Part 16

Mr. Strachan, in his able pamphlet on “Weather Forecasts,” puts the matter thus: “It follows from Ballot’s Law that in the northern temperate zone the winds will circulate around an area of low atmospherical pressure in the reverse direction to the movement of the hands of a watch, and that the air will flow away from a region of high pressure, and cause an apparent circulation of the winds around it, in the direction of watch hands.” And as the result of a careful digest of data contained in the eleventh number of meteorological papers, published by the Board of Trade, he has established the following valuable propositions. As introductory to the propositions, it should be stated that the positions of observations were the following:—

Places. Latitude. Longitude. Nairn 57° 29´ N. 4° 13´ W. Brest 48 „ 28 4 „ 29 W. Valentia 51 „ 56 10 „ 19 W. Yarmouth 52 „ 37 1 „ 44 E. Portrush (or Greencastle) 55 „ 12 6 „ 40 W. Shields 55 „ 0 1 „ 27 W.

Nairn and Brest are situated nearly on the same meridian, about 540 geographical miles apart. Valentia and Yarmouth are nearly on the same parallel of latitude, about 450 miles apart. Portrush and Shields, distant 180 miles, are on a parallel which is nearly as remote from the parallel of Nairn as that of Valentia and Yarmouth is from the one passing through Brest; and Shields is about as much to the westward of Yarmouth as Portrush is to the eastward of Valentia. When observations have not been obtainable for Brest, those made at Penzance have been used instead.

Proposition 1.—Whenever the atmospherical pressure is greater at Brest than at Nairn, while it is of the same or nearly the same value at Valentia and Yarmouth, being gradually less from south to north, the winds over the British Isles are westerly.

Proposition 2.—Whenever the pressure at Nairn is greater than at Brest, while its values at Valentia and Yarmouth are equal, or nearly so, the winds over the British Isles are easterly.

Proposition 3.—Whenever the pressure at Valentia is greater than at Yarmouth, while its values at Brest and Nairn are nearly equal, the winds over the British Isles are northerly.

Proposition 4.—Whenever the pressure at Yarmouth exceeds that at Valentia, while there is equality of pressure at Nairn and Brest, the winds of the British Isles are southerly.

Proposition 5.—Whenever the pressure of the atmosphere is equal, or nearly so, at Brest, Valentia, Nairn, and Yarmouth, and generally uniform, the winds over the British Isles are variable in direction and light in force.

The data from which the foregoing propositions were deduced, and indeed all other cases calculated by Mr. Strachan, show in every well marked instance that when the atmospherical pressure was

(1) greater in the south than in the north, the wind had westing;

(2) greater in the north than in the south, the wind had easting;

(3) greater in the east than in the west, the wind had southing;

(4) greater in the west than in the east, the wind had northing;

(5) uniformly high, or uniformly low, variable light winds (with fine weather in the former case, and vapoury or wet weather in the latter).

Conditions (1) and (3) give winds from the S.W. quarter.

Conditions (1) and (4) give winds from the N.W. quarter.

Conditions (2) and (4) give winds from the N.E. quarter.

Conditions (2) and (3) give winds from the S.E. quarter.

These principles may be employed to set forth the mode of foretelling the impending change of wind as regards its direction and force; for the atmospherical pressure may change—

(a) uniformly over the whole area of observation;

(b) by increasing in the south, or (which causes a similar statical force) by decreasing in the north;

(c) by increasing in the north, or (which has the same effect) by decreasing in the south;

(d) by increasing in the west, or (which has the same effect) by decreasing in the east;

(e) by increasing in the east, or (which has the same effect) by decreasing in the west;

------------------------------------------------------------------------ Scale, 0 to 6. | |Pressure in pounds | per square foot. | | | |Miles per hour. | | | | | |Seaman’s Nomenclature. | | | | | | | |Scale, 0 to 12. | | | | | | | | | | Beaufort Scale. ---+-----+---+---------+----+------------------------------------------- 0·0| 0·00| 2 |Calm | 0 | | | | | | 0·5| 0·25| 5 |Light Air| 1 | Just sufficient to make steerage way. | | | | | | | | Breeze | | 1·0| 1·00|10 | Light | 2 | ⎧With which a ship with ⎫ 1 to 2 knots. 1·5| 2·25|15 | Gentle | 3 | ⎨ all sail set would go ⎬ 3 to 4 „ 2·0| 4·00|20 | Moderate| 4 | ⎩ in smooth water. ⎭ 5 to 6 „ 2·5| 6·25|27 | Fresh | 5 | ⎧ ⎫ Royals, &c. 3·0| 9·00|35 | Strong | 6 | ⎪In which ⎮ Single Reefs and T.G. Sails. -- | -- |42 | -- -- | | ⎪ ⎮ Double Reefs and Jib, &c. | | | | | ⎪ ⎮ | | | Gale | | ⎨ she could ⎬ 3·5| -- |50 |Moderate | 7 | ⎪ ⎮ 4·0|16·00|60 |Fresh | 8 | ⎪ ⎮ Triple Reefs, &c. 4·5|20·25|-- |Strong | 9 | ⎩ just carry⎭ Close Reefs and Courses. 5·0|25·00|70 |Whole | 10 | ⎧In which she could just bear close-reefed | | | | | ⎩ Maintopsail and reefed Foresail. 5·5|30·25|80 |Storm | 11 | Under Storm Staysails or Trysails. 6·0|36·00|90 |Hurricane| 12 | Bare Poles. ---+-----+---+---------+----+-------------------------------------------

With (a) similar wind and weather will continue. „ (b) winds will veer towards west. „ (c) „ „ east. „ (d) „ „ north. „ (e) „ „ south.

“The probable strength of wind will be in proportion to the rate of increase of statical force, or differences of barometrical readings. The position of least pressure must be carefully considered; as, in accordance with the law, the wind will blow around that locality. The same remark applies to areas of high pressure, which, however, very rarely occur in a well-defined manner over the British Isles.”

Referring to the table on page 76, the scale 0 to 6 was formerly used by meteorological observers at land stations, and it was intended to express, when the square of the grade was obtained, the pressure of the wind as given in the second column.