Lecture 8 — On the local motion of the winds.

The course of winds is oblique: for though the evaporation rises straight up from the earth, they blow round it because all the surrounding air follows the motion of the heavens.

Hence the question might be asked whether winds originate from above or from below. The motion comes from above: before we feel the wind blowing the air betrays its presence if there are clouds or a mist, for their motion shows that the wind has begun to blow before it has actually reached us; and this implies that the source of winds is above. But since wind is defined as 'a quantity of dry evaporation from the earth moving round the earth', it is clear that while the origin of the motion is from above, the matter and the generation of wind come from below. The oblique movement of the rising evaporation is caused from above: for the motion of the heavens determines the processes that are at a distance from the earth, and the motion from below is vertical and every cause is more active where it is nearest to the effect; but in its generation and origin wind plainly derives from the earth.

The facts bear out the view that winds are formed by the gradual union of many evaporations just as rivers derive their sources from the water that oozes from the earth. Every wind is weakest in the spot from which it blows; as they proceed and leave their source at a distance they gather strength. Thus the winter in the north is windless and calm: that is, in the north itself; but, the breeze that blows from there so gently as to escape observation becomes a great wind as it passes on.

We have explained the nature and origin of wind, the occurrence of drought and rains, the reason why rain stops wind and wind rises after rain, the prevalence of north and south winds and also why wind moves in the way it does.

188. After determining about the generation of winds, the Philosopher now determines about their movement. About this he does two things:

first, he describes their movement, at (361a22);

second, he inquires into the source of their movement, at hence the question might be asked whether winds originate (361a25).

He says therefore first (361a22) that although the exhalation which is the principle of winds is lifted on high in a straight line, yet the motion of winds is not in a straight line: for the winds blow around the earth from one direction to the other, as from east to west, or vice versa. The cause of such a course is that, as previously stated, the upper region of air is moved circularly according to the motion of the heaven; and although winds do not blow in that upper air, as stated above, but in the lower air which is below the altitude of the highest mountains, yet this air also participates somewhat in the motion of the higher, although this circling is not completed. From this it comes about that the exhalations moving the air do not move it up or down (which the subtlety of the warmed exhalation or the cold of the condensed seem to demand); rather they move the air obliquely, as though the air were being affected by both motions. Hence, a wind's course need not be always westward, as is the course of the heavens, but in a direction opposite to that of the exhalation impelling it; which impulsion, however, becomes oblique on account of the influence exerted by the movement of the heaven. However, the fact that its obliqueness is caused by the motion of a heavenly body does not make the wind's motion unnatural. This is so both because motions caused in lower bodies by the heavenly body are called natural, though they may not be according to the nature of the lower body, as is plain in the ebb and flow of the sea—since the lower bodies are by nature subject to the higher, and because whatever results in a thing from the cause of its generation is natural to it. Hence, since the agent cause of winds is the motion of the sun, as was said above, it follows that the obliqueness of the motion, since it is caused by the motion of the heaven, is natural to it.

189. Then when he says, hence the question might be asked whether winds originate (361a25), he inquires about the starting-point of the movement of the winds:

first, as to whence they begin their movement, at (361a25);

second, as to how they proceed from that starting-point, at the facts bear out the view that winds (361b1).

He says therefore first (361a25) that because the course of a wind is not straight up or down, but oblique, someone will wonder whence the starting-point of a wind's movement is, whether from above or below. That the starting-point of the movement of winds is above, is manifested by the air itself, in which the wind's movement appears before the wind blows on earth. For if a cloud or mist is seen in the sky, it is seen to be moved by the wind already present in the air even before it manifestly reached the earth, as a wind having its source of movement from above. But because a wind is generated from the mass of dry exhalation resolved from the earth, it is plain that, although the source of its motion is from above, nevertheless the material principle of its generation is from below. The reason for this is that the motion of a wind originates from that place toward which the elevated dry exhalation tends, just as rain begins to descend from the place whither the vapor has ascended. And this is evident from the fact that the motion of a wind is more vigorous in places high above the earth; moreover, since the exhalation in a straight line is borne upwards, it is from there that the motion originates; and in a place more approaching that beginning, there is more of a chance for wind. However, it is plain that the principle of the generation of a wind is from the earth.

190. Then when he says, the facts bear out the view that winds (361b1), he shows how the winds come forth from their principle. And he says that just as the sources of rivers are collected together little by little from various sectors of the earth, so too a wind is gathered together bit by bit from exhalations that are blended together. And he manifests this with two signs. One of these is that winds appear feeblest in the places in which they arise, but take on strength as they move along. The other sign is that, in northern regions, in winter there is calm and there are no boreal winds there; but as you leave those northern regions, the wind gradually increases and becomes very strong.

Finally he summarizes what has been said—and it is plain in the text at (361b8). But it should be noted that the winds which he here calls Noti are the ones he above called austral.