Encyclopaedia Britannica, 11th Edition, "Atherstone" to "Austria" / Volume 2, Slice 8

by Various

Table II.—Diurnal Variation Potential Gradient.

Station.	Karasjok.	Sodankylä.	Kew(19, 12).		Greenwich.	Florence.	Perpignan.	Lisbon.	Tokyo.	Batavia.		Cape Horn(20).
Period.	1903-4.	1882-83.	1862- 1864.	1898- 1904.	1893-96.	1883-85.	1886-88.	1884-86.	1897-98 1900-1.	1887- 1890.	1890- 1895.	1882-83.
Days.		All.	All.	Quiet.	All.	All.	Fine.	All.	All.	Dry.	Dry.	Pos.
h l	5.5	3.0 2.5	3.5 1.0	3.35 1.3	3.0 1.8		8.4 1.5	3.0 0.5	1.7 2.0	2	7.8	3.5 2.0
Hour.
1	83	91	87	93	97	92	78	84	101	147	125	82
2	73	85	79	88	89	83	72	80	98	141	114	73
3	66	82	74	84	87	77	71	78	97	135	109	85
4	63	84	72	83	86	75	72	81	99	128	102	81
5	60	89	71	85	86	74	77	83	121	127	101	85
6	68	91	77	93	92	82	92	92	154	137	117	95
7	81	97	92	103	100	100	107	101	167	158	147	106
8	87	100	106	112	102	112	114	105	149	104	119	118
9	94	98	107	115	100	113	111	104	117	67	82	119
10	101	102	100	112	101	107	100	104	87	42	55	123
11	99	98	90	101	96	100	96	102	70	35	46	123
Noon.	103	102	92	94	97	95	99	108	61	30	43	115
1	106	105	90	89	96	92	99	111	54	30	42	112
2	108	107	91	87	94	90	97	114	49	30	43	94
3	108	108	92	88	95	89	99	109	53	33	46	89
4	109	108	98	93	97	89	105	108	61	41	53	88
5	110	108	108	99	102	94	113	108	76	67	73	84
6	119	110	121	108	108	113	126	111	95	91	108	110
7	129	102	134	115	111	121	131	116	107	120	145	107
8	136	111	139	118	115	129	129	114	114	137	155	123
9	139	111	138	119	117	132	120	109	119	146	155	112
10	133	104	128	115	117	127	109	102	120	148	147	99
11	121	108	113	108	111	114	97	92	119	151	143	85
12	102	93	99	99	104	100	86	85	112	147	130	98

Table III.—Diurnal Variation Potential Gradient.

Station.	Karasjok.		Sodankylä.		Kew.			Greenwich.		Bureau Central (21).		Eiffel Tower (21).	Perpignan (21).		Batavia. (2 m.)
Period.	1903-4.		1882-83.		1898-1904.			1894 and ’96.		1894-99.		1896-98.	1885-95.		1887-90.
	Winter.	Summer.	Winter.	Summer.	Winter.	Equinox.	Summer.	Winter.	Summer.	Winter.	Summer.	Summer.	Winter.	Summer.	Winter.	Summer.
Hour.
1	76	104	90	99	91	93	96	87	110	79	102	90	72	88	145	149
2	66	96	79	84	86	88	90	84	101	71	92	83	67	83	139	142
3	57	89	78	90	82	85	85	76	98	70	88	79	66	81	137	135
4	55	83	74	99	81	84	84	77	96	69	84	76	67	83	131	127
5	50	79	74	111	82	87	90	78	94	75	94	78	72	92	132	123
6	61	83	80	114	86	97	101	82	101	83	106	87	84	107	138	136
7	78	89	86	117	95	109	113	94	107	98	118	97	104	114	166	153
8	82	93	95	122	104	118	120	97	111	111	120	103	122	108	118	92
9	90	93	91	109	111	119	119	98	102	113	106	110	126	100	74	64
10	104	93	106	101	114	110	110	102	98	111	94	109	114	93	43	40
11	102	92	98	97	107	95	97	103	86	108	84	107	98	90	35	36
Noon.	119	90	98	100	102	86	87	107	94	106	77	104	99	95	31	30
1	116	94	116	97	99	81	80	107	85	112	79	107	96	93	29	33
2	118	97	113	97	97	80	76	109	82	112	81	110	94	90	28	32
3	119	100	121	93	99	82	76	111	78	111	78	107	95	88	24	41
4	115	99	111	96	103	88	80	116	81	113	80	105	102	92	30	49
5	120	106	105	106	108	96	87	112	93	120	85	106	115	98	60	74
6	131	104	115	92	111	109	98	114	98	124	97	109	128	110	88	94
7	136	110	118	102	114	120	111	117	99	124	123	113	133	122	119	122
8	134	113	117	106	112	124	123	113	108	116	134	110	131	127	138	135
9	137	125	115	90	111	123	129	111	118	104	130	109	124	125	145	147
10	125	135	112	90	108	118	125	110	124	97	122	105	111	117	148	148
11	114	126	113	103	103	109	116	102	120	90	115	101	96	108	149	152
12	96	111	95	85	96	99	105	93	116	83	108	94	83	95	148	146

