Heat index
The heat index combines air temperature and relative humidity to estimate how hot conditions feel to the human body in the shade.
What the heat index is
The heat index is a calculated temperature that describes how hot the air feels when humidity is included. It is often called the apparent temperature or feels-like temperature. A day with moderate humidity can feel close to the measured air temperature, while a very humid day can feel much hotter because sweat evaporates more slowly.
Why humidity matters
Sweating cools the body only when sweat evaporates. When the air is humid, evaporation slows because the air already contains a lot of water vapor. Heat then builds up more easily in the body. This is why two places with the same thermometer reading can feel very different.
How it is calculated
The National Weather Service heat-index formula uses air temperature and relative humidity. Operational charts and calculators convert those two inputs into an apparent temperature. The formula is designed for hot conditions and does not include every factor that affects comfort, such as full sun, wind, clothing, workload, age, or health.
Heat index and warnings
Weather services use heat index values as one input for heat advisories, excessive heat watches, and warnings. Local thresholds vary because people, buildings, infrastructure, and acclimatization differ by region. A heat index that is routine in one climate may be unusually dangerous in another.
Shade, sun, and wind
Heat-index values assume shade and light wind. Direct sunlight can add significant radiant heat to the body, while a breeze can help sweat evaporate. Pavement, buildings, and dark surfaces can also raise local heat exposure, especially in urban heat islands.
Heat index versus wet-bulb temperature
Heat index and wet-bulb temperature both involve humidity, but they are not the same measurement. Heat index estimates how hot conditions feel using air temperature and relative humidity. Wet-bulb temperature measures evaporative cooling potential, while wet-bulb globe temperature adds sun, wind, and radiant heat for heat-stress assessment.
Health and safety uses
High heat-index values can increase the risk of heat cramps, heat exhaustion, heat stroke, dehydration, and work injuries. Public-health guidance often focuses on drinking water, taking breaks, using shade or cooling, checking on vulnerable people, and adjusting outdoor labor, sports, and events during dangerous heat.
Why it matters
Heat index turns a pair of weather numbers into a more human signal. It helps explain why humid heat can be dangerous, why nighttime cooling matters during heat waves, and why climate adaptation has to consider moisture as well as temperature. It is useful, but it works best when paired with local forecasts and common-sense heat safety.