Overview

The heat index is an index that expresses how hot outdoor conditions feel to a typical person by combining air temperature with relative humidity. Often called the apparent temperature or "felt air temperature," the heat index translates a given temperature–humidity pair into a single number that approximates human perception of heat. For example, an air temperature near 90 °F (32 °C) with very high humidity can produce a heat index around 105 °F (41 °C), indicating that the body will experience conditions similar to a dryer but hotter day.

How it works and physiological basis

Human bodies lose heat primarily through the evaporation of sweat from the skin. When relative humidity is high, the air contains more water vapor and evaporation slows; sweat does not evaporate as efficiently, so the body retains heat. The heat index captures this interaction: as humidity increases at a given temperature, the index rises because evaporative cooling is reduced. Conversely, on dry days the index will be closer to the actual air temperature because sweat evaporates more readily.

Calculation, assumptions and practical limits

The heat index is computed from temperature and relative humidity using empirical formulas derived from human perception studies and meteorological data. Operational versions used by weather services apply regression equations that match subjective comfort surveys and laboratory results. Those formulae assume a person of average size and clothing, in the shade, with light activity and a light wind. Because of these assumptions, the heat index is most reliable under standard outdoor conditions; direct sun, heavy exertion, or unusual clothing can make the index understate actual stress.

Agencies commonly do not report a heat index when conditions fall outside conservative bounds: many products restrict calculation to temperatures above about 80 °F (27 °C) and relative humidity values in a range where meaningful differences occur. Dew point and mixing ratio descriptions are related ways to represent atmospheric moisture; at some temperatures a relatively low humidity is sufficient to raise the heat index above the air temperature.

History and development

The concept of apparent temperature grew from research into human thermal comfort. A modern practical index called the heat index emerged in the late 20th century and was adopted by national weather services to provide a single, understandable measure of heat stress for the public. The index builds on earlier scientific work and on laboratory studies that matched subjective sensations with measured air temperature and humidity. Over time, operational heat-index charts and calculators have been refined to reflect typical conditions and to align with public-health guidance.

Uses, guidance and health considerations

The heat index is widely used in public weather forecasts, workplace safety, and sports planning because it helps people judge when heat poses a risk. Elevated heat-index values correlate with increased likelihood of heat-related illnesses such as heat cramps, heat exhaustion, and heat stroke. Heat-safety advice tied to the index typically includes staying hydrated, scheduling strenuous activity for cooler hours, taking frequent breaks in shaded or air-conditioned spaces, and checking on vulnerable individuals such as children, older adults, and people with chronic illnesses.

It is important to remember the index assumes shaded conditions. Exposure to direct sunlight can add several degrees to perceived heat, and vigorous exercise or heavy clothing increases heat stress beyond what the index predicts. Occupational safety standards and athletic organizations sometimes use modified guidance that incorporates exertion level, clothing, and solar load.

Different countries and organizations may use alternate but related metrics. For example, Canada uses the humidex, which expresses discomfort using temperature and dew point rather than relative humidity. The wind chill index, by contrast, combines temperature and wind speed to estimate how cold it feels, and is often described together with heat index under the broader concept of apparent temperature. Each of these indices simplifies complex heat-transfer processes into a single value and therefore depends on standardized assumptions; awareness of those assumptions helps people apply the indices safely.