Tier 04 · Local Risks
Not hurricanes. Not tornadoes. Heat kills more Americans every year than any other weather event. It does so quietly, without a visible flood line or a wall of wind, and it disproportionately takes the people least likely to ask for help.
Make a heat planUnderstanding the hazard
A heat wave is two or more consecutive days of temperatures significantly above a region's normal range, typically combined with high humidity that prevents the body from cooling through sweat.
Unlike a tornado or a hurricane, there is no visible event to galvanize action. The sky is clear. The damage is internal. Heat works by overwhelming the body's ability to regulate its core temperature. When that system fails, the result is heat exhaustion and, if untreated, heat stroke: a medical emergency with a narrow treatment window.
600+
Americans die each year
Direct heat-related deaths per CDC data. The true toll, including heat-worsened conditions, is estimated two to three times higher.
103°F
Heat index warning threshold
When the heat index reaches 103°F, the NWS issues an Excessive Heat Warning. Outdoor activity becomes dangerous for healthy adults and potentially life-threatening for those at risk.
30 min
Heat stroke treatment window
Once heat stroke sets in (core temp above 104°F, confusion, no sweating), cooling must begin within minutes. Beyond 30 minutes of untreated heat stroke, organ damage becomes likely.
Regional reality
Every region of the United States experiences heat waves now. The Southwest and South have the most frequent and severe events. The Pacific Northwest had its worst heat wave on record in 2021, with temperatures exceeding 116°F in areas with virtually no air conditioning.
Desert Southwest
Arizona, Nevada, and southern California regularly see heat index values above 110°F. June through September is the core season. Dry heat masks the danger: perspiration evaporates immediately, creating a false sense of comfort while the body continues to heat.
Gulf Coast and Southeast
High humidity makes 95°F feel like 115°F. New Orleans, Houston, and Miami routinely post heat index values above 105°F from late spring through early fall. The moisture prevents sweat from evaporating, accelerating heat illness even in shade.
Urban cores everywhere
The urban heat island effect adds 5 to 10°F to city temperatures versus surrounding areas. Asphalt, concrete, and reduced tree cover store heat through the day and release it at night, eliminating the overnight recovery that rural areas get.
Pacific Northwest
A historically mild climate means low AC penetration and limited community experience with extreme heat. When events occur, they are disproportionately deadly. The 2021 heat dome is the clearest example on record.
Excessive Heat Watch
Conditions are favorable for a Warning within the next 24 to 72 hours. Not yet confirmed. Start preparing now: pre-cool your home, locate your nearest cooling center, and begin your check-in with at-risk neighbors.
Excessive Heat Warning
Dangerously hot conditions are occurring or imminent. Typically issued when the heat index will exceed 105°F for at least two hours. This is the highest tier. If you or anyone you know lacks AC access, act immediately.
Heat Advisory
Less severe than a Warning. Heat index will be uncomfortable but below the Warning threshold. Still dangerous for at-risk populations. Reduce strenuous outdoor activity and increase hydration through the advisory period.
NWS thresholds vary by region. What triggers a Warning in Oregon may be routine in Texas. Tune to local NWS forecasts, not just national headlines.
At-risk populations
Heat disproportionately kills people who are less able to regulate their own temperature, less able to recognize warning signs, or less able to act when they do. Most heat deaths are preventable. A check-in protocol is one of the highest-impact interventions available to any household.
The aging body loses its ability to regulate temperature efficiently. Older adults sweat less, feel thirst less acutely, and are more likely to be on medications that compound heat risk. A neighbor without air conditioning during an Excessive Heat Warning is in immediate danger.
Children's bodies heat up three to five times faster than adult bodies. They cannot communicate distress reliably. Never leave a child in a parked vehicle on any warm day, even briefly: a car interior reaches 130°F on a 90°F day within 30 minutes.
Diuretics, antihistamines, beta-blockers, antipsychotics, and some antidepressants can impair the body's heat response. If you or a family member takes any of these, ask a pharmacist about heat safety precautions before the season begins.
Roughly 10 to 12% of U.S. households lack air conditioning. In a prolonged heat event, an un-air-conditioned home can reach 115°F or higher. These households are the primary reason cooling centers exist.
