Ever wondered why one day feels hotter or cooler than the next, even though the temperature is the same on both days? The answer is not as complicated as you might think. There is a formula for working out what the temperature of a given day might actually feel like.
The heat index (HI) is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature. In other words, how hot it feels.
The result is also known as the “felt air temperature” or “apparent temperature”. In Australia, the Bureau of Meteorology uses the term ‘feels like’. For example, when the temperature is 32 °C with very high humidity, the heat index can be about 41 °C. That is why one day at 32 °C can ‘feel’ hotter or cooler than another day at 32 °C.
Screenshot from my personal weather station showing ‘feels like’ temperature. Note the high humidity.
But why? The human body normally cools itself by perspiration, or sweating. Heat is removed from the body by evaporation of that sweat. However, relative humidity reduces the evaporation rate because the higher vapour content of the surrounding air does not allow the maximum amount of evaporation from the body to occur. This results in a lower rate of heat removal from the body, hence the sensation of being overheated. However, it is important to note that this effect is subjective. How one person ‘feels’ might be completely different to another person. The ‘feels like’ temperature measurement system has been based on subjective descriptions of how hot subjects feel for a given temperature and humidity. This results in a heat index that relates one combination of temperature and humidity to another.
This table is from the U.S. National Oceanic and Atmospheric Administration, and has been adapted to reflect temperature in degrees Celsius.
To find the ‘feels like’ temperature, look at the Heat Index chart above. For example, if the air temperature is 36°C and the relative humidity is 65%, the heat index—how hot it feels—is 51°C.
The table below highlights the potential effects of heat on the human body.
27–32 °C Caution: fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps.
32–41 °C Extreme caution: heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke.
41–54 °C Danger: heat cramps and heat exhaustion are likely; heat stroke is probable with continued activity.
over 54 °C Extreme danger: heat stroke is imminent.
As air temperature is usually measured in the shade, it is important to note that exposure to full sunshine can increase heat index values by up to 8 °C.
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