Heat Index

How to read the chart: Find the temperature on the left hand side, then move to the right until you find the column for the approximate relative humidity. That number will be the temperature that it will "feel" like. For example, a temperature of 95°F and relative humidity of 50% will "feel" like 107°. Add up to 15° if in the direct sun.

Heat Index Chart
Temperature (°F) vs. Relative Humidity
  10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 80%
115 111 115 120 127 135 143 151
110 105 108 112 117 123 130 137 143 151
105 100 102 105 109 113 118 123 129 135 142 149
100 95 97 99 101 104 107 110 115 120 126 132 136 144
95 90 91 93 94 96 98 101 104 107 110 114 119 124 130 136
90 85 86 87 88 90 91 93 95 96 98 100 102 106 109 113
85 80 81 82 83 84 85 86 87 88 89 90 91 93 95 97
80 75 76 77 77 78 79 79 80 81 81 82 83 85 86 86
75 70 71 72 72 73 73 74 74 75 75 76 76 77 77 78
Heat Index/Heat Disorders
Heat Index Possible heat disorders for people in higher risk groups
130 or higher Heatstroke/sunstroke highly likely with continued exposure.
105-130 Sunstroke, heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity.
90-105 Sunstroke, heat cramps and heat exhaustion possible with prolonged exposure and/or physical activity.
80-90 Fatigue possible with prolonged exposure and/or physical activity.
Source: National Weather Service

Related Links
The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science – R.G. Steadman
The Assessment of Sultriness. Part II: Effects of Wind, Extra Radiation, and Barometric Pressure on Apparent Temperature – R.G. Steadman
Heat Index “Equation” (or, More Than You Ever Wanted to Know About Heat Index) – Lans P. Rothfusz, NWS Technical Attachment SR 90-23 (PDF 15KB 2 pages)
Heat Index Calculator – National Weather Service, El Paso, Texas
Thermal Comfort Observations – Australia Bureau of Meteorology