps
Dehydration occurs when the body loses more water than it takes in. During a heat wave, this can happen through sweating, particularly during physical activity or prolonged outdoor exposure.
Early symptoms may include thirst, dry mouth, headache, fatigue, dizziness, and reduced urination. Dehydration can also contribute to weakness and make it more difficult for the body to regulate temperature.
Heavy sweating can cause the loss of electrolytes as well as water. This may contribute to painful muscle cramps, often called heat cramps, during or after physical exertion in hot conditions.
Moving to a cooler place, resting, and drinking fluids can help address mild dehydration and heat cramps. People with medical conditions that require fluid restriction should follow their clinician’s guidance rather than increasing fluid intake without considering those restrictions.
Heat exhaustion
Heat exhaustion develops when the body experiences substantial heat stress and loses water and salts through sweating. It may occur after prolonged exposure to high temperatures, particularly during exercise, manual labor, or other physically demanding activities.
Symptoms can include heavy sweating, weakness, dizziness, headache, nausea, vomiting, muscle cramps, a rapid pulse, and cool or clammy skin. A person may feel faint or have difficulty continuing normal activities.
Heat exhaustion requires prompt attention. Move the affected person to an air-conditioned or shaded area, loosen unnecessary clothing, and cool the body with cool water, wet cloths, or fans. If the person is fully alert and able to swallow safely, offer cool water or an appropriate electrolyte drink.
Seek medical care if symptoms worsen, vomiting prevents drinking, symptoms persist despite cooling, or there is uncertainty about the severity of the illness. Heat exhaustion can progress to a medical emergency, so it should never be ignored.
Heat stroke
Heat stroke is the most dangerous heat-related illness. It occurs when the body’s temperature regulation fails and internal temperature rises to a dangerous level, threatening the brain and other vital organs.
Warning signs may include confusion, unusual behavior, slurred speech, seizures, loss of consciousness, or collapse. The skin may be hot, but heavy sweating can still occur, so dry skin is not a necessary sign of heat stroke.
Heat stroke is a medical emergency. Call 911 immediately in the United States and begin rapid cooling while waiting for emergency services. Move the person to a cooler location, remove excess clothing, and use the fastest practical cooling method available. Cold-water immersion can be especially effective when it can be performed safely. Wetting the skin and using fans, applying cool wet cloths, or placing cold packs near the neck, armpits, and groin can also help.
Do not give fluids to a person who is confused, unconscious, having seizures, or unable to swallow safely. Do not delay emergency care while trying home treatments or waiting for the person’s temperature to return to normal.
Heat stroke can cause permanent injury or death without prompt treatment. Early recognition and rapid cooling are critical.
Cardiovascular and respiratory strain
Hot weather places additional demands on the cardiovascular system. The heart must circulate more blood toward the skin to release heat, while sweating reduces the body’s fluid volume. Together, these changes can increase strain on the heart and make it harder to maintain stable blood pressure.
People with heart disease, high blood pressure, or other cardiovascular conditions may be particularly vulnerable. Extreme heat can also aggravate some respiratory conditions, especially when hot weather coincides with air pollution or wildfire smoke.
Heat can contribute to kidney stress as well. Reduced fluid volume and prolonged dehydration can impair kidney function, particularly in people who already have kidney disease or who perform strenuous work in hot environments.
Effects on sleep, concentration, and mental well-being
High nighttime temperatures can disrupt sleep because the body needs to release heat to support normal sleep processes. When bedrooms remain hot, people may have difficulty falling asleep, wake frequently, or experience less restorative rest.
Poor sleep can affect concentration, reaction time, mood, and decision-making. These effects may increase the likelihood of accidents and make it harder for people to recognize or respond appropriately to heat-related symptoms.
Prolonged heat exposure can also intensify discomfort, irritability, and psychological stress. People experiencing displacement, power outages, or concerns about vulnerable family members may face additional emotional strain during severe heat events.
Who is most vulnerable during a heat wave?
Extreme heat can affect anyone, but some groups face a greater risk of illness or death.
Older adults may have a reduced ability to regulate body temperature, recognize thirst, or respond quickly to changing conditions. Some older people also have chronic illnesses or take medications that affect fluid balance, circulation, or sweating.
Infants and young children depend on adults to provide fluids, suitable clothing, and a cool environment. Their vulnerability makes supervision particularly important during hot weather. Children should never be left unattended in a parked vehicle, even briefly.
People with chronic medical conditions may be more susceptible to heat-related complications. Conditions affecting the heart, kidneys, lungs, or nervous system can interfere with the body’s ability to tolerate heat. Certain medications can also alter sweating, blood pressure, alertness, or fluid regulation. People should not stop or change prescribed medications on their own; a clinician or pharmacist can advise them about heat-related precautions.
Outdoor workers and people who exercise in the heat may accumulate substantial body heat through physical activity. Construction workers, agricultural workers, delivery personnel, emergency responders, and others who work outdoors may have prolonged exposure with limited opportunities to cool down.
