🌡️ Heat Index Calculator

What Can the Heat Index Calculator Do?

The Heat Index Calculator uses the US National Weather Service (NWS) standard algorithm to calculate the temperature the human body actually feels based on current air temperature and relative humidity. In hot and humid environments, the heat index can be significantly higher than the actual air temperature. Understanding the heat index helps prevent heat exhaustion and heat stroke.

Core Features

• Heat Index Calculation: Based on the NWS Rothfusz regression equation for accurate feels-like temperature
• Heat Risk Warnings: Four-level warning system (Caution → Extreme Caution → Danger → Extreme Danger) with safety advice
• Celsius/Fahrenheit Toggle: Support for both temperature units in input and display
• Humidity Comparison Table: Generate heat index at different humidity levels at a glance
• Real-time Calculation: Instant results as parameters change, no manual clicking needed

How to Use

1. Enter the current temperature value and select Celsius or Fahrenheit unit

2. Adjust the humidity slider or enter the relative humidity percentage

3. The heat index is calculated automatically, showing the risk level and safety advice

4. Check the comparison table to see how heat index changes at different humidity levels

Use Cases

Case 1: Outdoor Exercise Planning

Before running, cycling, or other outdoor activities, check the heat index to determine if it's safe. When the heat index exceeds 32°C, reduce exercise intensity; above 39°C, avoid outdoor activities entirely.

Case 2: Construction Site Safety

Construction sites and outdoor workplaces use the heat index to schedule work and rest periods, preventing heat-related illness among workers. At dangerous heat index levels, rest frequency must be increased.

Case 3: School Activity Planning

Schools adjust PE classes and outdoor activities based on the heat index. When the heat index is too high, activities are moved indoors to protect student health.

Case 4: Travel Planning

When traveling to tropical or hot summer destinations, check the heat index in advance to plan your itinerary and heat protection measures accordingly.

Technical Background

The Heat Index calculation is based on Steadman's (1979) human heat exchange model and the NWS Rothfusz (1990) regression equation. This equation accounts for the reduced evaporative cooling efficiency of the human body in hot, humid environments. When relative humidity increases from 40% to 80%, the heat index at 35°C can rise from approximately 36°C to about 50°C — a significant difference. The NWS specifies that the Heat Index should not be calculated when the temperature is below 27°C (80°F) or humidity is below 40%, as the difference between actual and perceived temperature is minimal in those conditions.