Final answer:
The Heat Index (HI) is calculated by combining air temperature with relative humidity, as humidity affects how hot the air feels by inhibiting sweat evaporation from the skin.
Step-by-step explanation:
The Heat Index (HI), also known as the "feels like" temperature, is calculated by combining the air temperature with relative humidity. This measure helps to describe how hot it truly feels, taking into account the moisture content of the air. People tend to feel much hotter on humid days than on dry days because the high moisture content in the air hinders the evaporation of sweat from the skin, which is our body's natural way to cool down. Hence, the correct factor used in the calculation of the Heat Index alongside air temperature is B) Humidity. On humid days, when the relative humidity is high, evaporation is inhibited, which means our bodies cannot cool off as effectively. This phenomenon explains why high humidity can make us feel more uncomfortable at the same temperature compared to days with lower humidity. The relative humidity plays a critical role in determining how hot the air truly feels to us, leading to the coined phrase "it's not the heat, it's the humidity."