During the dog days of summer, you might hear the term "heat index." You might already know heat index measures how hot it actually feels outside — but how is it determined?
Heat index is a mathematical formula that combines air temperature and relative humidity to calculate an apparent temperature — how hot it feels. The human body normally cools itself by sweating, as the water in sweat evaporates and carries heat away from the body. But when relative humidity is high, water doesn't evaporate as quickly, so the body retains more heat.
Scientists have created the heat index based on subjective descriptions for various temperatures and humidities. This index relates each temperature and humidity combination to a higher temperature in dry air. In Canada, the similar "humidex" is used.
At higher temperatures, it doesn't take as much humidity for the Heat Index to rise. That explains the saying: It's not the heat, it's the humidity. In reality, it's a little of both.