The Heat Index (HI) is sometimes referred to as the "apparent temperature" and is a measure of how hot it feels outside to the human body. The HI includes the influence of both the actual air temperature and relative humidity. As either value increases, the apparent temperature also increases. The reason why the apparent temperature increases with relative humidity is due to how the body cools itself. The body dissipates almost 90% of its heat through skin perspiration (sweat). Sweating by itself does nothing to cool the body unless the water is removed by evaporation--and high relative humidity hinders evaporation.
To figure out the HI, reference the Heat Index Chart below and find the intersection of the air temperature and relative humidity. The shaded zones on the chart correspond to the probabilities of developing heat-related disorders. It is important to note that the HI values were devised for shady, light wind conditions, thus, exposure to full sunshine can increase HI values by up to 15 degrees.
Heat disorders are generally a result of the body's inability to shed excess heat by sweating or a chemical (salt) imbalance caused by too much sweating. When heat gain exceeds the level the body can remove, or when the body cannot compensate for fluids and salt lost through perspiration, the temperature of the body's inner core begins to rise and heat-related illness may develop. The table below explains the risk to the body from continued exposure to excessive heat and is color coded to match the HI chart above.
|Heat Index/Apparent Temperature (°F)||Possible Heat Disorders for People in High Risk Groups|
|130°F or Higher||Heat/Sunstroke HIGHLY LIKELY with continued exposure|
|105°F - 130°F||Sunstroke, heat cramps, or heat exhaustion LIKELY, and heatstroke POSSIBLE with prolonged exposure and/or physical activity|
|90°F - 105°F||Sunstroke, heat cramps, or heat exhaustion POSSIBLE with prolonged exposure and/or physical activity|
|80°F - 90°F||Fatigue POSSIBLE with prolonged exposure and/or physical activity|
|Mean Heat Index||Maximum Heat Index||Minimum Heat Index|
|Additional Forecasts||Additional Forecasts||Additional Forecasts|