Dew-point.

—Dew is formed whenever falling temperature of the air passes the point where saturation occurs. The reduction of the temperature of air raises the relative humidity because of the diminished capacity to contain moisture. As the temperature declines there will come a point at which the air is saturated and any further decrease of temperature will cause supersaturation. At this point the moisture will be deposited on the cooler surfaces in the form of drops. The temperature at which dew begins to form is known as the dew-point. The sweating of cold water pipes, the dew that forms on a water glass and other relatively cold surfaces is caused by a temperature below the dew-point of the air.

Dew-point Table
Dew-point in degrees Fahrenheit, barometer pressure 29 inches

Air temp. t	Vapor press. e	Depression of wet-bulb thermometer (t-t´)
Air temp. t	Vapor press. e	1.0	2.0	3.0	4.0	5.0	6.0	7.0	8.0
30	0.164	27	25	22	18	14	9	+3	-5
31	0.172	29	26	23	20	16	11	5	-2
32	0.180	30	27	24	21	17	13	8	+1
33	0.187	31	28	25	22	19	15	10	3
34	0.195	32	29	27	24	20	16	12	6
35	0.203	33	30	28	25	22	18	14	8
36	0.211	34	31	29	26	23	20	15	11
37	0.219	35	32	30	27	24	21	17	13
38	0.228	36	33	31	28	26	23	19	14
39	0.237	37	34	32	29	27	24	21	16
40	0.247	38	35	33	31	28	25	22	18
41	0.256	39	37	34	32	29	26	23	20
42	0.266	40	38	35	33	30	28	25	21
43	0.277	41	39	36	34	31	29	26	23
44	0.287	42	40	38	35	32	30	27	24
45	0.298	43	41	39	36	34	31	29	26
46	0.310	44	42	40	37	35	32	30	27
47	0.322	45	43	41	39	36	34	31	28
48	0.334	46	44	42	40	37	35	32	30
49	0.347	47	45	43	41	39	36	34	31
50	0.360	48	46	44	42	40	37	35	32
51	0.373	49	47	45	43	41	39	36	34
52	0.387	50	48	46	44	42	40	37	35
53	0.402	51	49	47	45	43	41	39	36
54	0.417	52	50	49	47	44	42	40	38
55	0.432	53	52	50	48	46	43	41	39
56	0.448	54	53	51	49	47	45	43	40
57	0.465	55	54	52	50	48	46	44	42
58	0.482	56	55	53	51	49	47	45	43
59	0.499	57	56	54	52	50	48	46	44
60	0.517	58	57	55	53	51	49	47	45
61	0.536	59	58	56	54	52	51	49	46
62	0.555	60	59	57	55	54	52	50	48
63	0.575	61	60	58	56	55	53	51	49
64	0.595	62	61	59	58	56	54	52	50
65	0.616	63	62	60	59	57	55	53	51
66	0.638	64	63	61	60	58	56	54	53
67	0.661	65	64	62	61	59	57	56	54
68	0.684	67	65	63	62	60	58	57	55
69	0.707	68	66	64	63	61	60	58	56
70	0.732	69	67	66	64	62	61	59	57
71	0.757	70	68	67	65	63	62	60	58
72	0.783	71	69	68	66	65	63	61	60
73	0.810	72	70	69	67	66	64	62	61
74	0.838	73	71	70	68	67	65	64	62
75	0.866	74	72	71	69	68	66	65	63
76	0.896	75	73	72	70	69	67	66	64
77	0.926	76	74	73	71	70	68	67	65
78	0.957	77	75	74	72	71	69	68	66
79	0.989	78	76	75	73	72	70	69	67
80	1.022	79	77	76	75	73	72	70	69

Air temp. t	Vapor press. e	Depression of wet-bulb thermometer (t-t´)
Air temp. t	Vapor press. e	9.0	10.0	11.0	12.0	13.0	14.0	15.0	16.0
30	0.164	-20
31	0.172	-14	-50
32	0.180	-9	-29
33	0.187	-5	-20
34	0.195	-2	-14	-50
35	0.203	+1	-8	-28
36	0.211	4	-4	-19
37	0.219	7	-1	-12	-44
38	0.228	9	+3	-7	-25
39	0.237	12	6	-3	-16
40	0.247	14	8	+1	-10	-35
41	0.256	16	11	4	-5	-21
42	0.266	17	13	7	-1	-13	-59
43	0.277	19	15	10	+3	-7	-28
44	0.287	21	17	12	6	-2	-17
45	0.298	22	19	14	8	+2	-9	-37
46	0.310	24	20	16	11	5	-4	-20
47	0.322	25	22	18	13	8	+0	-12	-53
48	0.334	27	23	20	15	10	+4	-6	-25
49	0.347	28	25	21	17	13	7	-2	-15
50	0.360	29	27	23	19	15	9	+2	-8
51	0.373	31	28	25	21	17	12	6	-3
52	0.387	32	29	26	23	19	14	9	+1
53	0.402	34	31	28	24	21	16	11	5
54	0.417	35	32	29	26	23	19	14	8
55	0.432	36	34	31	28	24	21	16	11
56	0.448	38	35	32	29	26	23	19	14
57	0.465	39	36	34	31	28	24	21	16
58	0.482	40	38	35	32	29	26	22	18
59	0.499	42	39	37	34	31	28	24	20
60	0.517	43	41	38	35	32	29	26	22
61	0.536	44	42	39	37	34	31	28	24
62	0.555	46	43	41	38	35	32	30	26
63	0.575	47	45	42	40	37	34	31	28
64	0.595	48	46	44	41	38	36	33	30
65	0.616	49	47	45	43	40	37	34	31
66	0.638	51	48	46	44	42	39	36	33
67	0.661	52	50	48	45	43	40	38	35
68	0.684	53	51	49	47	44	42	39	36
69	0.707	54	52	50	84	46	43	41	38
70	0.732	55	53	51	49	47	45	42	40
71	0.757	57	55	53	51	49	46	44	41
72	0.783	58	56	54	52	50	48	45	43
73	0.810	59	57	55	53	51	49	47	44
74	0.838	60	58	56	54	53	50	48	46
75	0.866	61	60	58	56	54	52	50	47
76	0.896	62	61	59	57	55	53	51	49
77	0.926	64	62	60	58	56	54	52	50
78	0.957	65	63	61	59	58	56	54	52
79	0.989	66	64	62	61	59	57	55	53
80	1.022	67	65	64	62	60	58	56	54

The temperature at which dew forms will depend on the amount of moisture present in the air, but with a definite humidity and air pressure it will always occur at the same temperature. If the dew-point is above freezing, the dew will form as drops of water, but if it is at or slightly below the freezing point, the dew will appear as frost. White frost is formed when the dew-point is only a few degrees below the freezing point. A Black frost occurs when the atmospheric humidity is so low that dew does not form until the temperature is much below the freezing point.