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.