What is the heat index?

The heat index is the perceived temperature when humidity is factored in. High humidity prevents sweat from evaporating, reducing your body's ability to cool itself. At 30°C with 70% humidity, the heat index can reach 36°C — significantly increasing the risk of heat-related illness.

When is heat dangerous for hiking?

A heat index above 33°C requires extreme caution. Above 41°C is dangerous for any sustained outdoor activity. In the UK, summer heatwaves can push temperatures above 30°C with moderate humidity, creating heat index values in the caution to extreme caution range.

How do I prevent heat illness while hiking?

Drink 500ml+ of water per hour. Start early in the morning. Take shade breaks every 30-45 minutes. Wear light, loose clothing and a sun hat. Wet your hat and neck for evaporative cooling. Know the signs of heat exhaustion: heavy sweating, weakness, nausea, dizziness, headache.

What is the difference between heat exhaustion and heat stroke?

Heat exhaustion: heavy sweating, cool pale skin, weakness, nausea, headache. Treatment: move to shade, drink water, cool the person. Heat stroke: hot dry skin (no sweating), confusion, rapid pulse, temperature above 40°C. Heat stroke is a medical emergency — call 999 immediately and cool the person rapidly.

CampCalcs

Heat Index Calculator

Calculate the heat index from temperature and humidity with safety warnings.

Air Temperature

Relative Humidity

How We Calculate This

Rothfusz Heat Index Formula

Uses the Rothfusz regression equation with adjustments for low humidity/high temperature and high humidity/moderate temperature conditions.

Risk Levels

27-33°C: Caution. 33-41°C: Extreme caution. 41-54°C: Danger. 54°C+: Extreme danger.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify conditions and use proper safety equipment outdoors.