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CHAPTER VII.

TEMPERATURE-ITS RELATION TO ATMOSPHERIC PRESSURE.

279. The relation which exists between the temperature of a portion of the earth's surface and the atmospheric pressure at that place compared with the atmospheric pressure of neighbouring regions at the same time is all-important, especially in its bearings on many questions affecting the practical business of life. The relation is a simple one, and admits of clear illustration. The examples will be taken from the recent numbers of the Journal of the Scottish Meteorological Society, and almost all of them will be selected from the weather of 1867—a year so remarkable for illustrations of violent alternations of heat and cold, drought and rain, brilliant sunshine and the dullest weather, as well as for the many great storms which have swept all parts of the globe, that it may well be regarded as the annus mirabilis of meteorology.

280. Examples 1 and 2.-During the severe frost which prevailed in Great Britain from the 1st to the 21st of January 1867, the following are the mean pressures, reduced to 32° and sea-level, at different places from the English Channel to Iceland for these three weeks :

Inches.
Reykjavik, Iceland,

30.262
Thorshavn, Faro,

29.941 Inverness, .

29.758 Edinburgh,

29.692 Jersey,

29.604 Now, since air flows from the region of high to that of low

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pressure, these figures show at a glance the immediate cause of the singularly low temperature which prevailed; for it is plain that Great Britain was then in the stream of a powerful polar current descending from Iceland over Western Europe. The geographical distribution of the cold is instructive. Thus in Orkney the temperature of the month was 69.9 below the average, whilst on the Solway Firth it was only 4°.2; on the Moray Firth it was 89.4, but on the Firth of Forth it was only 4°.6 below the average, and at Jersey 3o. Hence places nearer the source of the cold suffered a greater depression of temperature than places further south. The wind during the time blew from the N., N.E., and E. ten days more than the average of the month. The same principle is illustrated by the equally severe weather of March 1867. From the 15th to the 18th, during which the severity of the cold was greatest, the pressure in Iceland was 30.191 inches; in the south of Norway, 29.941 inches; in Orkney, 29.943 inches; and at Brest, in France, 29.619 inches; and as a consequence, the mean temperature of Scotland fell to 299.7, being about 11° below the average temperature of the season-a degree of cold which, in our equable climate, is happily of rare occurrence in this or any other season.

281. Example 3.—During the mild weather of February 1867 the following were the mean pressures :

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Thus during the month there prevailed over this part of Europe a remarkably strong equatorial current, bringing over Great Britain the warmth of southern latitudes. The mean temperature of Scotland for the month rose to 4° above the average, being absolutely the highest mean temperature for

February hitherto recorded. The winds, it need scarcely be added, were almost wholly from the S., S.W., and W. The greatest excess of temperature, 4o.7, occurred in the south, and the least, 2°.6, in the extreme north of Scotland. The mildness of the month was all the more remarkable from its being immediately preceded and followed by the almost unprecedentedly cold weather already described.

282. Example 4.-During April 1867 the following were the mean pressures :

Inches.
Stykkisholm, Iceland,

29.657
Thorshavn, Faro,

29.522 North Unst, Shetland,

29.521 Sandwick, Orkney,

29.520 Dundee,

29.588 Silloth,

29.640 Jersey,

29.937

Here it will be observed that there was not, as in the previous examples, a regular decrease of atmospheric pressure all the way from south to north, or vice versa, but that the lowest mean pressure occurred over a somewhat broad area, extending from Orkney to Faro, south and north of which the pressure rose. To the north, the temperature was below the average, to the extent of 5° in Iceland, and 2° at Fari; whereas to the south the temperature was above the average, being 2.6 at Jersey, and 10.8 in the south of Scotland, from which it diminished northward to 0°.7 in Orkney. Over the region of low pressure a good deal more rain fell than fell to the north and to the south of it. It is under such circumstances that long-continued rains take place—viz., in an area of low pressure having a higher pressure on each side of it; on the northern side the rain is cold and the weather raw, and on the southern side the weather is close and

warm.

283. Examples 5 and 6.The fine weather of November 1867, and the warm weather of September 1865.—During these two months the pressures were as under :

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It will be seen that during November 1867, which may be regarded as one of the finest Novembers on record, the pressure over the British Islands was unusually high, and varied little from the Channel to Iceland, but that such variation as was, showed a gradual increase on proceeding southwards. Hence the gradual movement of the whole atmosphere from south to north kept up the temperature, and frost, which might have been expected at the season, with the still atmosphere which prevailed at the time, rarely occurred—its occurrence being checked, no doubt, by the amount of vapour in the air brought by the southerly current. If this state of things had occurred in summer, when the sun's heat is great, we should have experienced a tract of the finest summer weather, accompanied by great heat. This is what actually took place during September 1865 (see the above barometric readings). On the fortnight ending the 16th, during which the barometric relations were most completely fulfilled, the mean temperature rose above the average 59.3 in Orkney, 89.5 at Belfast, 70.3 at Aberdeen, 8° at Manchester, 89.6 at Yarmouth, 6o.9 at Brussels, and 10°.3 at Paris.

284. An important and vital distinction must be here drawn. When high pressures, with little variation, prevail over the British Islands, the temperatures which accompany them are determined by this circumstance-viz., if the pressure increases, however little, towards the south, so as to appear to issue from or have its origin in that quarter, the temperature will be relatively high ; but if, on the other hand, the

high pressures increase as we go northwards, then the temperature will be low. Thus, if during November 1867 the order of the pressures had been reversed—that is, if instead of becoming lower in advancing northward they had got higher -severe frosts would no doubt have been the consequence.

285. It is the general relation of the barometric pressure to that of surrounding regions which gives to the climate of Great Britain its distinctive features. These general relations during January and July may be seen on examining Plates I. and II., from which the south-westerly direction of the winds which prevail in these months is the natural consequence, as well as the peculiar distribution of the temperature as figured on the small charts on page 121. In addition to the examples already adduced, I shall give other two instances which place in a remarkably strong light the intimate connection between these two important meteorological elements.

286. Example 7.The Great Frost of December 1860.This memorable frost, unquestionably the severest that has occurred in Scotland during at least the present century, may be considered as having commenced with a heavy fall of snow on the 18th, and to have continued till the 26th. I have laid down the isobarometric curves for Europe and Western Asia for each of these nine days. The general result is shown by a chart showing the average pressures during these nine days. These I have calculated for a large number of places, and thence drawn the isobarometric lines on the accompanying chart (fig. 20), in which the solid lines show a pressure of 30 inches and upwards, and the dotted lines a pressure of less than 30 inches. The mean direction of the wind during the time is represented by arrows. Compare this chart of the pressure with Plate II., representing the mean pressure for January, which is nearly that of the period of the year when this great frost occurred. It is seen that from the 18th to the 26th December 1860, the isobarometric lines did not lie as usual from W.S.W. to E.N.E., and show a gradual diminution of pressure from the south of Europe to Iceland, but that they indicated an enormously high pressure in

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