Heat Index

Heat index or HI is sometimes referred to as the "apparent Temperature". The HI, given in degrees F, is a measure of how hot it feels when relative humidity (RH) is added to the actual air temperature.

Calculate the heat index using the Meteorological calculator Javascript .

                              HEAT INDEX CHART

                              AIR TEMPERATURE (F)

          70   75   80   85   90   95   100  105  110  115  120  125  130

     0    64   69   73   78   83   87   91   95   99   103  107  111  117
     5    64   69   74   79   84   88   93   97   102  107  111  116  122
R    10   65   70   75   80   85   90   95   100  105  111  116  123  131
E    15   65   71   76   81   86   91   97   102  108  115  123  131
L    20   66   72   77   82   87   93   99   105  112  120  130  141
A    25   66   72   77   83   88   94   101  109  117  127  139
T    30   67   73   78   84   90   96   104  113  123  135  148
I    35   67   73   79   85   91   98   107  118  130  143
V    40   68   74   79   86   93   101  110  123  137  151
E    45   68   74   80   87   95   104  115  129  143
     50   69   75   81   88   96   107  120  135  150
H    55   69   75   81   89   98   110  126  142
U    60   70   76   82   90   100  114  132  149
M    65   70   76   83   91   102  119  138
I    70   70   77   85   93   106  124  144
D    75   70   77   86   95   109  130
I    80   71   78   86   97   113  136
T    85   71   78   87   99   117
Y    90   71   79   88   102  122
     95   71   79   89   105
(%)  100  72   80   91   108

To find the heat index value or HI, look at the heat index chart. For example, if the air temperature is 95 degrees fahrenheit (found at the top of the table) and the relative humidity or RH is 55% (found on the left side of the table). The HI, "or how hot it really feels" is 110 degrees Fahrenheit. This is at the intersection of the 95 fahrenheit row and the 55% Column.


Important:

since heat index values were devised for shady, light wind conditions, exposure to full sunshine can increase heat index values by up to 15 degrees fahrenheit.


Note:

heat index values above 105 degrees fahrenheit corresponds to a level that may cause increasingly severe heat disorders with continued exposure and/or physical activity.


The following equation approximates the heat index. There are many assumptions made produce this, far too many to list here. The equation was obtain by multiple regression analysis and there is a +-1.3 degree F error.

HI = -42.379 + 2.04901523T + 10.1433127R - 0.22475541TR - 6.83783x10 -3 T 2 - 5.481717x10 -2 R 2
+ 1.22874x10 -3 T 2R + 8.5282x10 -4 TR 2 - 1.99x10 -6 T 2 R 2

where

T = ambient dry bulb temperature degrees Farhrenheit
R = relative humidity

References
Rothfusz, L. P., 1990:The heat index equation (or, more than you ever wanted to know about heat index). NWS Southern Region Technical Attachment, SR/SSD 90-23, Fort Worth, TX.


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Last modified 05/31/96