People without reliable air conditioning or safe housing can face elevated risks even if they do not spend much time outdoors. Poor insulation, limited ventilation, crowded housing, high energy costs, and power outages can prevent indoor spaces from cooling adequately.
People experiencing homelessness may have limited access to shade, drinking water, sanitation, and cooling facilities. People who live alone or have limited mobility may also be less likely to receive timely help when symptoms develop.
Recognizing these differences is essential for public health planning. Effective heat protection depends not only on individual behavior but also on access to safe housing, cooling, health care, and community support.
How heat waves affect cities and the environment
Heat waves can produce consequences beyond immediate health effects. They influence electricity use, transportation, agriculture, ecosystems, and water availability, often placing several systems under stress at once.
The urban heat island effect
Cities often remain warmer than surrounding rural areas, particularly at night. This pattern is known as the urban heat island effect.
Roads, rooftops, parking lots, and other built surfaces absorb and store solar energy. Many urban materials release that heat slowly after sunset, while buildings and vehicles also add waste heat to the local environment. Cities may have fewer trees and less vegetation than nearby natural areas, reducing shade and evaporative cooling.
The urban heat island effect can intensify an existing heat wave, making it harder for residents to recover overnight. Neighborhoods with limited tree cover, extensive paved surfaces, and poor-quality housing may experience especially high temperatures.
Planting trees where appropriate, expanding shaded public spaces, using reflective roofing materials, and designing buildings to limit unwanted heat gain can help reduce some of these effects. The effectiveness of each measure depends on local climate, available water, building design, and neighborhood conditions.
Electricity demand and infrastructure
During a heat wave, air conditioners and other cooling systems run more frequently, increasing electricity demand. If demand approaches or exceeds the available capacity of the power system, utilities may face operational challenges, and outages can become especially dangerous when cooling is urgently needed.
Extreme temperatures can also reduce the efficiency of some electrical equipment and increase stress on transmission and distribution systems. Roads and rail infrastructure may experience heat-related deformation or operating restrictions, while some industrial and commercial activities may need to reduce operations for safety.
These risks highlight the importance of reliable electricity, resilient infrastructure, backup plans for critical facilities, and accessible cooling centers during severe heat events.
Agriculture and water supplies
High temperatures increase evaporation from soil and water surfaces and can accelerate water loss from plants. When heat coincides with drought, crops may struggle to obtain enough moisture to maintain normal growth.
Extreme heat can interfere with flowering, pollination, and grain development in some crops. It can also reduce livestock productivity and increase the need for drinking water and shade for animals.
Heat waves may place additional pressure on reservoirs, rivers, and groundwater supplies, particularly in areas already experiencing drought. The degree of impact depends on the timing of the heat, the crop or ecosystem involved, available irrigation, and the amount of moisture present before the event.
Air quality and wildfire conditions
Heat waves can worsen air quality in several ways. Sunlight and high temperatures can accelerate the chemical reactions that produce ground-level ozone, a pollutant that irritates the lungs. The severity of ozone pollution also depends on emissions, atmospheric chemistry, and weather patterns.
Hot, dry conditions can increase the risk of wildfire ignition and support the rapid spread of fires when fuels are dry and other conditions are favorable. Wildfires can then release smoke containing fine particles that travel long distances and harm respiratory and cardiovascular health.
Heat and poor air quality can therefore occur together, creating additional challenges for people with asthma, heart disease, and other health conditions. During such events, outdoor activity decisions should account for both heat conditions and air quality.
How to stay safe during a heat wave
The most effective way to prevent heat-related illness is to reduce exposure, support the body’s cooling mechanisms, and recognize symptoms early. Preparation is especially important when forecasts indicate that extreme temperatures will persist for several days.
Stay in a cool environment
Spend as much time as possible in an air-conditioned location during the hottest periods. If air conditioning is unavailable at home, consider a public library, shopping center, community facility, or designated cooling center that is open and accessible.
Fans can improve comfort by increasing air movement, but they are not equally effective under all conditions. When air temperatures become extremely high, fans alone may not adequately prevent dangerous heat buildup, particularly when humidity is high. A cooler indoor environment is preferable when available.
Close curtains or blinds exposed to direct sunlight during the day. Limit the use of ovens and other appliances that generate substantial indoor heat. If outdoor air is cooler than indoor air, ventilation may help remove accumulated heat, provided outdoor air quality is acceptable.
During a power outage, identify alternative cooling locations in advance. If your home becomes dangerously hot, do not assume that opening windows or using a fan will be sufficient. Seek a safer, cooler place when possible.
Drink fluids regularly
Drink water throughout the day rather than waiting until you feel very thirsty. Fluid needs vary with body size, activity, temperature, health, and sweating, so there is no single amount that is appropriate for everyone.
During prolonged sweating or strenuous activity, drinks containing electrolytes may help replace salts lost through sweat. Regular meals can also supply electrolytes. Avoid relying on alcohol as a source of hydration, and be cautious with drinks containing substantial caffeine if they interfere with sleep, fluid intake, or other health needs.
