Heat Index and Heat Safety: NWS Heat Alerts and Signs of Heat Illness
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How we wrote this: A plain-English summary of National Weather Service (NWS) heat safety pages, checked October 29, 2026. This is general information, not medical advice. Heat stroke is an emergency: call 911.
A hot beach day can turn dangerous fast, especially when it's humid. Here is how the National Weather Service measures heat and what its alerts mean.
What the heat index is
NWS defines the heat index as how hot it really feels when relative humidity is factored in with the air temperature. Its example: at 96°F and 65% relative humidity, the heat index is 121°F.
Two caveats NWS gives:
- Heat index values were devised for shady, light-wind conditions. Full sunshine can raise the heat index by up to 15°F, which matters on an open beach.
- Strong winds with very hot, dry air can also be extremely hazardous.
Other NWS heat tools
- Wet Bulb Globe Temperature (WBGT) combines temperature, humidity, wind and solar radiation. NWS calls it particularly effective for active people such as outdoor workers and athletes.
- HeatRisk is an experimental NWS forecast that rates heat concern for each location for seven days, taking into account how unusual the heat is for the time of year, how long it lasts and whether nights stay warm.
What heat alerts mean
| Alert | NWS meaning |
|---|---|
| Extreme Heat Warning | Extremely dangerous heat is expected or occurring. Take action |
| Extreme Heat Watch | Conditions are favorable for extreme heat, but timing is uncertain. Be prepared |
| Heat Advisory | Dangerous heat that is not expected to reach warning criteria. Take action |
NWS notes local offices set criteria with local partners, since places differ in how used to heat they are. Under a warning or advisory, NWS advises postponing outdoor activities during the heat of the day, drinking plenty of water, taking breaks in the shade and checking on family and neighbors.
Signs of heat illness (NWS)
| Condition | Symptoms | First aid |
|---|---|---|
| Heat cramps | Painful muscle cramps and spasms, heavy sweating | Firm pressure or gentle massage; sips of water unless nauseated. Get medical help if cramps last over 1 hour |
| Heat exhaustion | Heavy sweating; weakness; cool, pale, clammy skin; fast, weak pulse; dizziness; nausea; headache; fainting | Move to a cooler place, loosen clothing, cool wet cloths, sips of water. Get medical help if vomiting, symptoms worsen or last over 1 hour |
| Heat stroke | Throbbing headache, confusion, slurred speech, body temperature above 103°F, hot red dry or damp skin, rapid strong pulse, loss of consciousness | Call 911. Move to a cooler place and cool with cloths or a bath. Do not give fluids |
NWS also warns that a fan can make someone hotter when heat index values are above the 90s.
For the rest of a beach day, see our sun protection checklist, beach weather safety and beach water quality guides.