Heat Index: It Makes You Feel Even Hotter

Here’s a fact that can make your blood run cold: Heat is the most lethal weather condition, killing more people than tornadoes, hurricanes, floods and lightning combined, according to the National Weather Service.

The heat wave punishing the United States has left people confused about the difference between the actual temperature and the “feels-like” temperature.

Here’s how the NWS explains the phenomenon: “The heat index,  sometimes referred to as the apparent temperature and given in degrees Fahrenheit, is a measure of how hot it really feels when relative humidity is factored with the actual air temperature.

“To find the heat index, look at the Heat Index Chart. As an example, if the air temperature is 96 degrees F (found on the top of the table) and the relative humidity is 65% (found on the left of the table), the heat index — how hot it feels — is 121 degrees F.”



The weather service issues alerts when the heat index is expected to exceed 105 to 110 degrees (depending on local climate) for at least two consecutive days.

The heat index comes into play when the hot temperatures combine with high humidity and foil the body’s attempts to cool down.

When people get hot, the body tries to cool down by moving extra blood to the skin and by sweating. Blood in the tiny vessels near the skin can dissipate heat into the air, if the air is cooler than the body.

But that doesn't work if the air is as hot as the body or hotter. Sweat helps, because when water evaporates it removes heat. But the more moisture already in the air — the higher the humidity — the less evaporation can occur.

Those two processes account for more than 90 percent of the body's ability to dissipate heat, and when they aren't working, trouble can come from heat exhaustion, heat stroke, and even death.