Pressure of the Wind.
| Velocity. | Pressure per sq. foot. Rouse and Smeaton. | Description of wind. | |
|---|---|---|---|
| Miles per hour. | Feet per second. | ||
| 1 | 1·47 | 0·005 | Very gentle. |
| 2 | 2·93 | 0·020 | |
| 3 | 4·40 | 0·044 | Light airs. |
| 4 | 5·87 | 0·079 | |
| 5 | 7·33 | 0·123 | Light breeze |
| 10 | 14·67 | 0·492 | |
| 15 | 22·00 | 1·107 | Brisk breeze |
| 20 | 29·34 | 1·968 | |
| 25 | 39·67 | 3·075 | Strong breez |
| 30 | 44·01 | 4·429 | |
| 35 | 51·34 | 6·027 | High wind. |
| 40 | 58·68 | 7·873 | |
| 45 | 66·01 | 9·963 | Gale. |
| 50 | 73·35 | 12·300 | |
| 60 | 88·02 | 17·715 | Heavy Gale. |
| 80 | 117·36 | 31·490 | Hurricane. |
| 100 | 146·70 | 49·200 | Tornado. |
The south-west wind is more constant than any other, and the west wind in our islands is more frequent than the east; tables have been compiled showing the average number of days upon which the winds blow from different quarters, but need not be quoted. Storms can generally be anticipated by the barometer, which falls very quickly for “wind.” The quarter whence the breeze may be expected is often indicated by the streamers of clouds, or “mare’s tails,” across the sky; though we must admit the opposite direction to that anticipated by casual observers may often prove the right one.
Hurricanes and tornadoes are really whirlwinds in motion. The rotatory movement of the air is from right to left in the northern hemisphere, and from left to right in the southern—that is, in the opposite and same directions respectively as the hands of a watch move. The whirlwinds are caused by two currents of air meeting at a certain angle, just as a whirlpool is the result of opposing currents of water.
Fig. 716.—Effects of storm at Halligen in 1834.
The use of the wind in nature cannot be over-estimated. It is frequently destructive and terrible in its effects, but these comparatively trifling damages are as nothing when weighed against the advantages conferred upon mankind by the wind and the currents of the atmosphere. The north cold is tempered by the warm south wind. The pollen and the seeds of plants are borne on the wings of the wind, and the clouds are carried over the land to “drop fatness” upon our fields. The want of free circulation of air is very injurious. Witness the terrible affliction of goitre, so prevalent in the closely shut-in valleys such as the Rhone Valley, where cretinism or congenital idiotcy is distressingly prevalent.
Vapour and Clouds.
Vapour, as we have heard, is invisible, and is produced by heat. As the visible steam (which is invisible as it issues from the safety valve at the actual aperture, and nearly invisible altogether on a hot day) is produced by combustion, so vapour is produced by the heat of the sun’s rays. But there are some observations to be made respecting these rays, which are the cause of vapour, and therefore of cloud, rain, dew, frost, ice, snow, and water all over the earth; and we must look at the circumstances closely.