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Heat Index Calculator
Calculate how hot it really feels with humidity. Know when it's dangerous.
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Why Heat Index Matters
Your body cools itself through sweat evaporation. High humidity blocks evaporation. That's why 90°F with 90% humidity feels like 122°F—your body can't cool down, and heat stroke becomes a real danger.
🚨 Heat Stroke Warning Signs
Call 911 if you see:
- Confusion or altered mental state
- Hot, dry skin (stopped sweating)
- Rapid, strong pulse
- Throbbing headache
Move to shade, cool with water, do NOT give fluids if unconscious.
Heat Index Danger Zones
- 80-90°F: ⚠️ Caution. Fatigue possible with prolonged exposure.
- 90-105°F: ⚠️ Extreme Caution. Heat cramps and heat exhaustion possible.
- 105-130°F: 🔥 Danger. Heat exhaustion and heat stroke likely.
- 130°F+: 🔥 Extreme Danger. Heat stroke imminent. Stay indoors.
💡 Expert Tip from Sarah Jenkins, RD
"If the heat index is above 90°F, move your run to 6 AM. I've seen runners hospitalized for heat stroke at noon. Your ego isn't worth kidney failure."
How to Stay Safe
1. Time Your Activity: Exercise before 10 AM or after 6 PM when it's cooler.
2. Hydrate Aggressively: Drink 16oz water 2 hours before, 8oz every 15 minutes during exercise.
3. Watch for Warning Signs: Dizziness, nausea, confusion = stop immediately, get to shade.
4. Wear Light Colors: Dark clothing absorbs heat. White/light colors reflect it.
5. Know Your Limits: Elderly, children, and those on medications (beta-blockers, diuretics) are at higher risk.
⚠️ The "It's a Dry Heat" Myth
Phoenix at 110°F with 20% humidity (HI: 106°F) is survivable. Houston at 90°F with 90% humidity (HI: 122°F) sends people to the ER. Humidity is the killer, not just temperature.
⚠️ Common Mistakes to Avoid
- Ignoring humidity: "It's only 90°F." Yes, but with 80% humidity, it FEELS like 113°F. That's the "Danger" zone. Your sweat won't evaporate, and heat stroke can hit fast.
- Thinking shade saves you: Heat index assumes SHADE. If you are in direct sunlight, add 15°F to the "feels like" temperature. 113°F becomes 128°F. Deadly.
- Drinking only when thirsty: By the time you're thirsty, you're already dehydrated. In high heat index weather, drink water every 15 minutes, even if you don't feel like it.
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❓ Frequently Asked Questions
What is heat index?
Heat index is what the temperature feels like when humidity is factored in. 90°F with 60% humidity feels like 100°F.
At what heat index should you not exercise?
Above 90°F, exercise caution. Above 105°F, cancel outdoor workouts. Heat stroke can happen in minutes at 130°F+.
What are the signs of heat stroke?
Confusion, hot dry skin (no sweating), rapid pulse, headache, dizziness, nausea. Call 911 immediately.
Why is humidity dangerous?
High humidity prevents sweat evaporation, trapping heat. 90°F at 90% humidity feels like 122°F—your body can't cool itself.