Our bodies dissipate heat by varying the rate of blood circulation, losing water through the skin and sweat glands—and, as a last resort, by panting—when the blood is heated above 98.6 degrees Fahrenheit (37 degrees Celsius), the average body temperature. Sweating cools the body through evaporation. You can get the same feeling when you put alcohol on your skin, because as the alcohol evaporates, the skin is cooled.

The Heat Index (HI) is an index that combines air temperature and relative humidity to estimate how hot it actually feels. It is based on a mathematical concept called the heat index equation, a long equation that includes the dry air temperature, relative humidity (in percent form), and many biometeorological factors too long to list here. The resulting heat index table represents the apparent, or “feels like,” temperature. For example, if the air temperature is 90 degrees Fahrenheit, with the relative humidity at 60 percent, it will feel like 100 degrees Fahrenheit.

Why do meteorologists want people to pay attention to the heat index? The major reason involves how the body responds to high heat-value numbers: If the relative humidity is high, it curtails evaporation on the skin, and the body is unable to effectively cool itself (and a person will perceive that the air is warmer). When heat index values grow higher, conditions exceed the level a body can remove heat, causing the body temperature to rise. This can cause heat-related illnesses, such as sunstroke or heat exhaustion. For example, according to the United States National Weather Service, exposure to direct sunlight can increase the HI by up to 15 degrees Fahrenheit (9.4 degrees Celsius). And when a heat index between a mere 90 degrees Fahrenheit (32.2 degrees Celsius) to 105 degrees Fahrenheit (40.6 degrees Celsius) can cause possible sunstroke, heat exhaustion, and heat cramps, it is easy to see the meteorologists’ concerns.

The following table shows how the heat we actually experience changes with temperature and humidity (humidity is expressed as a percentage; temperatures are in degrees Fahrenheit).

According to the National Weather Service, sunstroke, heat cramps, and heat exhaustion are possible above 90 degrees Fahrenheit; temperatures above 105 degrees can also lead to heat stroke; and above 130 degrees heat stroke is likely if exposure to such temperatures is prolonged.