Heat Index Calculator
Calculate the "feels like" temperature based on air temperature and humidity. Know when outdoor conditions become dangerous.
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What This Means
Enter temperature and humidity to see heat index.
Quick Answer: Heat Index Danger Levels
The heat index indicates how hot it feels when humidity is combined with temperature. Take precautions when the heat index is above 90°F (32°C).
Dr. Snezana Lawrence
Mathematical Historian
15+ years experience
PhD from Yale University. Published mathematical historian ensuring precision in all calculations.
Education
PhD in Mathematical History - Yale University
Table of Contents
What is Heat Index?
The heat index, also known as the apparent temperature or "feels like" temperature, is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature.
When humidity is high, the body's natural cooling mechanism (sweating) becomes less effective because sweat evaporates more slowly. This means your body has a harder time cooling down, making it feel hotter than the actual temperature.
Why It Matters
High heat index values can lead to heat-related illnesses including heat cramps, heat exhaustion, and heat stroke. Understanding the heat index helps you take appropriate precautions.
How It's Calculated
The National Weather Service uses the Rothfusz regression equation, which combines air temperature and relative humidity with various adjustments for extreme conditions.
Heat Index Chart
Reference table showing heat index values for various temperature and humidity combinations:
| Temp (°F) | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|
| 80°F | 80 | 81 | 82 | 83 | 85 | 87 |
| 85°F | 85 | 87 | 90 | 93 | 97 | 102 |
| 90°F | 91 | 95 | 100 | 106 | 113 | 122 |
| 95°F | 98 | 104 | 113 | 123 | 135 | -- |
| 100°F | 109 | 118 | 129 | 142 | -- | -- |
| 105°F | 121 | 133 | 148 | -- | -- | -- |
Yellow = Caution, Orange = Extreme Caution, Red = Danger, Purple = Extreme Danger
Heat Safety Tips
Stay Hydrated
Drink plenty of water before you feel thirsty. Avoid alcohol and caffeine.
Seek Air Conditioning
Spend time in air-conditioned spaces during the hottest parts of the day.
Wear Light Clothing
Choose lightweight, light-colored, loose-fitting clothes.
Limit Outdoor Activity
Avoid strenuous activity during peak heat (10am-4pm).
Take Breaks
If working outdoors, take frequent breaks in shade or AC.
Know the Signs
Watch for heat exhaustion: heavy sweating, weakness, nausea, dizziness.
Frequently Asked Questions
What is the heat index?
The heat index, also called apparent temperature or "feels like" temperature, combines air temperature and relative humidity to show how hot it actually feels to the human body. High humidity prevents sweat from evaporating effectively.
At what heat index is it dangerous?
Heat index of 90°F (32°C) requires caution for prolonged exposure. At 105°F (41°C), heat exhaustion is likely with prolonged activity. Above 130°F (54°C) is extreme danger with high risk of heat stroke.
Why does humidity make it feel hotter?
Your body cools itself through sweating. When humidity is high, sweat evaporates more slowly, reducing your body's ability to cool down. This makes you feel hotter than the actual air temperature.
What's the difference between heat index and wind chill?
Heat index measures how hot it feels with humidity (summer). Wind chill measures how cold it feels with wind (winter). Both are "feels like" temperatures but for opposite conditions.
What are signs of heat exhaustion?
Warning signs include heavy sweating, cold/pale/clammy skin, fast/weak pulse, nausea, muscle cramps, tiredness, weakness, dizziness, and headache. Move to a cool place, drink water, and seek medical help if symptoms worsen.
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Dr. Snezana Lawrence
Mathematical Historian | PhD from Yale
Dr. Lawrence is a published mathematical historian with a PhD from Yale University. She ensures mathematical precision and accuracy in all our calculations, conversions, and academic score calculators. Her expertise spans computational mathematics and educational assessment.
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