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Heat Index

What is Heat Index?

Heat Index uses temperature and relative humidity to determine how hot the air actually "feels." When humidity is low, the apparent temperature will be lower than the air temperature, since perspiration evaporates rapidly to cool the body. However, when humidity is high (i,e., the air is saturated with water vapor) the apparent temperature "feels" higher than the actual air temperature, because perspiration evaporates more slowly.

Formula:

The formula for heat index is based upon the lookup table presented by Steadman (1979). This table was derived with a complicated set of measurements. The table can be mathematically analyzed to produce an approximate equation for the heat index:

Heat Index = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783x10-3T2 - 5.481717x10-2R2 + 1.22874x10-3T2R + 8.5282x10-4TR2 - 1.99x10-6T2R2
Where:
T = ambient dry bulb temperature
R = relative humidity

 Steadman Heat Index Table Relative Humidity (%) Temp °F 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 110 100.3 102.1 104.6 108.0 112.0 116.8 122.3 128.6 135.7 109 99.7 101.3 103.6 106.6 110.4 114.9 120.1 126.0 132.7 108 99.2 100.5 102.6 105.4 108.8 113.0 117.9 123.5 129.8 136.7 107 98.6 99.7 101.6 104.1 107.3 111.2 115.7 121.0 126.9 133.5 106 98.0 98.9 100.6 102.8 105.8 109.4 113.6 118.6 124.2 130.4 137.3 105 97.4 98.2 99.6 101.6 104.3 107.6 111.6 116.2 121.5 127.4 133.9 104 96.8 97.4 98.6 100.4 102.9 106.0 109.6 114.0 118.9 124.4 130.6 137.4 103 96.1 96.6 97.6 99.2 101.5 104.3 107.7 111.7 116.4 121.6 127.4 133.8 102 95.5 95.8 96.6 98.1 100.1 102.7 105.9 109.6 113.9 118.8 124.3 130.3 136.9 101 94.8 95.0 95.7 97.0 98.8 101.1 104.1 107.5 111.5 116.1 121.2 126.9 133.1 100 94.1 94.2 94.8 95.8 97.5 99.6 102.3 105.5 109.3 113.5 118.3 123.6 129.5 135.9 99 93.5 93.4 93.8 94.8 96.2 98.2 100.6 103.6 107.0 111.0 115.5 120.5 126.0 132.0 138.5 98 92.8 92.6 92.9 93.7 95.0 96.7 99.0 101.7 104.9 108.6 112.8 117.4 122.6 128.2 134.3 97 92.0 91.8 92.0 92.7 93.8 95.4 97.4 99.9 102.9 106.3 110.2 114.5 119.3 124.5 130.3 136.4 96 91.3 91.0 91.1 91.6 92.6 94.0 95.9 98.2 100.9 104.0 107.6 111.7 116.1 121.0 126.4 132.1 138.3 95 90.6 90.2 90.2 90.6 91.5 92.7 94.4 96.5 99.0 101.9 105.2 108.9 113.1 117.6 122.6 128.0 133.8 140.0 94 89.8 89.4 89.3 89.7 90.4 91.5 93.0 94.9 97.2 99.8 102.9 106.3 110.2 114.4 119.0 124.0 129.4 135.2 141.3 93 89.0 88.6 88.5 88.7 89.3 90.3 91.6 93.4 95.4 97.9 100.7 103.8 107.4 111.3 115.5 120.1 125.1 130.5 136.2 142.3 92 88.3 87.8 87.6 87.8 88.3 89.2 90.4 91.9 93.8 96.0 98.5 101.4 104.7 108.3 112.2 116.4 121.0 126.0 131.3 136.9 91 87.5 86.9 86.7 86.9 87.3 88.0 89.1 90.5 92.2 94.2 96.5 99.2 102.1 105.4 109.0 112.9 117.1 121.6 126.5 131.7 137.1 90 86.6 86.1 85.9 86.0 86.3 87.0 87.9 89.2 90.7 92.5 94.6 97.0 99.7 102.7 105.9 109.5 113.3 117.5 121.9 126.6 131.6 89 85.8 85.3 85.1 85.1 85.4 86.0 86.8 87.9 89.3 90.9 92.8 94.9 97.4 100.0 103.0 106.2 109.7 113.5 117.5 121.8 126.3 88 85.0 84.5 84.3 84.3 84.5 85.0 85.7 86.7 87.9 89.4 91.0 93.0 95.1 97.6 100.2 103.1 106.3 109.6 113.2 117.1 121.2 87 84.1 83.7 83.5 83.5 83.7 84.1 84.7 85.6 86.6 87.9 89.4 91.1 93.1 95.2 97.6 100.2 102.9 106.0 109.2 112.6 116.3 86 83.3 82.9 82.7 82.7 82.8 83.2 83.8 84.5 85.4 86.6 87.9 89.4 91.1 93.0 95.1 97.3 99.8 102.5 105.3 108.3 111.5 85 82.4 82.1 81.9 81.9 82.0 82.4 82.9 83.5 84.3 85.3 86.5 87.8 89.3 90.9 92.7 94.7 96.8 99.1 101.6 104.2 107.0 84 81.5 81.2 81.1 81.1 81.3 81.6 82.0 82.6 83.3 84.1 85.1 86.3 87.5 88.9 90.5 92.2 94.0 95.9 98.0 100.3 102.7 83 80.6 80.4 80.3 80.4 80.6 80.8 81.2 81.7 82.3 83.1 83.9 84.9 85.9 87.1 88.4 89.8 91.3 92.9 94.7 96.5 98.5 82 79.7 79.6 79.6 79.7 79.9 80.1 80.5 80.9 81.5 82.1 82.8 83.6 84.4 85.4 86.4 87.6 88.8 90.1 91.5 93.0 94.5 81 78.7 78.8 78.8 79.0 79.2 79.5 79.8 80.2 80.7 81.2 81.7 82.4 83.1 83.8 84.6 85.5 86.4 87.4 88.5 89.6 90.8 80 77.8 77.9 78.1 78.3 78.6 78.9 79.2 79.5 79.9 80.3 80.8 81.3 81.8 82.4 83.0 83.6 84.2 84.9 85.6 86.4 87.2 With Prolonged Exposure and/or Physical Activity: Extreme Danger: Heat Stroke or Sunstroke likely Danger: Sunstroke, muscle cramps, and/or heat exhaustion likely Extreme Caution: Sunstroke, muscle cramps, and/or heat exhaustion possible Caution: Fatigue possible

The Davis implementation simply extends the range of use of this table to make it usable at temperatures beyond the scope of the table. The formula Davis uses is also used by the US National Weather Service. Heat Index can also be used to determine indoor comfort levels. Note, Heat Index has also been referred to as "Temperature-Humidity Index" and "Thermal Index" in some Davis products

REFERENCE

Steadman, R.G., 1979: The Assessment of Sultriness, Part 1: A Temperature-Humidity Index Based on Human Physiology and Clothing Science. Journal of Applied Meteorology, July 1979

"Media Guide to NWS Products and Services", National Weather Service Forecast Office, Monterey, CA, 1995.

 The Exploratorium would like to thank Davis Instruments for their generous support in setting up this weather station.