Lecture 7 — On the generation of winds.

Let us proceed to the theory of winds. Its basis is a distinction we have already made. We recognize two kinds of evaporation, one moist, the other dry. The former is called vapor: for the other there is no general name but we must call it a sort of smoke, applying to the whole of it a word that is proper to one of its forms. The moist cannot exist without the dry nor the dry without the moist: whenever we speak of either we mean that it predominates.

Now when the sun in its circular course approaches, it draws up by its heat the moist evaporation: when it recedes the cold makes the vapor that had been raised condense back into water which falls and is distributed through the earth. (This explains why there is more rain in winter and more by night than by day: though the fact is not recognized because rain by night is more apt to escape observation than by day.) But there is a great quantity of fire and heat in the earth, and the sun not only draws up the moisture that lies on the surface of it, but warms and dries the earth itself.

Consequently, since there are two kinds of evaporation, as we have said, one like vapor, the other like smoke, both of them are necessarily generated. That in which moisture predominates is the source of rain, as we explained before, while the dry evaporation is the source and substance of all winds.

That things must necessarily take this course is clear from the resulting phenomena themselves, for the evaporation that is to produce them must necessarily differ; and the sun and the warmth in the earth not only can but must produce these evaporations.

Since the two evaporations are specifically distinct, wind and rain obviously differ and their substance is not the same, as those say who maintain that one and the same air when in motion is wind, but when it condenses again is water. Air, as we have explained in an earlier book, is made up of these as constituents. vapor is moist and cold (for its fluidity is due to its moistness, and because it derives from water it is naturally cold, like water that has not been warmed): whereas the smoky evaporation is hot and dry. Hence each contributes a part, and air is moist and hot.

It is absurd that this air that surrounds us should become wind when in motion, whatever be the source of its motion on the contrary the case of winds is like that of rivers. We do not call water that flows anyhow a river, even if there is a great quantity of it, but only if the flow comes from a spring. So too with the winds; a great quantity of air might be moved by the fall of some large object without flowing from any source or spring.

The facts bear out our theory. It is because the evaporation takes place uninterruptedly but differs in degree and quantity that clouds and winds appear in their natural proportion according to the season; and it is because there is now a great excess of the vaporous, now of the dry and smoky exhalation, that some years are rainy and wet, others windy and dry. Sometimes there is much drought or rain, and it prevails over a great and continuous stretch of country. At other times it is local; the surrounding country often getting seasonable or even excessive rains while there is drought in a certain part; or, contrariwise, all the surrounding country gets little or even no rain while a certain part gets rain in abundance. The reason for all this is that while the same affection is generally apt to prevail over a considerable district because adjacent places (unless there is something special to differentiate them) stand in the same relation to the sun, yet on occasion the dry evaporation will prevail in one part and the moist in another, or conversely. Again the reason for this latter is that each evaporation goes over to that of the neighboring district: for instance, the dry evaporation circulates in its own place while the moist migrates to the next district or is even driven by winds to some distant place: or else the moist evaporation remains and the dry moves away. Just as in the case of the body when the stomach is dry the lower belly is often in the contrary state, and when it is dry the stomach is moist and cold, so it often happens that the evaporations reciprocally take one another's place and interchange.

Further, after rain wind generally rises in those places where the rain fell, and when rain has come on the wind ceases. These are necessary effects of the principles we have explained. After rain the earth is being dried by its own heat and that from above and gives off the evaporation which we saw to be the material cause of. wind. Again, suppose this secretion is present and wind prevails; the heat is continually being thrown off, rising to the upper region, and so the wind ceases; then the fall in temperature makes vapor form and condense into water. Water also forms and cools the dry evaporation when the clouds are driven together and the cold concentrated in them. These are the causes that make wind cease on the advent of rain, and rain fall on the cessation of wind.

The cause of the predominance of winds from the north and from the south is the same. (Most winds, as a matter of fact, are north winds or south winds.) These are the only regions which the sun does not visit: it approaches them and recedes from them, but its course is always over the west and the east. Hence clouds collect on either side, and when the sun approaches it provokes the moist evaporation, and when it recedes to the opposite side there are storms and rain. So summer and winter are due to the sun's motion to and from the solstices, and water ascends and falls again for the same reason. Now since most rain falls in those regions towards which and from which the sun turns and these are the north and the south, and since most evaporation must take place where there is the greatest rainfall, just as green wood gives most smoke, and since this evaporation is wind, it is natural that the most and most important winds should come from these quarters. (The winds from the north are called Boreae, those from the south Noti.)

178. After determining about the sea, whose saltiness is caused from an admixture of the dry earthy exhalation, the Philosopher subsequently determines about the winds, which are caused by the same dry exhalation. And it is divided into two sections:

in the first he determines about the winds themselves, at (358a27);

in the second about certain phenomena caused from winds at we must go on to discuss earthquakes next (365a14).

