Heat Index

On warm summer days, it can feel stifling and seem much hotter than what the thermometer indicates. This sensation is caused by the combination of temperature and humidity. While the term "wind chill" is used during cold periods, taking into account the air temperature and wind speed, the heat index is used during warmer periods.

Due to the combination of warm and humid air, we experience heat more intensely than it actually is. The temperature our body perceives is indicated by the heat index (also known as HI or Humiture). A similar measure is the Humidex used in Canada, which combines temperature and dew point, often resulting in higher values.

When the temperature rises above 30 degrees Celsius, the human body struggles to dissipate heat. This is only possible through perspiration, which cools the body by evaporating sweat. However, when the humidity is very high, our sweat evaporates less efficiently, making it harder for the body to release heat and causing heat to remain trapped inside the body.

When we are physically active in such conditions (high temperature + high humidity), for example during work or sports, there is a risk of the body overheating. This can be particularly dangerous for the elderly and individuals with poor health. The KNMI (Royal Netherlands Meteorological Institute) uses a table for the heat index (developed by Robert Steadman, who is also known for the wind chill/feels-like temperature concept). The heat index is based on low wind conditions and shaded environments. In direct sunlight, the heat index can be up to 10 degrees Celsius higher.

With a heat index between 25 and 30 degrees Celsius, accelerated fatigue may occur. During work and sports activities, frequent breaks should be taken, and direct sunlight should be avoided.

With a heat index between 30 and 40 degrees Celsius, there is a risk of heatstroke, cramps, and/or dehydration. Physical exertion should be minimized, plenty of water should be consumed, and sun exposure should be avoided.

With a heat index between 40 and 55 degrees Celsius, the risk of heatstroke, cramps, and/or dehydration increases significantly. Physical activity should be avoided, plenty of water should be consumed, and sufficient rest should be taken. Try to stay in the shade as much as possible.

With a heat index above 55 degrees Celsius, there are serious health risks.