Cardiovascular disease, kidney disease, obesity, and diabetes all increase heat risk. The cardiovascular system works harder to dissipate heat, and kidney function is especially sensitive to the dehydration that accompanies extreme events.
Farm laborers, construction workers, landscapers, and utility crews bear a disproportionate heat burden. OSHA's three-point standard: water, rest, shade. If you manage people who work outdoors, implement it seriously and track symptoms.
The check-in protocol
Write down the names and contact information of at-risk neighbors, elderly relatives, and anyone in your network who might need a welfare check. You will not have time to compile this list in the middle of an Excessive Heat Warning.
Understanding the numbers
You have seen "heat index" on weather apps. Wet-bulb temperature is the number underneath it, and it tells you whether your body can cool itself at all.
Your body cools by sweating. Sweat evaporates, carrying heat away with it. But evaporation only works when the air is not already saturated with moisture. In high humidity, sweat does not evaporate. It sits on your skin, and your core temperature keeps rising.
Wet-bulb temperature is measured by wrapping a wet cloth around a thermometer. It captures the combined effect of air temperature and humidity in a single number that reflects what evaporative cooling actually has to work with.
The body can cool itself effectively with adequate hydration and rest. Uncomfortable, but manageable for most people.
Heat illness risk rises significantly, especially during exertion. At-risk populations should avoid prolonged outdoor exposure. Everyone should slow down and hydrate frequently.
Dangerous for everyone. Even fit, healthy adults will experience heat illness with moderate activity. Go inside and stay inside.
The physiological ceiling. Even at rest in the shade, the human body cannot maintain a safe core temperature. Conditions at this level are lethal within hours without mechanical cooling.
Quick reference
Heat index is the "feels like" temperature that accounts for humidity. It is what weather apps display. Wet-bulb temperature is the physical limit of cooling. They are related but not the same number.
Heat Index 91°F
NWS Heat Advisory range. Caution advised for at-risk populations. Reduce prolonged outdoor exertion.
Heat Index 103°F
NWS Excessive Heat Warning threshold. Dangerous for everyone outdoors. Outdoor work should stop or be heavily restricted.
Heat Index 115°F+
Extreme danger. Heat stroke possible with minimal exertion for any person at any fitness level.
Wet-Bulb 95°F
The human survival limit. Not a comfort threshold: a physiological ceiling. No amount of rest or shade compensates once this threshold is reached without AC.
Wet-bulb temperatures approaching 95°F have been recorded in South Asia and the Persian Gulf. Climate projections show them becoming possible in parts of the U.S. Gulf Coast and Southeast by mid-century.
Preparation
Most heat wave preparation takes 30 minutes or less. The goal is to remove friction from decisions you will have to make quickly once the Warning is active.
Fans do not lower air temperature. They cool you by accelerating sweat evaporation. At the same time, the motor generates heat. In moderate conditions, the evaporation benefit outweighs the motor output.
Here is the threshold: when ambient air temperature exceeds approximately 95°F, a fan blowing on you adds net heat to your body. At that point, a fan makes conditions worse, not better.
Air conditioning lowers the actual air temperature. It works regardless of ambient conditions. If your home reaches 95°F and you have only a fan, the safer choice is finding a cooler environment.
Practical rule
Fans are effective below 95°F with good hydration. Above 95°F, or for any at-risk person: get to air conditioning.
Thirst is a lagging indicator. By the time you feel thirsty in extreme heat, you are already mildly dehydrated, and mild dehydration impairs thermoregulation before you notice any other symptoms.
The baseline target during heat: 1 cup (8 oz) per hour during light outdoor activity. During strenuous activity in direct sun, 1 cup every 15 to 20 minutes. At rest indoors, the standard 8 to 10 cups per day still applies.
Avoid alcohol and high-caffeine drinks during heat events. Both have diuretic effects that accelerate fluid loss. Sports drinks with electrolytes are appropriate for anyone sweating heavily for more than an hour.
Warning sign
Dark yellow or amber urine means significant dehydration. Drink water now, before your next outdoor exposure.
Before the season
During-event protocol
An Excessive Heat Warning means dangerous conditions are occurring now or within hours. The following sequence moves from the most time-sensitive actions to ongoing monitoring.
