What Is a Heat Wave? Causes, Heat Domes, Risks, and Safety

A temperature that is routine in Phoenix could set records in Seattle. That is why no single number defines every heat wave. The useful question is not only “How hot will it get?” but also “How unusual is this heat here, how long will it last, and will people and buildings cool at night?”

How is a heat wave defined?

Meteorological agencies use thresholds suited to their regions and responsibilities. Broadly, the term describes unusually hot conditions that persist for two or more days. Some definitions use daily maximum temperature, some incorporate minimum temperature or humidity, and warning systems may consider health outcomes and local acclimatization.

The National Weather Service issues heat products based on expected risk, but exact criteria vary among forecast offices. That local approach makes sense: residents, infrastructure, vegetation, and typical summer conditions are different across the country. A formal watch or warning is therefore more useful than applying a national rule of thumb.

Climate scientists compare an event with a historical distribution for the place and time of year. Public-health officials also care about duration and nighttime relief. A three-day spell that never lets homes cool can create more cumulative stress than one isolated, slightly hotter afternoon.

What causes a heat wave?

Many heat waves develop beneath a persistent area of high pressure in the middle and upper atmosphere. Sinking air warms and dries as it is compressed. The pattern suppresses clouds and rain, allowing strong sunshine to heat the ground day after day. Light winds and a slow-moving circulation can keep the same region under the hot air mass.

The popular phrase heat dome describes this broad setup: high pressure aloft helps trap and maintain hot conditions below. It is a useful picture, but not a literal lid. The atmosphere is moving in three dimensions, and individual events reflect the position of ridges, jet-stream waves, soil moisture, ocean temperatures, and air flowing from surrounding regions.

Dry ground can intensify heat because less incoming energy is used to evaporate water and more goes into sensible heating. Cities add the urban heat-island effect: dark roofs and pavement absorb solar energy, limited vegetation reduces evaporative cooling, and buildings release stored heat after sunset. Neighborhood-level temperatures can therefore differ significantly within one forecast zone.

Why humidity and warm nights matter

The body cools itself partly through sweat evaporation. High humidity slows that evaporation, raising the heat index and making activity more stressful. In dry heat, sweat may evaporate so quickly that a person does not notice how much fluid is being lost. Both environments can be dangerous.

Warm nights remove a critical recovery period. People without effective air conditioning may carry heat stress from one day into the next, while buildings and pavement release stored energy overnight. Minimum temperature is therefore one of the most revealing observations during a prolonged event. A “low” in the upper 70s or 80s can be a serious warning in a place unaccustomed to it.

Heat watch, warning, and advisory

Names and thresholds may differ locally, and newer experimental or operational tools may express risk by category. Read the full message from your local National Weather Service office. Timing, overnight conditions, location, and expected impacts matter more than the headline alone.

Who is most at risk?

Anyone can become ill during extreme heat, especially during strenuous activity, but risk is not evenly distributed. Older adults, infants, pregnant people, outdoor workers, athletes, people with chronic illness, and people taking certain medications may have greater difficulty regulating heat. People without stable housing or reliable cooling face prolonged exposure. Never leave a child, dependent adult, or pet in a parked vehicle, even briefly.

Heat exhaustion may involve heavy sweating, weakness, nausea, dizziness, headache, or fainting. Move the person to a cooler place, loosen clothing, apply cool wet cloths, and offer sips of water if they are alert. Heat stroke can include confusion, loss of consciousness, seizures, or very abnormal behavior and is a medical emergency. Call emergency services and begin rapid cooling, do not wait for the body temperature to “prove” the diagnosis.

How to prepare before a heat wave

• Identify an air-conditioned place and how you will reach it if home cooling fails.
• Test air conditioning early and learn whether your community operates cooling centers.
• Make a check-in list for relatives, neighbors, and anyone who may need help.
• Plan outdoor work for cooler hours and build in water, shade, and rest.
• Close blinds on sun-facing windows and reduce unnecessary indoor heat sources.
• Charge devices and keep several ways to receive official alerts.

Fans can improve comfort and help sweat evaporate, but they do not lower room temperature. During very high indoor heat, moving hot air may not provide enough protection. Follow local public-health guidance and relocate to a cooler space when advised. Never run a generator or fuel-burning appliance indoors or near openings.

What to do during extreme heat

Drink water regularly rather than waiting until you feel thirsty, unless a clinician has given you fluid restrictions. Wear light, loose clothing, seek shade, and take more breaks than usual. Avoid alcohol and be cautious with very sugary drinks. Do not abruptly ask an unacclimatized person to perform heavy work in peak afternoon heat.

Check people in person or by phone, with special attention to homes without cooling. Pets need shade and fresh water, and pavement can burn paws. If the power fails, report the outage, preserve phone battery, and move to a designated cooling location if indoor conditions are rising.

How to observe a heat wave well

Use a shaded, ventilated thermometer or a properly sited weather-station radiation shield. A sensor on a sunny wall, deck, roof, or inside a closed porch may read the heated surface environment rather than representative air temperature. Record daily maximum and minimum temperatures, humidity or dew point, cloud cover, and notable impacts. Our guide to daily weather observations explains how to keep a consistent log.

Your backyard reading adds local context but does not replace an official warning. Compare it with nearby National Weather Service or mesonet observations, note siting differences, and focus on the trend. During a heat wave, the night's minimum and the number of consecutive hot days often tell the deeper story.

Sources and further reading

• NOAA NESDIS: What is a heat wave?
• National Weather Service: Heat safety
• National Weather Service: During a heat wave