People with heart failure, advanced kidney disease, or other conditions requiring fluid restrictions should follow individualized medical advice. Anyone who becomes confused, collapses, or shows signs of heat stroke needs emergency assistance rather than simply being encouraged to drink more water.
Adjust outdoor activities
Whenever possible, schedule exercise, yardwork, and other strenuous tasks for cooler parts of the day. Early morning is often cooler than afternoon, although local conditions vary and nighttime temperatures may remain high.
Reduce the intensity and duration of physical activity during extreme heat. Take frequent breaks in a cool or shaded location, drink fluids regularly, and use a gradual adjustment period when beginning outdoor work in hot weather. This process, known as heat acclimatization, allows the body to adapt to repeated heat exposure over time, but it does not eliminate the risk of heat illness.
Wear lightweight, breathable clothing and a wide-brimmed hat when appropriate. Use sunscreen to reduce sunburn risk, remembering that sunscreen does not prevent the body’s internal temperature from rising.
Workers and supervisors should take heat exposure seriously. Access to water, rest breaks, shade or cooling areas, and procedures for responding to symptoms can reduce risk. New workers and those returning after time away from hot conditions may require especially careful monitoring.
Use water and other cooling methods
A cool shower or bath, wet cloths, and water applied to the skin can help remove body heat. Moving to a cooler environment remains important, because cooling the skin may be insufficient if the surrounding air and indoor space remain excessively hot.
If someone feels overheated, stop physical activity, move out of the sun, loosen unnecessary clothing, and begin cooling promptly. Do not wait for severe symptoms to appear.
For suspected heat stroke, call 911 and begin rapid cooling immediately. A person who is confused, unconscious, or having seizures should not be given anything to drink.
Protect children, older adults, and pets
Check on older relatives, neighbors, and people who may have difficulty accessing cooling. Do not assume that someone will ask for help if they become unwell. A phone call or in-person visit can help identify unsafe indoor conditions or early signs of illness.
Never leave a child or another person unattended in a parked vehicle. Temperatures inside vehicles can rise rapidly, even when the outside air does not feel exceptionally hot and even when a window is partly open.
Pets also need access to shade, fresh water, and a cool environment. Avoid strenuous exercise with animals during hot periods, and remember that pavement can become hot enough to injure their paws. Never leave pets unattended in a parked vehicle.
Monitor forecasts and heat alerts
Pay attention to local weather forecasts and official heat advisories or warnings. These alerts account for regional conditions and can indicate when the risk of heat-related illness is elevated.
Plan ahead if a heat wave is expected to last several days. Make sure drinking water is available, cooling arrangements are in place, essential medications are accessible, and there is a plan for power outages or transportation difficulties.
People who work outdoors, exercise regularly, or care for vulnerable family members should take preventive action before temperatures peak. Heat illness can develop quickly, and waiting until someone feels seriously unwell can delay necessary treatment.
How communities can reduce heat-related harm
Individual precautions are important, but communities can also reduce the health effects of extreme heat through planning and infrastructure improvements.
Early-warning systems help residents prepare before dangerous conditions arrive. Public agencies can communicate heat risks, extend access to cooling centers, coordinate outreach to vulnerable residents, and ensure that emergency services are prepared for increased demand.
Urban planning can reduce heat exposure through trees, shade structures, reflective surfaces, and building designs that limit heat absorption. Green spaces can support cooling through shade and evapotranspiration, the release of water vapor from plants and soil. These benefits depend on suitable plant selection, maintenance, and adequate water availability.
Building improvements can also make a difference. Effective insulation, external shading, reflective roofing, and well-designed ventilation can reduce indoor heat gain. However, ventilation strategies must account for outdoor temperatures and air quality, since bringing in very hot or polluted air may worsen indoor conditions.
Reliable electricity is particularly important for households that depend on air conditioning or powered medical equipment. Emergency planning should consider residents who cannot easily travel, people with disabilities, those without stable housing, and households unable to afford high cooling costs.
Workplace protections, access to drinking water, public education, and coordination among health departments, utilities, employers, and emergency services can further reduce preventable harm. As extreme heat becomes more challenging in many regions, effective responses require both immediate safety measures and long-term preparation.
Understanding heat waves in a changing climate
Heat waves result from the interaction of atmospheric conditions, land surfaces, moisture, and regional geography. Persistent high-pressure systems often help initiate or maintain them, while dry soils, warm air masses, and urban heat island effects can intensify local temperatures. Human-caused climate change increases the likelihood and severity of many heat extremes by raising the underlying temperature of the climate system.
Their consequences extend well beyond discomfort. Extreme heat can cause dehydration, heat exhaustion, heat stroke, cardiovascular strain, and other serious health problems. It can also disrupt electricity supplies, damage crops, worsen air quality, and place vulnerable communities under considerable pressure.
The central principle of heat safety is straightforward: reduce exposure before the body becomes overwhelmed. Stay in a cool environment when possible, drink fluids appropriate to your health needs, adjust physical activity, monitor weather alerts, and seek help promptly when symptoms develop. Recognizing heat as a serious environmental hazard—and preparing for it at home, at work, and across communities—can prevent illness and save lives.
