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What Is Heat Index?

The Heat Index (HI) gives a measure of how hot it actually feels due to the combined effect of the air temperature and the relative humidity. Hot, humid air actually feels hotter than hot, dry air.

Heat Index Table

The table below gives the heat index for different values of air temperature and relative humidity.

To determine the Heat Index, locate the actual temperature along the top of the table and find the relative humidity along the left side of the chart. The value at the intersection of the column and row is the Heat Index expressed in degrees Fahrenheit.

  Air Temperature (°F)
Relative
Humidity
70° 75° 80° 85° 90° 95° 100° 105° 110° 115°
20%  66°  72°  77° 82° 87° 93° 99° 105° 112° 120°
25%  66°  72°  77° 83° 88° 94° 101° 109° 117° 127°
30%  67°  73°  78° 84° 90° 96° 104° 113° 123° 135°
35%  67°  73°  79° 85° 91° 98° 107° 118° 130° 143°
40%  67°  74°  79° 86° 93° 101° 110° 123° 137° 151°
45%  68°  74° 80° 87° 95° 104° 115° 129° 143°  
50%  69°  75° 81° 88° 96° 107° 120° 135° 150°  
55%  69°  75° 81° 89° 98° 110° 126° 142°    
60%  70°  76° 82° 90° 100° 114° 132° 149°    
65%  70°  76° 83° 91° 102° 119° 138°      
70%  70°  77° 85° 93° 106° 124° 144°      
75%  70°  77° 86° 95° 109° 130°        
80%  71°  78° 86° 97° 113° 136°        
85%  71°  78° 87° 99° 114°          
90%  71°  79° 88° 102° 122°          
95%  71°  79° 90° 105°            
100%  72° 80° 91° 106°            

 

Key:

  Category Heat Index Possible effects
  Caution 80 - 90°F
(27 - 32°C)
Fatigue possible with prolonged exposure and/or physical activity.
  Extreme Caution 90 - 105°F
(32 - 41°C)
Sunstroke, muscle cramps, and/or heat exhaustion possible with prolonged exposure and/or physical activity.
  Danger 105 - 129°F
(41 - 54°C)
Sunstroke, muscle cramps, and/or heat exhaustion likely. Heatstroke possible with prolonged exposure and/or physical activity.
  Extreme Danger 130°F or higher (54°C or higher) Heat stroke or sunstroke likely.

 

Find out about:

Wind Chill
Beaufort wind scale