Heat Index: Feels-Like Temperature Guide
Quick Answer
Heat index is the apparent temperature when humidity is factored in with air temperature.
NOAA danger thresholds:
| Heat Index | Risk Level |
|---|---|
| 27 to 32°C (80 to 90°F) | Caution. Fatigue possible. |
| 32 to 41°C (90 to 105°F) | Extreme caution. Heat cramps and exhaustion. |
| 41 to 54°C (105 to 130°F) | Danger. Heatstroke possible. |
| Above 54°C (130°F) | Extreme danger. Heatstroke very likely. |
For temperature unit conversions, use the free Temperature Converter on CalConvs.
Why Humidity Makes Heat More Dangerous
The human body cools itself primarily through sweat evaporation. When humidity is high, sweat evaporates more slowly because the air is already saturated with water vapour. The body cannot cool itself efficiently, causing core temperature to rise.
- 38°C at 20% relative humidity: Heat index approximately 36°C (feels cooler than actual)
- 38°C at 50% relative humidity: Heat index approximately 44°C (Danger zone)
- 38°C at 80% relative humidity: Heat index approximately 58°C (Extreme danger)
Heat Index Table: Temperature vs Relative Humidity
| Air Temperature | Relative Humidity | Apparent Temperature (Heat Index) |
|---|---|---|
| 30°C | 30% | Feels like 28°C (feels cooler, dry air) |
| 30°C | 80% | Feels like 37°C (moderate effect) |
| 35°C | 50% | Feels like 40°C (Extreme caution) |
| 35°C | 80% | Feels like 51°C (Extreme danger) |
| 40°C | 50% | Feels like 51°C (Extreme danger) |
| 40°C | 70% | Feels like 67°C+ (Life-threatening) |
Heat Safety by Country and Region
- India (May to June): Dry heat in Rajasthan, Punjab and UP can exceed 45°C. Mumbai and coastal cities combine 33 to 36°C with 80 to 90% humidity. Both are dangerous for different reasons.
- Pakistan (May to June): Interior Sindh and Punjab regularly see 42 to 48°C with low humidity. Karachi experiences deadly humid heat spells.
- UAE and Saudi Arabia (July to August): Air temperature 42 to 46°C with 50 to 70% coastal humidity. Heat index routinely above 50°C. Outdoor work restricted by law during peak hours.
- Australia (summer): Southern and inland Australia: dry heat above 40°C. Northern Australia and Queensland: tropical humidity adds heat index risk.
- United Kingdom: 2022 saw temperatures above 40°C for the first time on record. No widespread air conditioning makes even moderate heat a serious health risk.
Heat Safety Guidelines
- Drink water before you feel thirsty. By the time thirst appears, mild dehydration has already begun.
- Avoid outdoor physical activity between 11am and 3pm during extreme heat.
- Wear light coloured, loose, breathable clothing.
- Seek shade or air-conditioned spaces. A 15 to 20 minute cooling break reduces core temperature significantly.
- Never leave children or animals in parked cars. Temperature inside a car can exceed 60°C within minutes in the sun.
Frequently Asked Questions
What is the difference between heat index and temperature?
Air temperature measures the actual temperature of the air. Heat index combines temperature with relative humidity to reflect how the human body perceives the heat. A high-humidity day feels much hotter than the air temperature alone would suggest.
At what temperature does heatstroke become a risk?
Heatstroke becomes a significant risk when the heat index exceeds 41°C (105°F). Warning signs include hot dry skin, confusion, rapid heartbeat and loss of consciousness. This is a medical emergency.
How do I convert heat index from Fahrenheit to Celsius?
Celsius = (Fahrenheit - 32) ÷ 1.8. For example, a heat index of 105°F = (105 - 32) ÷ 1.8 = 40.6°C. Use the Temperature Converter for any conversion instantly.
Related Tools
- Temperature Converter: Celsius, Fahrenheit and Kelvin
- All Unit Converters: 74 converters on CalConvs