srjskam's site

Usually someone has created and put on the net everything you might possibly want, but there doesn't seem to be a formula for heat index in Celsius yet. So here it is:

HI=
-0.000003582 * RH² * T²
0.0022117 * RH * T²
-0.0123 * T²
0.0007254 * RH² * T
-0.14611 * RH * T
1.611 * T
-0.01643 * RH²
2.34 * RH
-8.9               

where RH is relative humidity in percents, and T is temperature in Celsius. Notice that the formula stops working below ~26 °C and ~40 %.

The above is slightly prettified from the original formula, but is accurate to 0.03 °C, which should be accurate enough, considering the formula is based on subjective impressions of heat.

This corresponds with the original formula:

HI=
-0.000003582 * RH² * T²
+0.002211732 * RH * T²
-0.012308094 * T²
+0.00072546 * RH² * T
-0.14611605 * RH * T
+1.61139411 * T
-0.0164248278 * RH²
+2.3385488389 * RH
-8.7846947556

Yet another formula, that works on values less than 40 % humidity:

HI=
-1.561599e-10 * RH³ * TC³
-7.0770996e-8 * RH² * TC³
+4.6241604e-6 * RH * TC³
-1.2521304e-4 * TC³
-6.8105952e-9 * RH³ * TC²
+1.460390688e-5 * RH² * TC²
+8.68291488e-4 * RH * TC²
+0.0102724812 * TC²
+1.03391224e-7 * RH³ * TC
-2.286195888e-4 * RH² * TC
-0.07376580288 * RH * TC
+0.669176288 * TC
+1.931219745777775e-5 * RH³
-0.005028108526222222 * RH²
+1.428727636266666 * RH
-0.429784071111111

The table is from the simpler formula.

RH (%)
100	28	36	44	54	66	78	92	108	125	143	163
95	28	35	42	51	62	74	87	101	117	134	152
90	28	34	41	49	58	69	81	95	109	125	142
85	28	33	39	47	55	65	76	88	102	116	132
80	28	32	38	44	52	61	71	83	95	108	123
75	27	31	36	42	49	58	67	77	88	101	114
70	27	31	35	40	47	54	63	72	82	94	106
65	27	30	34	39	44	51	59	67	77	87	98
60	27	29	33	37	42	48	55	63	71	81	91
55	27	29	32	36	40	46	52	59	66	75	84
50	27	28	31	34	38	43	49	55	62	69	78
45	26	28	30	33	37	41	46	51	57	64	72
40	26	28	30	32	35	39	43	48	54	60	66
	26	28	30	32	34	36	38	40	42	44	46	T (°C)