The heat index is a calculated temperature value of how hot it really feels when the amount of moisture in the air is factored in with the actual air temperature outside.
This value is calculated by taking the relative humidity or dewpoint value for a specific location and factoring it into the air temperature reading.
The heat index was created to help better convey the dangers of excessive heat on the human body and relay how hot the current weather conditions are really making it feel to the average person.
The heat index takes into account the not only the science of meteorology, but biology as well.
To help regulate its internal temperature, the human body will sweat and rely on the evaporation of that sweat to help cool down.
However, when there is a lot of moisture in the air -- “it’s very humid outside” -- the evaporation process is not as effective and the body cannot cool down as efficiently.
Depending on how humid it is, the difference between the actual air temperature and the heat index can be rather significant.
Below are the forecasted actual air temperatures for Saturday, June 30th across the eastern US. Much of the region will be in the 90s.
However, when the amount of moisture in the air is factor, it feels much hotter than that to the human body. In many cases 5 to 10 degrees warmer and sometimes more than 15 even 20 degrees warmer if it is very humid.
You can see in this example that the difference is much greater in the central US rather than for cities in the Northeast. This is because the humid air is focused in the center of the country.