Auto A/C Terms - Glossary
Heat Index

Also referred to as the humidex, the heat index is a number that represents the effect of both the ambient air temperature and the humidity on humans. Remember that humidity in the air makes it feel hotter or warmer than it really is.

The heat index is a number that combines both variable of temperature and relative humidity to provide an apparent temperature. As an example an ambient air temperature of 90 degrees F and a relative humidity of 65 percent would combine to produce a heat index of 102. In other words, with those temperatures and humidity levels, it would feel like 102 degrees F, but it is only 90 degrees F.

Humidity not only effects how we feel, it also adds heat load to an A/C system. In fact, on hot humid days, having a duct outlet temperature of 55 or 60 degrees F could be considered normal for a properly operating A/C system. The increased duct outlet temperatures are a direct result of the added heat load that humidity apples to the A/C system.