Frost Prediction.

—The formation of dew is always attended with a liberation of heat—the heat of vaporization—which tends to check the further decline of temperature. The heat thus developed is usually sufficient to prevent the fall of temperature beyond a very few degrees, but at times when there is little moisture in the air the fall of several degrees of temperature is necessary before the heat liberated by the forming dew balances the heat lost by radiation and the temperature remains stationary.

This condition of things was pointed out many years ago by Tyndall, who in his book on “Heat” states: “The removal for a single summer’s night of the aqueous vapor which covers England would be attended by the destruction of every plant which a freezing temperature would kill.”

The frosts of late spring and early fall which occur at times of dry air and cloudless sky are often caused by local conditions that are not forecasted by the weather department and often may be successfully combated.

At the time of suspected frost, the temperature of the dew-point in relation to the freezing point determines the probability of a freezing temperature. If the dew-point occurs at 10° or more above the freezing point there will be little danger of a killing frost. As the difference in temperature between the dew-point and the frost point decreases, the danger of frost increases. If the dew-point falls at the freezing point, frost is a certainty.

In using the table on [page 214], the open diagonal line may be considered the danger line and any dew-point falling below the temperature thus indicated will be considered dangerously near the frost point. This table differs from the other dew-point table only in the range of temperature. The dew-point is found in exactly the same way as before. In the use of the psychrometer and table as a means of frost prediction it is first necessary to make a reading of the wet-bulb and dry-bulb temperature described above. The dry-bulb reading is found in the left-hand column of the table; then follow the horizontal line opposite the figure, till the perpendicular column is reached indicating the difference in reading between the dry and wet bulb. The number at the meeting will be the temperature of the dew-point. For example, suppose the dry bulb stands at 65° and the wet bulb at 55°, the difference being 10° and dew-point under these conditions will be 47°.

If the dew-point is 10° or more above the freezing point there is no danger of a frost, but if the conditions are such as to give a temperature difference less than 10° above the freezing point there would be danger. If the dew-point falls below the open diagonal line of the table there is danger and that danger increases as the difference in degrees between the freezing point and the dew-point becomes less.

As another illustration, suppose that at sunset at the time of suspected frost the dry-bulb thermometer read 54 and the depression of the wet bulb showed 10°. Referring to the table it will be seen that for these conditions the dew-point falls at 33 which is only 1° above the freezing point. It is highly probable that frost would form.

Dew-point Table for Frost Prediction
Depression of the wet-bulb thermometer

Dry-bulb temp.	1	2	3	4	5	6	7	8	9	10	11	12	13
70	69	67	66	64	62	61	59	57	55	53	51	49	47
69	68	66	64	63	61	59	58	56	54	52	50	48	46
68	67	65	63	62	60	58	57	55	53	51	49	46	44
67	66	64	62	61	59	57	55	54	52	50	47	45	43
66	64	63	61	60	58	56	54	52	50	48	46	44
65	63	62	60	59	57	55	53	51	49	47	45	42	41
64	62	61	59	57	56	54	52	50	48	46	43		40
63	61	60	58	56	55	53	51	49	47	44	42	41	38
62	60	59	57	55	53	52	50	48	45	43		39	37
61	59	58	56	54	52	50	48	46	44	42	41	38	35
60	58	57	55	53	51	49	47	45	43		39	36	33
59	57	56	54	52	50	48	46	44		40	38	43	32
58	56	55	53	51	49	47	45	42	41	39	36	33	30
57	55	54	52	50	48	46	44		40	37	35	31	28
56	54	53	51	49	47	44	42	41	39	36	33	30	26
55	53	52	50	48	46	43		40	37	34	31	28	25
54	52	50	49	46	44	42	41	39	36	33	30	27	23
53	51	49	47	45	43		40	37	34	31	28	25	20
52	50	48	46	44	42	41	38	36	33	30	27	23	18
51	49	47	45	43		40	37	34	31	28	25	21	16
50	48	46	44	42	41	38	36	33	30	27	23	19	14
49	47	45	43		40	37	34	31	28	25	21	17	11
48	46	44	42	41	38	36	33	30	27	23	19	14	9
47	45	43		40	37	35	32	29	25	22	17	12	6
46	44	42	41	39	36	33	30	27	24	20	15	10	3
45	43		40	37	35	32	29	26	22	18	13	7	-1
44	42	41	39	36	33	30	27	24	20	16	11	4	-5
		40	37	35	32	29	26	23	19	14	8	1	-9
43	41	39	36	34	31	28	25	21	17	12	6	-2	-15
42	40	38	35	33	29	26	23	19	15	9	3	-6	-22
41	39	36	34	31	28	25	22	17	13	7	0	-11	-32
40	38	35	33	30	27	24	20	16	11	4	-4	-16	-74
39	37	34	32	29	26	22	18	14	8	2	-8	23
38	36	33	31	28	24	21	17	12	-6	-1	-12	-35