“In the accompanying figure the solid arrows in the interior part represent the resultant motions of the winds (longer arrows indicating greater velocities), in case of an earth with a homogeneous surface over both hemispheres, in which the motions would be symmetrical in both and the same at all longitudes, and the equatorial and tropical calm belts would be situated at equal distances from each pole. The dotted arrows indicate the strong, almost eastern motion of the air at all latitudes at some high altitude, as that of the cirrus clouds.
“The outline of the outer part of the figure represents an isobaric surface high up where the bulging up near the parallel of 30° disappears and the maximum pressure at the same altitude is transferred to the equator. For lower altitudes the isobaric surfaces have a bulging up at the parallel of 30°, and a slight depression at and near the equator. The arrows in this part represent the polar and equatorial components of motion, the former above and the latter below, except near the earth’s surface on the polar sides of the tropical calm-belts, where there is a polar component of motion arising from the air’s being pressed out from under the belt of high pressure. This, perhaps, does not extend beyond the polar circles, beyond which there can be little motion in any direction, except from abnormal disturbances.
“For reasons given in § 103, the actual mean position of the equatorial and tropical calm-belts are not precisely as here represented, but are all a little displaced toward the north pole, and the polar depression of the isobaric surfaces is greater in the southern than in the northern hemisphere.”
The conclusions from this approximate analysis are in the main supported by observation, except as modified by the heterogeneity of the earth’s surface. The sea-level distribution of barometric pressure between the equator and poles, as found by Ross’ long series of measurements, manifests a variation of about one inch of mercury, with maxima at about 30° of latitude, north and south, as required by Ferrel’s theory. As a further cause of the depression toward the poles, may be mentioned the greater speed of the permanent east wind with the consequent centrifugal lift in the atmosphere.
As to the general easterly direction of the winds at middle and higher latitudes, that is well known from observation of the motion of clouds and of the air near the earth. At the cirrus level the velocity in those latitudes is almost exactly eastward. But the flow in longitude, illustrated by the outer arrows in Fig. 45, has not been fully determined by observation. Moreover, as Ferrel himself showed, the unequal heating of continents and oceans sets up gradients in longitude, especially in the northern hemisphere, thus adding considerable disturbance to the general circulation. To this agency must be added also the latitudinal shifting of insolation, due to the annual march of the sun across the equator, entailing an oscillatory seasonal shift of the hot belt, and therefore of the twin-hemispheric cycle of the atmosphere.
Some currents of the general and permanent circulation are sufficiently prominent to have special names, such as the trade-winds, the antitrade-winds, the prevailing westerlies, and, in the lower latitudes, the calm belts, where the flow is exceptionally feeble. All these currents have been known to sailors since early times, and have been of considerable importance in marine navigation. Eventually, perhaps, they may be of like importance in aërial navigation.
The trade-winds are mild tropical surface currents of remarkably steady speed and direction. Springing from the high-pressure belts in either hemisphere, at about latitude 30°, they blow toward the equator with increasing westerly trend. As shown in charts 46 and 47 for midwinter and midsummer, the trade winds cover a large portion of the tropical zones in both oceans, and shift slightly in latitude with the sun. They are separated at the heat equator by the equatorial calm belts, or doldrums, and are bounded north and south respectively by the calms of Cancer and of Capricorn. Particularly interesting are the trade-winds blowing from Spain to the West Indies, which favored Columbus on his westward voyage, and which certain adventurous Germans have proposed using to duplicate that memorable voyage, in air ships.
Fig. 46.—Normal Wind Direction and Velocity for January and February. (Köppen.)
The antitrade-winds, or counter trades, are lofty winds blowing over and contrary to the trade winds. As some doubt regarding the direction of these counter trades had existed, an expedition was sent in 1905, by two distinguished meteorologists, Teisserenc de Bort of France, and A. Lawrence Rotch of America, to explore the atmosphere above the tropical Atlantic. Mr. Rotch has summarized their measurements and conclusions as follows:[63]
“Pilot balloons, dispatched from the island of Teneriffe and St. Vincent, were observed with theodolites at the ends of a base-line, and in this way the heights at which the balloons changed direction could be ascertained. Later the balloons were sent up from the yacht itself, which steamed after them, measurements being made of their angular elevation. The observations which are plotted in Fig. 46 prove conclusively the existence of the upper counter-trade. The courses of the balloons are represented as if projected upon the surface of the sea and show that the northeast trade-wind extended only to the height of 3,200 or 4,000 meters, and then gradually turned into a southerly current which, higher up, came from the southwest. The width of the dotted band represents approximately the varying velocity of the trade and counter-trade. Similar proofs of the northwest trade-wind, south of the equator were obtained by the same expedition during the following year, but the above suffices to show that it would be possible for an aëronaut in the ordinary balloon to start from the African coast, or from some of the islands in the trade-wind region, and, after drifting towards the southwest, to rise a few miles into the current, which would carry the balloon north and eventually northeast back to land. Nevertheless, it does happen in certain atmospheric situations over the tropical north Atlantic that the winds from the general northwesterly direction prevail up to great heights without any evidence of the return-trade. Near the equator the winds are easterly up to the greatest heights which have been attained.”
