Heat Index Calculator
Calculate how hot it really feels
⚠️ Warning: Heat index is calculated for temperatures at or above 80°F (27°C) in shaded conditions with light wind.
Must be ≥ 80°F
Heat Index Danger Chart
Heat Safety
🌊 Stay Hydrated
- • Drink water throughout the day
- • Don't wait until you're thirsty
- • Avoid sugary or alcoholic drinks
- • Use electrolyte drinks for intense activity
❄️ Stay Cool
- • Use air conditioning when possible
- • Take cool showers or baths
- • Wear light, loose-fitting clothing
- • Stay in shade outdoors
⏰ Time Activities
- • Exercise early morning or evening
- • Avoid midday sun (10 AM - 4 PM)
- • Take frequent breaks in shade/AC
- • Reduce intensity in extreme heat
⚠️ Warning Signs
- • Heat exhaustion: heavy sweating, weakness, nausea
- • Heat stroke: hot dry skin, confusion, seizures
- • Call 911 if heat stroke suspected
- • Move to cool place, apply cool water
About Heat Index
What is Heat Index?
The heat index (also called "apparent temperature") is what the temperature feels like to the human body when relative humidity is combined with air temperature. High humidity makes it harder for sweat to evaporate, preventing your body from cooling efficiently.
Why Humidity Matters
Your body cools itself through evaporation of sweat. In humid conditions, sweat doesn't evaporate as quickly, making you feel hotter. At 90°F with 90% humidity, your body struggles to cool down, making it feel like 122°F!
Important Notes
- Calculated for shaded areas with light wind
- Direct sunlight can increase heat index by up to 15°F
- Valid for temperatures ≥80°F (27°C)
- Individual sensitivity varies (age, health, fitness)
- Pregnant women, elderly, and children at higher risk
Frequently Asked Questions
Why does humidity make it feel hotter?
Your body cools through sweat evaporation. High humidity slows evaporation, reducing cooling efficiency. The sweat stays on your skin instead of evaporating.
Is 100°F with low humidity safer than 90°F with high humidity?
It can be! 100°F with 20% humidity (heat index ~96°F) is more tolerable than 90°F with 90% humidity (heat index ~122°F). Humidity is crucial.
Does heat index apply in the shade?
Yes, heat index assumes shade. Direct sun can add 10-15°F to the heat index. Always seek shade when possible in extreme heat.
What's the difference between heat index and humidex?
Both measure apparent temperature. Heat index is used in the US. Humidex (used in Canada) uses a different formula but serves the same purpose.
At what heat index should I cancel outdoor activities?
Above 103°F heat index, strenuous activities become dangerous. Above 125°F is extreme danger — stay indoors in AC. Always listen to your body.