Lecture 9 — On the increase and diminishing of winds

The sun both checks the formation of winds and stimulates it. When the evaporation is small in amount and faint the sun wastes it and dissipates by its greater heat the lesser heat contained in the evaporation. It also dries up the earth, the source of the evaporation, before the latter has appeared in bulk: just as, when you throw a little fuel into a great fire, it is often burnt up before giving off any smoke. In these ways the sun checks winds and prevents them from rising at all: it checks them by wasting the evaporation, and prevents their rising by drying up the earth quickly. Hence calm is very apt to prevail about the rising of Orion and lasts until the coming of the Etesiae and their 'forerunners'.

Calm is due to two causes. Either cold quenches the evaporation, for instance a sharp frost: or excessive heat wastes it. In the intermediate periods, too, the causes are generally either that the evaporation has not had time to develop or that it has passed away and there is none as yet to replace it.

Both the setting and the rising of Orion are considered to be treacherous and stormy, because they place at a change of season (namely of summer or winter; and because the size of the constellation makes its rise last over many days) and a state of change is always indefinite and therefore liable to disturbance.

The Etesiae blow after the summer solstice and the rising of the dog-star: not at the time when the sun is closest nor when it is distant; and they blow by day and cease at night.

The reason is that when the sun is near it dries up the earth before evaporation has taken place, but when it has receded a little its heat and the evaporation are present in the right proportion; so the ice melts and the earth, dried by its own heat and that of the sun, smokes and vapors.

They abate at night because the cold of the nights checks the melting of the ice. What is frozen gives off no evaporation, nor does that which contains no dryness at all: it is only where something dry contains moisture that it gives off evaporation under the influence of heat.

The question is sometimes asked: why do the north winds which we call the Etesiae blow continuously after the summer solstice, when there are no corresponding south winds after the winter solstice?

The facts are reasonable enough: for the so-called 'white south winds' do blow at the corresponding season, though they are not equally continuous and so escape observation and give rise to this inquiry. The reason for this is that the north wind I from the arctic regions which are full of water and snow. The sun thaws them and so the Etesiae blow: after rather than at the summer solstice. (For the greatest heat is developed not when the sun is nearest to the north, but when its heat has been felt for a considerable period and it has not yet receded far. The 'bird winds' blow in the same way after the winter solstice. They, too, are weak Etesiae, but they blow less and later than the Etesiae. They begin to blow only on the seventieth day because the sun is distant and therefore weaker. They do not blow so continuously because only things on the surface of the earth and offering little resistance evaporate then, the thoroughly frozen parts requiring greater heat to melt them. So they blow intermittently till the true Etesiae come on again at the summer solstice: for from that time onwards the wind tends to blow continuously.)

191. After determining about the generation and motion of winds, the Philosopher here determines concerning their increase and decrease. And it is divided into two parts:

in the first he determines about the decrease of winds, at (361b14);

in the second about their increase, at both the setting and the rising of Orion are considered to be treacherous and stormy (361b30).

About the first he does two things:

first, he shows how the sun is the cause of winds' diminishing;

second, he collects under one general heading the causes from which winds happen to cease or diminish, at calm is due to two causes (361b24).

He says therefore first (361b14) that just as the sun moves the winds, so also it makes them cease. For when exhalations are few and feeble, the heat of the sun draws out what is more hot in the exhalation, absorbing it and dissolving the exhalations, just as a larger flame destroys a smaller one by consuming its matter. And so the winds cease. Moreover, the sun not only makes them cease when already existing, but also prevents them from being formed: namely, when by drying out the earth it anticipates the massing together of the exhalation, which is the matter of winds (and this happens especially in those seasons and places that are hot and dry). It is as though, if someone should throw a small bit of fuel into a large fire, the strength of the fire were to dry out the moisture in the fuel before smoke could issue forth from it.

Therefore the sun both makes winds to cease by consuming the material already collected, and prevents them from forming by quickly drying out the earth. That is why, around Orion's rising, i.e., before the time when the constellation of Orion begins to appear, coming out from under the sun's rays, during the time of summer's heat, a great calm from the winds prevails in the air, until the Etesiae, which are the annual winds that are accustomed to blow annually in the summer, and the prodromes, i.e., the "forerunners"—because certain winds sometimes arrive before the Etesiae, due to suitable matter's being sometimes quickly prepared.

192. Then when he says, calm is due to two causes (361b24), he brings together the causes of winds' ceasing and says that a calm due to the absence of winds comes about from two causes: either from great cold quenching the warmness that resolves exhalations, as happens at the time of a sharp frost—since it has been said above that frost is prevented by winds, and therefore, when there is great cold and frost, there are no winds; or from excessive heat, which chokes and quenches the exhalation, as was said above. But even in intermediate seasons, i.e., between periods of maximum heat and maximum cold, there are very many calms: either when an exhalation has not yet been formed after heat and cold no longer block them, or when some exhalation has already been formed and has passed, and another has not yet come, after winds have been generated from the one now past.

193. Then when he says, both the setting and the rising of Orion are considered to be treacherous and stormy (361b30), he determines about the increase of winds.

First, about the increase which accompanies the rising of Orion, at (361b30);

second, about the increase occurring after the rising of the Dog Star, at the Etesiae blow after the summer solstice (361b35).

