Two days with the same air temperature can feel very different. On one the heat is bearable; on the other, a short walk leaves you drained. The difference is usually humidity, and the heat index is the measure designed to capture it.
Air temperature is only part of the picture
Air temperature is what a thermometer in the shade records. It describes the air, not the person standing in it.
The body's experience of heat depends on how well it can shed the heat it produces. That depends on the air temperature, but also on humidity, wind and direct sunlight.
How the body cools itself
The body's main way of cooling down in hot weather is sweating. Sweat cools the skin when it evaporates, because evaporation takes heat away from the surface.
Relative humidity is the amount of water vapour in the air compared with the most the air can hold at that temperature. When it is high, sweat evaporates slowly, so the body loses heat less effectively even though it keeps sweating.
What the heat index combines
The heat index, used by the US National Weather Service, combines air temperature and relative humidity into one number: an estimate of how hot the conditions feel to the human body. It is often shown in weather apps as the "feels like" temperature.
At the same air temperature, a higher humidity gives a higher heat index. This is why a humid morning can feel hotter than a drier afternoon with a higher reading on the thermometer.
What the heat index does not include
The heat index is calculated for shade and a light wind. Standing in direct sunlight can make conditions feel hotter than the index shows.
It also describes an average adult at rest. Physical work, heavy clothing, age and health all change how much heat a person can tolerate, so the same number does not carry the same risk for everyone.
Reading it for the day ahead
A useful habit is to compare the heat index with the air temperature, not only to read one of them. A large gap means humidity is doing much of the work, and the body will struggle more to cool down than the thermometer suggests.
Plans can then follow the number: outdoor work or exercise moved to the cooler hours, more water and rest in the shade, and extra attention to children, older people and anyone working outside.
When heat becomes a medical problem
Heat illness can become serious quickly. Confusion, very hot skin, a stop in sweating or loss of consciousness are signs of heat stroke, which needs urgent care.
In Thailand, call 1669 for medical emergencies. For heat advisories, follow the Thai Meteorological Department and the Department of Health.