The criteria for a heat advisory varies across the country, but generally refers to situations where the afternoon heat index values will exceed 105 degrees in addition to overnight lows not dropping below the upper 70s. This creates a 24-hour period of warm and humid conditions that could quickly lead to heat-related illnesses, including heat exhaustion and heat stroke.
Remember to drink plenty of water, wear light clothing and try to avoid being outdoors for a long period of time. The heat index, like the temperature, is taken in the shade. You can add 15 degrees to the heat index in direct sunlight!