A dangerous heat wave is rolling through much of the western and central U.S., with Texas experiencing triple-digit heat — making it the hottest week of summer so far.

From Amarillo to Austin, the National Weather Service is forecasting temperatures in major cities will not fall below 100 degrees this week, with heat indexes expected to reach as high as 110.

As of July 28, parts of the Texas Panhandle, North Texas, and the state's coastline have been placed under heat advisories throughout the day, with temperatures above 100 and a heat index around 105 to 110.

Here's what you need to know about the heat wave hitting Texas this week.

The temperature is the actual air reading — what a thermometer records in the shade. The heat index, on the other hand, represents what it feels like to the human body when both temperature and humidity are factored in, according to the National Weather Service.

When humidity is high, sweat doesn’t evaporate as easily, making it harder for your body to cool down. That’s why the heat index is often higher than the air temperature, and why it’s used to gauge the risk of heat-related illness.

The National Weather Service issues different alerts depending on how severe and certain the heat threat is, and the thresholds vary by region.

The map below identifies areas where the heat index is forecasted to reach NWS-defined levels from "Caution" (80 degrees Fahrenheit) to "Extreme Danger" (above 125 degrees).

The heat index measures how hot it actually feels when factoring for humidity. Prolonged exposure to a heat index above 80 degrees can lead to fatigue, as previously reported by USA TODAY. As it gets hotter, the risk continues to increase − potentially resulting in heat stroke or even death. Older adults, children, and outdoor workers are generally at higher risk of heat-related incidents.