Heat Index Explained
When the thermometer reads 95°F but the humidity is 70%, your body experiences it as 112°F. That's the heat index — and it's what determines whether heat is uncomfortable or deadly.
How the Heat Index Works
Your body cools itself by sweating. When sweat evaporates, it pulls heat away from your skin. But when the air is already saturated with moisture (high humidity), sweat can't evaporate as fast, and your body can't cool down effectively.
The heat index is a single number that combines air temperature and relative humidity to reflect the apparent temperature — how hot it actually feels to the human body. It's calculated using a regression equation developed by the NWS, and it's always measured in the shade. Direct sunlight can add 10–15°F to the apparent temperature.
Danger Levels
80–90°F — Caution
Fatigue possible with prolonged exposure and/or physical activity. Stay hydrated.
91–103°F — Extreme Caution
Heat cramps and heat exhaustion possible. Limit outdoor exertion, take frequent breaks in shade or AC.
104–124°F — Danger
Heat exhaustion likely, heat stroke possible. Outdoor work should be minimized. Check on elderly neighbors and anyone without air conditioning.
125°F+ — Extreme Danger
Heat stroke highly likely. This is a medical emergency waiting to happen. Stay indoors in air conditioning.
NWS Heat Alerts
Heat Advisory
Heat index of 100–104°F for 2+ hours (varies by region). Hot enough to cause heat illness in vulnerable people.
Excessive Heat Warning
Heat index of 105°F+ for 2+ hours (varies by region). Dangerous for everyone. This is the NWS's most serious heat alert.
Excessive Heat Watch
Excessive heat is possible in the next 24–72 hours. Prepare now — check your AC, stock water, plan to limit time outdoors.
Thresholds vary by NWS forecast office. Southern offices may use higher thresholds because populations are more acclimated. A 100°F heat index in Minnesota is more dangerous than the same reading in Houston because residents are less adapted.
Protecting Yourself
Hydrate Before You're Thirsty
By the time you feel thirsty, you're already dehydrated. Drink water consistently throughout the day.
Never Leave Kids or Pets in Cars
A car's interior can reach 140°F in minutes, even with windows cracked. This kills dozens of children every year.
Check on Vulnerable People
Elderly people, young children, people on certain medications, and those without AC are at highest risk. Check on them during heat waves.
Reschedule Outdoor Exertion
Move exercise, yard work, and outdoor events to early morning or evening. Heat illness can happen fast, even in fit people.
Get Heat Alerts on Your Phone
StormCast delivers NWS heat advisories, excessive heat warnings, and all other weather alerts to your Android phone. You'll know about dangerous heat before you walk outside.