The first is divided into two parts:

in the first he determines about winds in general;

in the second about the species of winds at let us now explain the position of the winds (363a21).

The first is divided into three parts:

in the first he determines about the generation of winds, at (358a27);

in the second about their local motion at the course of winds is oblique (361a22);

in the third about their increase and abatement at the sun both checks the formation of winds (361b14).

Regarding the first he does three things:

first, he lays down the principles of the generation of winds (358a27);

second, he describes the manner of their generation, at consequently, since there are two kinds of evaporation (360a8);

third, he manifests what has been said, at that things must necessarily take this course (360a13).

179. Regarding the first he does two things:

first he assigns the material principle of winds (359b27) and says that, since spirits, i.e., winds, are to be discussed, it is necessary to recall this principle, already enunciated, namely, that there are two kinds of exhalation: one, indeed, is the moist, which is called vapor; the other is the dry, which, having no common name, is called smoke from one of its forms: for smoke is, strictly speaking, the dry exhalation of burning wood. Now these two exhalations are not so independent of each other that the moist is without the dry, and the dry without the moist—rather they are denominated one or the other by that which is predominant in a given case.

180. second at now when the sun in its circular course approaches (359b34), he mentions the efficient principle, which is the motion of the sun. And he says that when the sun in its course approaches a given region of the earth, its warmth elevates the moist; as the sun recedes, this raised vapor is condensed into water on account of the cold. This is why there is more rain in winter than in summer, and more at night than during the day, although night-rains go unobserved because of sleep. The rain water is divided up in the earth and drunk in by it. In the earth much heat exists, caused by the action of the sun and other heavenly bodies. And the sun overhead, heating the earth, not only draws aloft the moisture resting on the surface of the earth—for example, the water of the sea, rivers and ponds—but also dries out the earth itself and draws up the moisture drunk by the earth. Consequently the exhalation it produces from the moisture resting on the earth is called "vapor," but the exhalation that results from its drying out the earth is called "smoke," just as in a parallel case, the exhalation from heated wood is called "smoke. "

181. Then when he says, consequently, since there are two kinds of evaporation (360a8), he determines the generation of winds and says that since exhalations are of two kinds, as has been said, one vaporous and one smoky, it is necessary that, from the sun's motion, both should come about. The one with more moisture is the source of rain water, as said above (which he says, because he had previously stated that some dry exhalation is mixed with it); but the dry exhalation is the source of winds.

182. Then when he says, that things must necessarily take this course (360a13), he manifests what has been said about the generation of winds. About this he does three things:

first, he manifests it with an argument;

second, from what has been said, he excludes false opinions about winds, at since the two evaporations are specifically distinct (360a17);

third, he manifests this with signs, at the facts bear out our theory (360a33).

He says therefore first (360a13) that since exhalations are of two kinds, on account of the two sources from which they are derived, namely, earth and water, it is possible, even necessary, that the sun and the heat which environs the earth can cause the release of both exhalations.

183. Then when he says, since the two evaporations are specifically distinct (360a17), he dismisses false theories about the winds. First, the opinion of those who said that the natures of wind and of rain are the same. This he excludes by the fact that the effects of diverse things are themselves diverse: hence, since the exhalations differ on the basis of dry and moist, it is necessary that the nature of wind and of rain water be not the same, as some supposed who said that it is the same air which, when moved, is wind, and when condensed, becomes water. But as stated in On Generation, air has something of vapor and of smoke. Its vapor is cold and moist and well-definable by its density; and this belongs to air in so far as it is moist. Thus also vapor, which is borne up from water, is cold by its very nature, as also is unwarmed water: just as warmed water remains cold according to nature, so also vapor. But smoke is hot and dry: because of the earth, it is dry; because of fire, it is hot. Hence it is manifestly plain that the upper air, which is hot and moist, bears a likeness to both.

184. second at it is absurd that this air (360a27), he dismisses the false opinion as to its tenet that wind is nothing more than air in motion. And he says that it is unacceptable for anyone to suppose that the air which surrounds each of us is, when in motion, wind; or that every movement occurring in air is wind; just as also we do not suppose any water at all that flows, even if it be a large amount, to be a river, but only when it flows from some determinate source, which is a spring gushing from the earth. The same applies to winds: it is not a wind, if air, even in large amounts, is moved in some chance way, but only when it has as its source, as though its spring, a raised dry exhalation. Consequently, it is not true that air in motion is wind: both because sometimes a small amount of air is in motion, and because it does not have a starting-point.

185. Then when he says, the facts bear out our theory (360a33), he manifests through signs what has been said about the generation of winds. And it is divided into three parts according to the three signs he gives:

the second part begins at further, after rain wind generally rises (360b26);

the third part at the cause of the predominance of winds (361a4).

