Heat Index Calculator

The heat index estimates how hot warm, humid air feels to the human body. High humidity slows the evaporation of perspiration, reducing one of the body's main cooling mechanisms. As a result, an air temperature that seems manageable in dry conditions can create much greater physiological stress when the atmosphere contains more moisture. This calculator combines air temperature in degrees Fahrenheit with relative humidity to report an apparent temperature in both Fahrenheit and Celsius. The calculation follows the regression used by the United States National Weather Service. It begins with a simpler estimate and applies the full Rothfusz regression when conditions are warm enough. The full equation includes temperature, relative humidity, squared terms, and interaction terms. Standard adjustments improve estimates for unusually dry conditions between 80°F and 112°F and very humid conditions between 80°F and 87°F. The method is designed primarily for temperatures of at least about 80°F and relative humidity of at least 40 percent. Heat index values assume shaded conditions with light wind. Direct sunshine can increase apparent conditions substantially, while strong wind may alter heat transfer. Clothing, physical effort, hydration, age, health, acclimatization, and medications also influence personal risk. Therefore, the number is a planning indicator rather than a diagnosis or a guarantee of safety. Always consider official local warnings and stop activity if symptoms such as dizziness, confusion, nausea, headache, or unusual weakness occur. Heat risk generally rises as the index moves through caution, extreme caution, danger, and extreme danger ranges. Even lower readings can affect people doing sustained work or exercise, especially without cooling breaks. Use the result to schedule demanding tasks for cooler hours, increase water and rest opportunities, provide shade, and monitor coworkers or teammates. For occupational decisions, follow applicable regulations and organization-specific heat plans. For weather monitoring, use current temperature and humidity measured at the same location and time rather than combining unrelated observations.