Heat Index Calculator

What Is Heat Index and Why Does It Matter?

The heat index, sometimes called the "apparent temperature" or "feels like" temperature, is a measure of how hot it truly feels when relative humidity is combined with the actual air temperature. When humidity is high, your body cannot cool itself as effectively through sweat evaporation, which is the primary cooling mechanism for humans. This means that a 90-degree day at 70 percent humidity can feel like 106 degrees to your body, dramatically increasing the risk of heat-related illness. The heat index is used by the National Weather Service to issue heat advisories and warnings that protect public health.

The Rothfusz Regression Equation

The Rothfusz regression equation is the standard formula used by the National Weather Service to calculate heat index values. This polynomial equation takes temperature and relative humidity as inputs and produces a value that represents the perceived temperature. The full equation uses nine terms with coefficients that were derived from extensive research on human heat exchange with the environment. For temperatures below 80 degrees Fahrenheit, the heat index is approximated using a simpler formula. When the basic equation produces a value below the simple average, adjustments are applied for low humidity conditions (below 13 percent) and high humidity conditions (above 85 percent).

Heat Index Danger Categories

The National Weather Service classifies heat index values into four danger categories. Caution (80 to 90 degrees) means fatigue is possible with prolonged exposure and physical activity. Extreme Caution (90 to 103 degrees) indicates that heat cramps and heat exhaustion are possible, and you should limit outdoor activity. Danger (103 to 124 degrees) means heat cramps and heat exhaustion are likely, and heat stroke is possible. Extreme Danger (above 124 degrees) indicates that heat stroke is highly likely, and outdoor activity should be avoided entirely. These thresholds are based on shade conditions — direct sunlight can increase heat index values by up to 15 degrees.

Health Risks of High Heat Index

Heat-related illness progresses through several stages, each more dangerous than the last. Heat cramps are the earliest warning sign, presenting as painful muscle spasms usually in the legs or abdomen. Heat exhaustion follows with heavy sweating, weakness, cold and clammy skin, nausea, and fainting. Heat stroke is a medical emergency characterized by a body temperature above 103 degrees, hot and dry skin with no sweating, rapid pulse, and possible unconsciousness. Each year, heat-related illness causes more deaths in the United States than any other weather phenomenon, making the heat index a critical tool for public safety.

Protecting Yourself in High Heat Index Conditions

When the heat index is elevated, drink plenty of water before you feel thirsty, as dehydration accelerates heat illness. Wear lightweight, light-colored, and loose-fitting clothing. Schedule outdoor activities for early morning or evening when temperatures are lower. Take frequent breaks in air-conditioned or shaded environments. Never leave children or pets in parked vehicles, as temperatures inside can reach lethal levels within minutes. Check on elderly neighbors and family members who may not have air conditioning. Workers in outdoor occupations should follow OSHA heat illness prevention guidelines, including acclimatization schedules and mandatory water and rest breaks.

When Is Heat Index Most Relevant?

The heat index calculation is most meaningful when the air temperature is above 80 degrees Fahrenheit and relative humidity is above 40 percent. In dry climates with low humidity, the actual temperature is generally close to or even above the heat index. In humid regions like the southeastern United States, Gulf Coast, and tropical areas, the heat index frequently exceeds the actual temperature by 10 to 20 degrees or more during summer months. Athletes, outdoor workers, and anyone spending extended time outside should monitor the heat index throughout the day, as conditions can change rapidly.