Heat Index Calculator
Heat index is calculated when temperature ≥ 80°F (27°C). At lower temperatures, feels-like temp equals actual temp.
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Understanding Heat Index
What is Heat Index?
The heat index, also known as apparent temperature, describes how hot it actually feels when humidity is factored in with the actual air temperature. High humidity prevents sweat from evaporating efficiently, reducing your body's ability to cool itself.
Why Humidity Matters
Your body cools itself through sweating. When sweat evaporates, it removes heat from your skin. High humidity means the air already contains a lot of water vapor, making it harder for sweat to evaporate. This makes hot days feel even hotter and more dangerous.
Heat Index Chart
| Temp/RH | 40% | 50% | 60% | 70% | 80% |
|---|---|---|---|---|---|
| 80°F | 80 | 81 | 82 | 84 | 86 |
| 85°F | 85 | 87 | 90 | 93 | 97 |
| 90°F | 91 | 95 | 100 | 106 | 113 |
| 95°F | 97 | 104 | 112 | 121 | 132 |
| 100°F | 105 | 114 | 124 | 136 | - |
Heat-Related Illness
Heat Cramps
- Muscle cramps or spasms
- Usually in legs or abdomen
- Often during heavy exercise
- Treatment: Rest, stretch, drink fluids
Heat Exhaustion
- Heavy sweating, weakness
- Cold, pale, clammy skin
- Nausea, dizziness, headache
- Treatment: Move to cool area, hydrate, rest
Heat Stroke (Medical Emergency)
- Body temperature above 103°F
- Hot, red, dry skin (no sweating)
- Rapid pulse, confusion, unconsciousness
- Treatment: Call 911 immediately, cool body with water/ice
Heat Safety Tips
- Stay hydrated—drink water before you feel thirsty
- Wear lightweight, light-colored, loose-fitting clothing
- Limit outdoor activities during peak heat (10am-4pm)
- Take frequent breaks in air conditioning or shade
- Never leave children or pets in parked vehicles
- Check on elderly neighbors and those without A/C
- Avoid alcohol and caffeine—they increase dehydration
NWS Heat Advisories
- Heat Advisory: Heat index expected to exceed 105°F for 2+ hours
- Excessive Heat Watch: Excessive heat possible in next 24-72 hours
- Excessive Heat Warning: Heat index expected to exceed 105°F for 2+ days
Frequently Asked Questions
What is the heat index?
Heat index (also called 'feels like' temperature) combines air temperature and relative humidity to describe how hot it actually feels to your body. High humidity prevents sweat from evaporating, making it feel hotter than the actual temperature.
At what heat index is it dangerous to be outside?
A heat index of 90-103°F requires caution. 103-124°F is dangerous with heat cramps and exhaustion likely. Above 125°F is extremely dangerous with high risk of heat stroke. Limit outdoor activity and stay hydrated in high heat.
Why does humidity make heat feel worse?
Your body cools itself through sweating. When sweat evaporates, it removes heat from your skin. High humidity means the air already holds lots of moisture, preventing your sweat from evaporating efficiently and making you feel hotter.
How is heat index different from dew point?
Heat index tells you how hot it feels based on temperature and humidity. Dew point measures the actual moisture content in the air. Dew point is better for absolute comfort levels (above 65°F feels muggy), while heat index is better for heat danger assessment.