Temperature, Dewpoint, and Humidity Conversions

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The old saying goes, "It's not the heat, it's the humidity". The way the human body reacts to warm temperatures is a function of both the actual temperature, and the moisture in the air. Low humidity lets the body cool off due to evaporation of perspiration. As the humidity rises, less perspiration evaporates, and it becomes more difficult to dissipate the heat.

The heat index is an attempt to measure the apparent temperature, or the way it feels outside. It uses a formula of both actual temperature, and absolute humidity. Generally, when the heat index rises above 90 degrees, people should avoid being outside for too long, and when the heat index is 105 degrees or higher, it's becoming dangerous. Note that the heat index is calculated as an 'in the shade' temperature, so if you are in the sun, it could actually feel warmer than the heat index indicates.

Another way to measure how hot it feels outside is to use the dew point temperature. The dew point temperature is an indicator of the absolute humidity — if the temperature drops to the dewpoint temperature, water vapor condenses to dew. No matter what the actual outside temperature is, most people begin to feel uncomfortable when the dew point approaches 70 degrees, and dewpoints above 70 degrees are opressive.

The table below shows the heat index and dew points for differing combinations of air temperature and relative humidity. To determine the heat index (and corresponding dew point), locate the column with the air temperature, and match it with the approprite row for humidity. For example, if it's 90 degrees and 55% relative humidity, the heat index is 97 degrees, and the dew point is 72 degrees. You'll likely feel uncomfortable. On the other hand, if it's 90, but the relative humidity is 30%, the heat index is only 88, and the dew point is 54. Most people wouldn't find this oppressive.

Keep in mind that different people react to heat and humidity differently, and that there is a real danger when you spend too much time outside during a heat wave.

	80		82		84		86		88
	HI	DP	HI	DP	HI	DP	HI	DP	HI	DP
10%	78	18	80	20	81	22	83	23	84	25
20%	79	35	80	37	84	39	83	40	85	42
30%	79	46	80	48	82	49	84	51	86	53
40%	80	54	81	55	84	57	87	59	89	61
45%	80	57	82	59	84	60	87	62	89	64
50%	81	60	83	62	85	63	88	65	91	67
55%	81	62	84	64	86	66	89	68	93	70
60%	82	65	84	67	88	69	91	70	95	72
65%	82	67	85	69	89	71	93	73	98	75
70%	83	69	86	71	90	73	95	75	100	77
75%	84	71	88	73	92	75	97	77	103	79
80%	84	73	89	75	94	77	100	79	106	81
85%	85	75	90	77	96	79	102	81	110	83
90%	86	77	91	79	98	81	105	83	113	85
95%	86	78	93	80	100	82	108	84	117	86
100%	87	80	95	82	103	84	112	86	121	88

	90		92		94		96		98
	HI	DP	HI	DP	HI	DP	HI	DP	HI	DP
10%	86	26	88	28	89	29	91	31	93	32
20%	86	44	88	45	90	47	93	48	95	50
30%	88	54	90	56	93	58	96	60	99	61
40%	93	62	94	64	97	66	101	68	105	70
45%	93	66	96	68	100	69	104	71	109	73
50%	95	69	99	71	103	73	108	74	113	76
55%	97	72	101	74	106	75	112	77	117	79
60%	100	74	105	76	110	78	116	80	123	82
65%	103	77	108	79	114	80	121	82	128	84
70%	105	79	112	81	119	83	126	85	134	87
75%	109	81	116	83	124	85	132	87
80%	113	83	121	85	129	87
85%	117	85	126	87	135	89
90%	122	87	131	89

	100		102
	HI	DP	HI	DP
10%	95	34	97	35
15%	96	44	98	45			80-90 Degrees -- Caution
20%	97	52	100	53			91-104 Degrees -- Extreme Caution
25%	100	58	103	60			105-125 Degrees -- Danger
30%	102	63	106	65			125+ Degrees -- Extreme Danger
35%	106	68	110	70
40%	109	71	114	73
45%	114	75	119	77
50%	118	78	124	80
55%	124	81	130	83
60%	129	84	137	85
65%	136	86