Check in with at-risk neighbors first
Do this before addressing your own comfort. An elderly neighbor without AC during an Excessive Heat Warning is in immediate danger. A phone call takes 90 seconds.
Cool the home before peak heat
Close blinds and curtains on south- and west-facing windows by 8am. Open windows overnight when it is cooler than inside, then close them again before the day heats up. Thermal mass in floors and walls takes hours to heat. If you can keep it cool overnight, the home stays significantly cooler the following afternoon.
Avoid outdoor activity 10am to 6pm
Peak heat load occurs in the early afternoon. If you must go outside, wear light-colored, loose-fitting clothing, apply sunscreen, and carry water. Take breaks in shade or AC every 15 to 20 minutes during any strenuous activity.
If no AC: go to a cooling center
Libraries, shopping malls, community centers, and designated cooling centers are real options. If your home reaches 95°F and you are without air conditioning, staying home is the higher-risk choice. Cooling centers exist for exactly this situation.
Know the difference between heat exhaustion and heat stroke
Heat exhaustion: heavy sweating, pale and moist skin, weakness, fast and weak pulse, nausea, possible fainting. Move the person to a cool place, loosen clothing, apply cool wet cloths, give water if they are conscious. This is not yet an emergency, but it requires immediate response.
Heat stroke: high body temperature above 104°F, skin that is hot and either dry or damp, fast and strong pulse, possible confusion or loss of consciousness. Call 911 immediately and begin cooling while you wait. This is a medical emergency.
Never leave children or pets in vehicles
A car interior reaches 130°F on a 90°F day within 30 minutes. There is no "briefly." There is no "windows cracked." If you are traveling with children or pets, they come with you when the car stops.
Call 911 immediately if you see
While waiting for EMS: move the person to the coolest available space, remove excess clothing, and apply ice packs to the neck, armpits, and groin. Fan the person while misting with cool water if both are available.
Recovery
The end of a heat warning does not mean conditions are immediately safe. Overnight temperatures may still be elevated, and anyone who experienced heat stress during the event needs time to recover.
Check in with the at-risk neighbors and relatives you contacted during the event. Ask how they are feeling. Someone who pushed through the event without help may be quietly recovering from heat exhaustion, and that condition can worsen over the following 24 hours.
If you were outdoors or in a warm environment during the event, rehydrate over 12 to 24 hours with water and electrolytes. Spread it out rather than drinking large amounts rapidly. Urine returning to pale yellow is a reliable indicator of adequate hydration.
If a power outage accompanied the heat event, follow food safety guidelines: refrigerator food is safe for four hours without power, and a full freezer holds for 48 hours. Do not rely on smell alone as an indicator of safety. When in doubt, discard it.
Heat exhaustion symptoms can persist for 24 to 48 hours after the event: fatigue, muscle cramps, headache, nausea. Rest, hydrate, and stay in a cool environment. If symptoms do not improve within 24 hours, or worsen, contact a healthcare provider.
Did the cooling center have the hours you expected? Did your AC hold up? Was there a neighbor on your list you could not reach? Write down what to address before next season. Heat events are recurring. Your preparation for next summer starts the day after this one ends.
Local resources
These links go directly to the source. The guidance changes and the originals are authoritative.
National Weather Service heat safety page. Includes current heat advisories, heat index charts by region, and local NWS office links for forecast-level detail.
weather.gov/safety/heat →
CDC guidance on heat-related illness: symptoms, treatment, and prevention for households and workers. Includes clinical guidance for healthcare providers.
cdc.gov →
Call or text 211 to find the nearest cooling center, shelter, or community resource. Available in all 50 states, 24 hours a day during declared heat emergencies.
211.org →
American Red Cross heat wave safety guidelines, including first aid instructions for heat exhaustion and heat stroke in accessible, practical language.
redcross.org →
FEMA's official heat preparation and response guidance, with state emergency management contact links for local resources and declarations.
ready.gov/heat →
NOAA's heat education resources: the heat index reference chart, excessive heat criteria by region, and urban heat island research for context on local risk.
noaa.gov →
Next steps
The best heat preparation runs on the same foundation as every other emergency: water stored, documents ready, a household plan in place. If you have not built that foundation yet, start there.