He says therefore first (360a33) that the phenomena attending winds and rains support what has been said about their generation. For because exhalations are continually occurring, now more, now less, clouds producing rain, and winds are forever being generated according to the nature of the season: for according to the varying condition of the season more occurs at one time and less at another. And because more vaporous exhalation is sometimes lifted up and more of the smoky at other times, depending on the diverse effects of the sun and stars, the consequence is that sometimes the years are more rainy and wet, and sometimes more windy and dry. Now this happens in two ways: in one way, with respect to some whole continuous region, where at one time there are many rainstorms and at another many winds; in another way, with respect to various parts. For sometimes in one area of one region many rainstorms occur, and in another area of the same region there is a great drought; at other times the opposite occurs, namely, that the entire surrounding region has middling rain or even drought, while the other has an excessive rainfall. And he assigns the reason, saying that the cause of this is that it is reasonable that the same passion of dryness or dampness should extend often to a whole region, because places that are close to one another have the same position in relation to the sun, which is the cause of rain or of winds—unless one place happens to have a special characteristic which changes its disposition, such as mountains or bodies of water. But although it most frequently happens that an entire region shares the same weather passion, yet it sometimes happens that one locality of a region has an abundance of dry exhalation to generate winds, whereas another abounds in the moist to generate rains; or the opposite happens, i.e., a region that once abounded in rain now abounds in wind. The reason for this diversity is that both types of exhalation can cross over into the exhalation of a had, i.e., following [adjacent] region: for example, the dry exhalation sometimes blows up a wind in the very region from which it was elevated, but the moist exhalation is blown to a region next to the windy one; while sometimes the moist exhalation may remain and the dry migrate. For, just as in the body of an animal the upper flatulence which exhales from the stomach is in a condition contrary to the lower which exhales from the intestines, so, with respect to places, it happens that from the interchanging of exhalations, they undergo a certain counter-state, so that while dryness prevails in the region from which the moist exhalation is transported, moisture abounds in the region to which it is carried.

186. Then when he says, further, after rain wind generally rises (360b26), he gives the second sign, and says that in the places in which it has rained, a wind often arises after the rain; and conversely, the winds stop when it rains. And this occurs because of what has been said about the sources of rain and winds, namely, that the one is from the dry exhalation and the other from the moist. For when the rain has fallen and wet the earth, the dry exhalation, which is the stuff of winds, is once more exhalated from the earth dried out by its own inherent warmth or by that of the sun above. And this is the cause of winds occurring after rains: namely, since winds are increased by the separation of such elevation from the earth. But they cease because the vigor of heat once more separates the warm vapor from the earth, and it is elevated to a higher place where the cold condenses it and forms rain—which is the reason for rains occurring after winds. Rains not only succeed winds but destroy them: for, when clouds are pushed together by a wind into one place, the surrounding cold condenses them and water is generated; the water then cools and dampens the dry exhalation which was the stuff of the winds. Hence it is plain that flowing waters make winds cease, and that, when they cease, the rain succeeds them, for the reasons given. And he takes this as a sign that wind and rain come from contrary causes.

187. Then when he says, the cause of the predominance of winds (361a4), he gives a third sign that winds are generated from the dry exhalation. For this is the cause why winds blow mostly from the Bear, i.e., from the north (described as from the Bear, because the two "bears," Big and Little, circle closely around the North Pole) and from the south — for it is a fact that most winds are boreal, i.e., from the north, or austral, i.e., from the south. The reason for this is that the sun is not moved over those places but approaches them and then departs. The closest it gets to the north pole is the beginning of Cancer; then it departs farther and farther, until it reaches the beginning of Capricorn, which is its maximum approach to the contrary pole, from which, departing once more, it cyclically returns to the beginning of Cancer. For this reason, these two points, namely, the beginnings of Cancer and of Capricorn, are called tropics, i.e., turning-points: when the sun is in the beginning of Cancer, the summer turn occurs; when it is in the beginning of Capricorn, the winter turn. Beyond these points the sun gets no closer to either pole. But it is always moving over the east and west. And therefore, in the places lateral to the sun's course many clouds gather, because, as the sun approaches, a moist exhalation is formed on account of the heat; after the sun recedes to its opposite place, rains and wintry cold arise.

And so, depending on whether the sun is approaching the tropics or departing, summer and winter are produced, and water is raised aloft by evaporation and once more descends. For when the sun in the heavens attains the beginning of Cancer, our summer is produced, and an abundance of vapors are elevated on account of the heat produced by the nearness of the sun; but when it comes to the beginning of Capricorn, cold and winter are produced for us, and many rains come, because of the distance of the sun from us. But the opposite is occurring in that part of the earth situated at the other pole. Since in those regions to the south and north the most water falls, the greatest amount of exhalation must also occur there, just as more smoke is produced from wood green and damp than from dry. Hence, since an exhalation of that type is the source of winds, it is reasonable that more winds and the strongest ones blow from the south and are called "austral," and from the north and are called "boreal." It should be noted that Aristotle here says that the south wind blows from the other pole according to the opinions of others; but later on he will give the opposite as his own opinion and will assign a different cause of the vigor of this wind.