Calculator Inputs
Formula Used
This calculator uses the widely cited regression that estimates apparent temperature from air temperature and relative humidity. It applies the Rothfusz regression in Fahrenheit for typical hot conditions, with standard adjustment terms for very dry or very humid cases.
For cooler conditions, a simplified approximation is used to avoid overstating heat stress. Results are shown in both Celsius and Fahrenheit for practical use.
How to Use This Calculator
- Enter the current air temperature from a reliable source.
- Select the correct temperature unit for your value.
- Enter relative humidity as a percent from 0 to 100.
- Choose exposure to add contextual guidance for sunlight.
- Press Calculate to view results above the form.
- Use CSV or PDF buttons to export your result.
Example Data Table
| Temperature (°C) | Humidity (%) | Typical Heat Index Range (°C) | Common Interpretation |
|---|---|---|---|
| 30 | 40 | 31–33 | Warm, mild stress possible |
| 32 | 60 | 38–41 | Noticeable heat stress risk |
| 35 | 50 | 45–49 | High risk with exertion |
| 38 | 70 | 58–64 | Severe risk, limit exposure |
Ranges are illustrative and depend on conditions.
Heat Index Insights
1) What the heat index represents
The heat index combines air temperature and relative humidity to estimate how hot conditions feel to the human body. When humidity is high, sweat evaporates more slowly and cooling becomes less effective, so the same temperature feels warmer.
2) Why humidity changes perceived heat
Evaporation is the body’s main cooling tool during hot weather. At 70% humidity, evaporation efficiency drops sharply. As humidity rises, even small temperature increases can produce a large rise in apparent heat.
3) Typical thresholds used in guidance
Many safety guides group heat index into bands. Below 32°C, heat stress may be limited for most people. From roughly 32°C to 39°C, prolonged exposure can cause fatigue and dehydration. Above 40°C, heat illness risk rises quickly.
4) Interpreting results during activity
Physical exertion increases internal heat production. A moderate walk can add meaningful strain when the heat index is already high. If your result approaches the “extreme caution” band, plan more rest breaks, reduce intensity, and increase hydration.
5) Sun, shade, and local surfaces
Heat index is often based on shaded measurements. Direct sunlight and reflective surfaces can feel hotter than the estimate. Asphalt, concrete, and metal can raise local radiant heat, especially in dense urban areas during midday.
6) Practical cooling and hydration steps
Drink water regularly, not only when thirsty. Light clothing, airflow, and cool breaks support thermoregulation. For long outdoor work, use a schedule that alternates effort and recovery, and watch for early symptoms like headache.
7) How this calculator supports decisions
This tool provides a quick estimate from two measurable inputs. It shows the heat index in Celsius and Fahrenheit and includes a risk statement. Use it to compare morning and afternoon conditions or to confirm if humidity is amplifying heat more than expected.
8) Limits and best practices
The heat index is an estimate, not a medical diagnosis. Wind, clothing, acclimatization, and health can change risk. For critical planning, combine the result with forecasts and local guidance, and prioritize personal symptoms over numbers.
FAQs
1) Is heat index the same as air temperature?
No. Heat index estimates how hot it feels by combining temperature and humidity. High humidity reduces sweat evaporation, so perceived heat can exceed the measured temperature.
2) Why does the calculator show Fahrenheit and Celsius?
The core regression is commonly applied in Fahrenheit. The calculator converts inputs and outputs so you can read results in both units without manual conversion.
3) What inputs give the most accurate results?
Use a shaded air temperature and a reliable humidity reading. Avoid surfaces heated by direct sun. If you only have a “feels like” value, use measured temperature instead.
4) Does wind speed change the heat index?
Wind can improve cooling, but classic heat index formulas do not include wind speed. This calculator accepts wind as a note only, not as a modifier.
5) Why is full sun mentioned in the results?
Direct sunlight can feel hotter than a shaded estimate because of radiant heating. The tool flags this context so you can plan more conservatively outdoors.
6) When should I be most cautious?
Use extra caution when the heat index reaches higher bands, especially with exercise. Reduce intensity, take breaks, seek shade, and monitor symptoms like dizziness or nausea.
7) Can I use this for indoor environments?
Yes, if you know indoor temperature and humidity. However, airflow, clothing, and heat sources can change comfort. Treat the result as an estimate for planning.