He says therefore first (361b30) that the configuration of Orion in its rising and setting, i.e., when it begins to appear and when it begins to disappear, is undiscerning, i.e., intolerable, and difficult, i.e., has severe and stormy winds. Nor is this contrary to his previous statement, for, before the rising of Orion, there is calm, but in the period of its rising and setting there is stormy weather. The cause of this is that its rising occurs when summer is changing to fall, and its setting when fall is changing to winter. Both times, i.e., that of its rising and that of its setting, cover a number of days because of the constellation's size, which does not appear all at once and disappear all at once. But when seasons are changing, many disturbances occur, for when a season is not determined to one, it inclines now in one direction, now to its contrary. That is why many rains and winds are produced on account of the exhalations.

194. Then when he says, the Etesiae blow after the summer solstice (361b35), he determines about the increase of wind after the rising of the Dog Star. About this he does three things:

first, he proposes the proposition, at (361b35);

second, he assigns the cause, at the reason is that when the sun is near (362a2);

third, he raises a doubt on this point, at the question is sometimes asked (362a11).

He says therefore first (361b35) that the Etesiae, i.e., certain annual winds, blowing, as it were, always at the same time, blow after the conversions, i.e., after the summer solstice—and not only right after the solstice, but also after the rising of the Dog Star. This is so because they do not blow when the sun is most close to us, namely, in the first turning [solstice], i.e., in the beginning of Cancer, nor when it is far away, as for example, when it is in the southern signs. Moreover, the Etesiae blow by day but stop at night.

195. Then when he says, the reason is that when the sun is near (362a2), he assigns the cause of the aforesaid.

First, why it is that the Etesiae blow during the day, and particularly in the morning and around evening. And he says that when the sun is closest, it dries out any moisture from which wind material might be assembled, if it could be resolved; but when it recedes a bit, then an exhalation is resolved in a moderate amount; the heat, likewise, is moderate, so that frozen waters liquefy; and the earth, being dried both by the sun's heat and by its own inherent warmth, begins to swell, as it were, and, as the resolved moisture accumulates, to exhale—and in this way winds are generated.

196. second at they abate at night (362a7), he shows why the Etesiae abate at night and says that the reason this happens is that the night cold freezes the liquefying humors, so that the exhalation stops. For it is plain that neither does what is frozen exhale, nor what is dry, having no moisture, but the dry which has moisture exhales when heated. Consequently, the Etesiae do not blow either when the sun is nearest us on account of the drying: nor at night, on account of the freezing. However, there is another explanation that could be given, namely, that at night the sun is most distant from us and therefore cannot lift exhalations.

197. Then when he says, the question is sometimes asked (362a11) he raises a doubt about what was just determined. About this he does three things:

first, he raises the doubt (362a11);

second, he solves it, at the facts are reasonable enough (362a13);

third, he dismisses something seemingly contrary to the solution, at but the south wind blows from the tropic of Cancer (362a31).

He says therefore first (362a11) that some wonder why the north winds blow as they do continuously after the summer solstice, while the Noti, i.e., the south winds, do not blow the same way after the winter solstice. For it seems that just as after the sun's approach to the north pole, winds blow from there, so after its approach to the opposite pole, winds should blow from the opposite direction.

198. Then when he says, the facts are reasonable enough (362a13), he solves this problem and says that at the corresponding season, winds do blow, called the leuconoti, so called because they blow in fair weather (leucos in Greek means "white"); but since they do not blow uninterruptedly, as do the Etesiae from the north — and therefore, because latent, it is the fact of their not being noticed that causes the present problem. Now the cause why they do not blow continuously is this: the north wind blows from places under the arctic pole where abound water and snow, which are melted by the sun more after the summer solstice than in the beginning of the turning, although the sun is then closest to us; and that is why it is after and not during the summer solstice that the Etesiae blow. Similarly, the greatest stifling heat occurs, not when the sun is nearest us in the north, but the greater heat comes later, because of the continuing of the warming process over a long time.

For first, when the sun approaches the tropic, it finds matter disposed, then, beginning to dominate it little by little, it imprints its effect more after it begins to recede, but while it is nevertheless still near. That is why after the rising of the Dog Star, i.e., during the dog-days, there is more heat than before the solstice, or during it. Water and snow melt more then also; for which reason more exhalations occur and more winds blow then. But it is true that at the solstice, when the sun is nearer, it causes more drying, as he said above, and disposes the matter more for exhalation; but the greater exhalation occurs after the rising of the Dog Star, and it is then that the Etesiae blow without interruption. And similarly, after the winter solstice the ornithiae blow (so called from a bird or hen), because they blow as some constellation of the "Bird" rises, just as the Etesiae blow after the rising of the Dog Star. (The "ornithiae" are the winds he previously called "leuconoti.") And he says that the Ornithiae are weak, because they are minor winds and blow later than the Etesiae: for they begin to blow on the seventieth day after the winter solstice, as though around the beginning of spring.

The reason for this is that the sun is more remote and less strong, so that it does not totally heat that region (from which the south winds blow) and enable exhalations to be lifted up to form winds. And the reason why they do not blow continuously is this: certain moistures, that are on the surface of the earth and are weak, exhale from that region of the earth, the sun being at a distance, but from these a continuous wind cannot be generated; the other moistures, being more frozen, need a greater heat, before they vaporize, whereas the heat which the sun then affords is slight, because it is so far away. That is why those winds do not blow continuously, but in fits, until the Etesiae blow again from the north after the summer solstice: for these northern winds are better suited for continuous blowing, for the cause stated above.