A heatwave is a stretch of abnormally hot days and nights where temperatures stay well above what’s typical for a given location. There’s no single universal temperature that triggers one. What counts as a heatwave in Phoenix would be different from what counts in London, because the definition is always relative to local climate norms. A heatwave can last anywhere from a few days to several weeks, and the danger comes not just from peak daytime heat but from overnight temperatures that stay elevated, preventing the body from recovering.
How Heatwaves Are Defined and Measured
The World Meteorological Organization describes a heatwave as a period where excess heat accumulates over a sequence of unusually hot days and nights. The key word is “unusually.” Each region sets its own thresholds based on historical temperature patterns, which is why a 95°F day might trigger warnings in Seattle but not in Dallas.
In the United States, the National Weather Service issues two levels of alerts. A Heat Advisory typically goes into effect when the temperature or heat index reaches 105 to 109°F. An Excessive Heat Warning kicks in at 110°F or higher. These thresholds vary by region and are periodically updated to reflect changing conditions.
Two metrics help communicate how dangerous the heat actually feels. The heat index, which most people encounter in weather forecasts, combines air temperature and humidity to estimate how hot it feels on your skin. One important detail: the heat index assumes you’re in the shade. If you’re standing in direct sunlight, conditions are significantly worse. The wet-bulb globe temperature, used more in occupational safety and military settings, factors in direct sun exposure, wind speed, and cloud cover, making it a better measure for anyone working or exercising outdoors.
Why Your Body Struggles in Extreme Heat
Your body cools itself through two main mechanisms: sweating and redirecting blood flow toward the skin. When you’re hot, blood vessels near the surface of your skin dilate, carrying heat from your core outward where it can escape. At the same time, sweat glands ramp up production, and as that sweat evaporates, it pulls heat away. Evaporation of sweat alone accounts for roughly 22% of the body’s total heat loss under normal conditions.
This system works well up to a point. When humidity is high, sweat can’t evaporate efficiently, so one of your primary cooling tools becomes much less effective. When temperatures stay elevated overnight, your body never gets a chance to fully reset. Over hours or days, this accumulated heat stress can overwhelm your thermoregulation entirely. Certain conditions that reduce or eliminate sweating are especially dangerous, because without the ability to sweat, the body essentially loses its emergency cooling system. The result is a core temperature that climbs unchecked, which can damage the brain, heart, kidneys, and muscles.
Heat Exhaustion vs. Heatstroke
Heat exhaustion is the warning stage. You might feel heavy fatigue, nausea, dizziness, or a headache. Your skin may feel cool and clammy despite the heat. At this point, moving to a cooler environment, drinking water, and resting can usually reverse the symptoms.
Heatstroke is a medical emergency. The defining sign is a core body temperature of 104°F (40°C) or higher, paired with changes in mental state: confusion, slurred speech, agitation, irritability, or even seizures and loss of consciousness. Heatstroke can cause permanent organ damage quickly, and the longer treatment is delayed, the worse the outcome. If someone shows these signs, the priority is cooling them as fast as possible while waiting for emergency help. Cold-water immersion, where the person is placed in ice water, is the most effective method. If that’s not possible, spraying their skin with cool water while fanning them, or applying ice packs to the neck, armpits, and groin, can help bring the temperature down.
Why Cities Get Hotter
Urban areas amplify heatwaves through what’s known as the urban heat island effect. Concrete, asphalt, and buildings absorb solar energy during the day and release it slowly after sundown. According to the U.S. Environmental Protection Agency, daytime temperatures in cities run 1 to 6°F higher than surrounding rural areas on average. The nighttime gap is far more dramatic: urban areas can be as much as 22°F warmer than nearby countryside after dark. That nighttime heat is what makes urban heatwaves so dangerous, because it eliminates the cooling relief that would normally come with sunset.
Who Faces the Greatest Risk
Older adults, young children, and people with chronic health conditions are most vulnerable during heatwaves. But one of the less obvious risk factors is medication. Several common drug classes interfere with the body’s ability to handle heat. Blood pressure medications like diuretics can cause dehydration and electrolyte imbalances. Beta-blockers reduce sweating and limit the blood vessels’ ability to dilate. Antipsychotic medications impair both sweating and the brain’s temperature-sensing ability. Even widely used drugs like antihistamines with anticholinergic properties (the kind that cause drowsiness, like diphenhydramine) decrease sweating and disrupt thermoregulation.
Antidepressants can go either direction. Some increase sweating, potentially accelerating fluid loss, while others, like tricyclic antidepressants, decrease sweating and trap heat. People taking anti-seizure medications, lithium, or common pain relievers like NSAIDs also face elevated risk, particularly if they become dehydrated. If you take any of these medications regularly, you need to be more cautious about heat exposure than the average person.
Staying Safe During a Heatwave
Hydration is the single most important thing you can control. OSHA recommends drinking about one cup (8 ounces) of water every 15 to 20 minutes during heat exposure, which works out to roughly 32 ounces per hour. Don’t wait until you feel thirsty, as thirst is a lagging indicator that your body is already behind on fluids. There is an upper limit, though: drinking more than 48 ounces per hour can dangerously dilute the sodium in your blood, a condition that itself requires emergency treatment.
Beyond hydration, the basics matter more than people expect. Staying indoors during peak afternoon heat, using fans and air conditioning, wearing lightweight and light-colored clothing, and avoiding strenuous outdoor activity during the hottest hours all reduce your heat load. If your home doesn’t have air conditioning, spending even a few hours in a cooled public space like a library or shopping center can make a meaningful difference in how your body copes over a multi-day heatwave. The goal isn’t just comfort. It’s giving your body enough of a break to keep its cooling systems functional.