At the temperate stations the maximum occurs near midwinter; in the Arctic it seems deferred towards spring.

6. Diurnal Variation.—Table II. gives the mean diurnal variation for the whole year at a number of stations arranged in order of latitude, the mean from the 24 hourly values being taken as 100. The data are some from “all” days, some from “quiet,” “fine” or “dry” days. The height, h, and the distance from the wall, l, were the potential is measured are given in metres when known. In most cases two distinct maxima and minima occur in the 24 hours. The principal maximum is usually found in the evening between 8 and 10 P.M., the principal minimum in the morning from 3 to 5 A.M. At some stations the minimum in the afternoon is indistinctly shown, but at Tokyo and Batavia it is much more conspicuous than the morning minimum.

7. In Table III. the diurnal inequality is shown for “winter” and “summer” respectively. In all cases the mean value for the 24 hours is taken as 100. By “summer” is meant April to September at Sodankylä, Greenwich and Batavia; May to August at Kew, Bureau Central (Paris), Eiffel Tower and Perpignan; and May to July at Karasjok. “Winter” includes October to March at Sodankylä, Greenwich and Batavia; November to February at Kew and Bureau Central; November to January at Karasjok, and December and January at Perpignan. Mean results from March, April, September and October at Kew are assigned to “Equinox.”

At Batavia the difference between winter and summer is comparatively small. Elsewhere there is a tendency for the double period, usually so prominent in summer, to become less pronounced in winter, the afternoon minimum tending to disappear. Even in summer the double period is not prominent in the arctic climate of Karasjok or on the top of the Eiffel Tower. The diurnal variation in summer at the latter station is shown graphically in the top curve of fig. 1. It presents a remarkable resemblance to the adjacent curve, which gives the diurnal variation at mid-winter at the Bureau Central. The resemblance between these curves is much closer than that between the Bureau Central’s own winter and summer curves. All three Paris curves show three peaks, the first and third representing the ordinary forenoon and afternoon maxima. In summer at the Bureau Central the intermediate peak nearly disappears in the profound afternoon depression, but it is still recognizable. This three-peaked curve is not wholly peculiar to Paris, being seen, for instance, at Lisbon in summer. The December and June curves for Kew are good examples of the ordinary nature of the difference between midwinter and midsummer. The afternoon minimum at Kew gradually deepens as midsummer approaches. Simultaneously the forenoon maximum occurs earlier and the afternoon maximum later in the day. The two last curves in the diagram contrast the diurnal variation at Kew in potential gradient and in barometric pressure for the year as a whole. The somewhat remarkable resemblance between the diurnal variation for the two elements, first remarked on by J.D. Everett (19), is of interest in connexion with recent theoretical conclusions by J.P. Elster and H.F.K. Geitel and by H. Ebert.