How to use a heat index chart: Locate today’s predicted high temperature across the top column (Air Temp.). Down the left side (Relative Humidity), locate today’s predicted humidity. Follow across and down to find the “Apparent Temperature”. Keep in mind, this heat index chart was devised for shady, light wind conditions, so exposure to full sun can increase values by up to 15°F. Strong winds, particularly with hot, dry air can be extremely hazardous.