During an anticyclone there is a descending movement of air currents from a centre of high pressure in all directions, and these currents or winds are deflected "clockwise" in the northern hemisphere; and when cyclonic conditions prevail the air currents are directed inwards towards a low-pressure central area, rotating spirally at the surface of the earth in the direction contrary to the hands of a watch. In the southern hemisphere the directions are reversed. A cyclonic system is usually carried forward by great drift winds like eddies upon a swift stream, in the North Atlantic as a rule from south-west to north-east.
Do we really know the force and direction of the winds at a high altitude during these movements? Are we not merely guessing at the real aerial conditions by the movements near the earth at the time of the departure of the birds? Is it fair, if I am right that the meteorological observations are founded upon only those observed at comparatively low altitudes, to lay down laws as to the particular conditions which are favourable or unfavourable, or the particular winds which are used or avoided? The direction of the wind may be the same up to a great height, many thousand feet, or it may vary within 500 feet of the earth. Nearly fifty years ago, when Glaisher made his great ascents, he sometimes met with three or four currents moving in opposite directions. The more recent upper air investigations show that though as a rule the wind at various elevations is in the main from one point of the compass, its degrees vary considerably, and its force at the various heights shows remarkable differences. Generally the force rises to about 5000 feet, but there is no invariable rule. I tabulate a few examples taken more or less at random from the Weather Reports for 1908. The altitudes above the ground are measured in metres, roughly converted into feet; the letters indicate the direction of the wind, the figures its speed in miles per hour. The last one in the table, observations made at Brighton on September 20th, is particularly useful. The conditions on this date were anticyclonic, and favourable to migration. At 400 feet above the sea the wind was blowing at 5 miles an hour; at between 5000 and 6000 feet its force was 20 miles per hour. What then would happen to a bird leaving Brighton for say the Spanish Peninsula? If it flew at 20 miles an hour towards the French coast about Dieppe, it would meet the wind blowing at 5 miles an hour, and take between five and six hours to reach the coast, head to wind. If it rose to the height of 3000 feet it would meet a wind blowing at the same speed as it was flying, and it could make no headway. If, however, it flew in a south-westerly direction the more it turned westerly the farther it would drift down channel towards Normandy or Brittany, and be carried out to sea! But this is exactly what would not have happened, for on this date a feeble cyclonic system was approaching from the Atlantic and extending its area of influence over southern England. In the Channel the bird would meet westerly winds which would bring it safely to the Brittany shores, or if it missed them, to the western shores of the Bay, where the wind was actually from the north. I mention this merely to show that apparently unfavourable winds may be really favourable.
| Date. | Station. | Ground Level. | 100 mtrs. (330 ft.) | 500 mtrs. (1660 ft.) | 1000 mtrs. (3320 ft.) | 1050 mtrs. (5000 ft.) |
|---|---|---|---|---|---|---|
| Jan. 2 | Petersfield | NE by E | ... | ENE | E by N | E ½ N |
| 30 | 50 | 13 | ||||
| " 2 | Glossop | E by N | ... | E | E by S | ... |
| 1100 ft. | 8 | 15 | 30 | |||
| " 3 | Pyrton Hill | ... | ... | E by N | E | ... |
| 500 ft. | 14 | 35 | 53 | |||
| " 4 | " | NE by E | ... | ... | E by S | E by S |
| 10 | 25 | 25 | 30 | |||
| " 11 | Petersfield | S by E | ... | S | SW by W | SW by W |
| 10 | 3 | 5 | ||||
| April 9 | " | SE | ... | N by W | ... | N ½ W |
| 7 | 20 | |||||
| " 18 | Glossop | N | ... | N by W | NW by N | W |
| 9 | 16 | 7 | ||||
| " 30 | " | S | S by E | S | W by N | ... |
| 14 | 27 | 30 | 46 | |||
| May 16 | " | WSW | W by S | W by S | W | W by N |
| 16 | 26 | 27 | 29 | 33 | ||
| Sept. 5 | " | WSW | W by S | W by S | W by N | W by N |
| 12 | 15 | 17 | 21 | 23 | ||
| " 7 | " | S by W | ... | ... | SW | ... |
| 9 | 16 | 20 | 33 | |||
| " 10 | " | NW by N | NW by N | NW by N | NW | NW |
| 8 | 16 | 21 | 34 | 36 | ||
| " 20 | Brighton | ... | ... | S | ... | ... |
| 380 ft. | 5 | 5 | 15 | 20 | 20 |
| 2000 mtrs. | 2500 mtrs. | 3000 mtrs. | 3500 mtrs. | |
|---|---|---|---|---|
| Date | (6,660 ft.). | (8,320 ft.). | (10,000 ft.). | (11,660 ft.). |
| Jan. 2 | ENE | NE | NE by N | NE by N |
| 23 | 22 | 18 | 25 | |
| " 2 | ||||
| " 3 | ||||
| " 4 | ESE | SE by E | SE by E | |
| 35 | 20 | 15 | ||
| " 11 | N by W | NW | NNW | N½W |
| 9 | 8 | 7 | 11 | |
| April 9 | N | NW½N | NW½W | ... |
| 14 | 9 | 12 | ... | |
| " 18 | NE | N by E | W by N | NNE |
| 6 | 8 | 8 | 9 | |
| " 30 | ||||
| May 16 | WNW | |||
| 35 | ||||
| Sept. 5 | W by N | |||
| 28 | ||||
| " 7 | ||||
| " 10 | ||||
| " 20 | ||||
| 4000 mtrs. | 4500 mtrs. | 5000 mtrs. | 6000 mtrs. | |
|---|---|---|---|---|
| Date | (13,320 ft.). | (15,000 ft.). | (16,700 ft.). | (20,000 ft.). |
| Jan. 2 | NE by N | |||
| 23 | ||||
| " 2 | ||||
| " 3 | ||||
| " 4 | ||||
| " 11 | E½N | E by S | E by NE | ENE |
| 8 | 14 | 13 | 14 | |
| April 9 | NW by W | W by N | ||
| 18 | 20 | |||
| " 18 | NW | SSW | NNW | SW |
| 1 | 3 | 5 | 7 | |
| " 30 | ||||
| May 16 | ||||
| Sept. 5 | ||||
| " 7 | ||||
| " 10 | ||||
| " 20 | ||||
Under ordinary circumstances are we justified in saying that birds make use of the winds blowing with a certain force at the point of departure, or that they ignore them? Certainly we cannot judge by either the force or direction of the wind at the point of arrival, as Mr Clarke points out. The bird may have dropped into most adverse currents.
In Hungary, where migration has been very carefully studied, we find evidence supporting Mr Clarke's theory, and yet giving it a slightly different complexion. Low atmospheric pressure, depression (the warm cyclonic conditions of spring) very often shows the greatest rate in the arrival of the swallow. If there is a centre of depression west of Hungary, and its path is directed north or north-east, swallows appear in crowds. The fair side of the depression, with its warm southerly winds, is therefore favourable. A list of twelve other birds, which also appear in spring under these conditions in greatest numbers, is added. The "bad" side, with cool northerly winds causes delays in the arrival of these thirteen species. The depressions often have a sphere of influence extending so far as North Africa, so that birds, on the fair side, can cross the Mediterranean with southerly winds all the way [(31)].