The heat index (or apparent temperature) is a metric indicating the perceived heat the human body experiences when high temperatures combine with elevated relative humidity. Unlike wind chill, which applies to cold, the heat index measures the combined effect of heat and humidity on the body's ability to cool itself through sweating. The physiological principle is straightforward: when it is hot, the body cools itself by evaporating sweat from the skin. If ambient humidity is high, sweat evaporates more slowly, reducing cooling capacity. At 35 °C with 30% humidity, the heat index is 36 °C (barely noticeable); but at 35 °C with 70% humidity, the index rises to 48 °C, an extreme danger level. The NWS heat index table defines four categories: caution (27–32 °C), extreme caution (32–40 °C), danger (40–54 °C), and extreme danger (>54 °C). The heat index is particularly relevant on Mediterranean coasts during heat waves: the combination of 35–40 °C temperatures with sea-borne humidity (60–80%) can generate heat indices exceeding 50 °C, potentially lethal conditions for vulnerable individuals. Interior regions, though reaching higher absolute temperatures, typically have low humidity (dry solano wind), which paradoxically results in lower heat indices.