A weather app may report an air temperature of 35°C while saying it “feels like” more than 40°C. The higher number is not a second thermometer reading. It estimates how temperature and humidity affect the human body's ability to cool itself.
One widely used measure is the heat index. It combines shade temperature with relative humidity to describe the strain a typical person may experience under specified conditions.
Sweat cools only when it evaporates
The body moves heat toward the skin and produces sweat. When that moisture evaporates, it carries heat away. Dry air can accept more water vapour, so evaporation happens efficiently.
Humid air is already holding a large amount of moisture. Sweat then evaporates more slowly, and the body retains more heat. This is why the same temperature can feel much more uncomfortable on a humid day.
How the heat index is calculated
Meteorological agencies use equations or tables based on temperature and relative humidity. The calculation models human heat balance rather than measuring an object outdoors.
Different countries may use different formulas, thresholds and labels. Some publish a humidex, apparent temperature or wet-bulb globe temperature instead. Values should therefore be read with the definition provided by the issuing agency.
Sunlight can make conditions worse
Standard heat-index values usually assume shade and light wind. Direct sunlight adds radiant heat and can make the body experience substantially harsher conditions.
Dark surfaces, concrete, vehicles and poorly ventilated buildings also store heat. A city street or factory floor can be more stressful than the official reading from a shaded weather station.
Wind is not always cooling
A breeze helps sweat evaporate, but extremely hot wind can add heat when the air is warmer than the skin. Fans move air; they do not lower air temperature.
During severe heat, agencies may warn that fans alone are insufficient, particularly for older adults and people with medical conditions. Air-conditioned or actively cooled spaces offer greater protection.
Who faces higher risk?
Age, health, medication, pregnancy, clothing, physical work and lack of acclimatisation change how a person responds. Infants, older adults, outdoor workers and people without reliable cooling can face greater danger.
The heat index is not a personal diagnosis. Two people in the same weather may experience different levels of strain.
Warning signs should not be ignored
Heavy sweating, weakness, headache, nausea and dizziness can indicate heat exhaustion. Confusion, loss of consciousness, seizures or very high body temperature can signal heat stroke, a medical emergency.
Move the person to a cooler place, begin rapid cooling and contact local emergency services when severe symptoms appear. Do not wait for the weather number to cross a particular threshold.
Practical ways to reduce exposure
Check local alerts, reschedule strenuous activity, drink water regularly and take breaks in shade or cooling. Light, breathable clothing can help. Never leave a child, older adult or animal in a parked vehicle.
Night-time temperature matters because the body needs a chance to recover. Several hot nights in a row can increase risk even when daytime readings seem familiar.
The bottom line
The heat index explains why humid heat can be more dangerous than the thermometer suggests. It estimates reduced evaporative cooling under shade conditions, while sunlight, buildings, work and health can raise real exposure further. Follow official local warnings and respond to symptoms, not only a single number. Find more clear guides in Explainers and read how emergency phone alerts work.
